Acronyms relevant to Executable and Linkable Format (ELF)
| ABI | Application binary interface |
| a.out | Assembler output file format |
| BSS | Block started by symbol. The uninitialized data segment containing statically-allocated variables. |
| COFF | Common object file format |
| DTV | Dynamic thread vector (for TLS) |
| DWARF | A standardized debugging data format |
| GD | Global Dynamic (dynamic TLS) One of the Thread-Local Storage access models. |
| GOT | Global offset table |
| IE | Initial Executable (static TLS with assigned offsets) One of the Thread-Local Storage access models. |
| LD | Local Dynamic (dynamic TLS of local symbols) One of the Thread-Local Storage access models. |
| LE | Local Executable (static TLS) One of the Thread-Local Storage access models. |
| Mach-O | Mach object file format |
| PC | Program counter. On x86, this is the same as IP (Instruction Pointer) register. |
| PE | Portable executable |
| PHT | Program header table |
| PIC | Position independent code |
| PIE | Position independent executable |
| PLT | Procedure linkage table |
REL
RELA | Relocation |
| RVA | Relative virtual address |
| SHF | Section header flag |
| SHT | Section header table |
| SO | Shared object (another name for dynamic link library) |
| VMA | Virtual memory area/address |
Useful books and references
ELF man page
System V Application Binary Interface
AMD64 System V Application Binary Interface
The gen on function calling conventions
Section II of Linux Standard Base 4.0 Core Specification
Self-Service Linux: Mastering the Art of Problem Determination by Mark Wilding and Dan Behman
Solaris Linker and Libraries Guide
Linkers and Loaders by John Levine
Understanding Linux ELF RTLD internals by mayhem (this article gives
you an idea how the runtime linker ld.so works)
ld.so man page
Prelink by Jakub Jelinek (and prelink man page)
Executable and Linkable Format
An ELF executable binary contains at least two kinds of headers: ELF file header
(see struct Elf32_Ehdr/struct Elf64_Ehdr in /usr/include/elf.h)
and one or more Program Headers (see struct Elf32_Phdr/struct Elf64_Phdr in /usr/include/elf.h)
Usually there is another kind of header called Section Header, which describe
attributes of an ELF section (e.g. .text, .data,
.bss, etc) The Section Header is
described by struct Elf32_Shdr/struct Elf64_Shdr in /usr/include/elf.h
The Program Headers are used during execution (ELF's "execution view"); it tells the kernel or the runtime linker
ld.so what to load into memory and how to find dynamic linking information.
The Section Headers are used during compile-time linking (ELF's "linking view"); it tells the link editor ld
how to resolve symbols, and how to group similar byte streams from different ELF binary
objects.
Conceptually, the two ELF's "views" are as follows (borrowed from Shaun Clowes's Fixing/Making Holes in Binaries slides):
+-----------------+
+----| ELF File Header |----+
| +-----------------+ |
v v
+-----------------+ +-----------------+
| Program Headers | | Section Headers |
+-----------------+ +-----------------+
|| ||
|| ||
|| ||
|| +------------------------+ ||
+--> | Contents (Byte Stream) |<--+
+------------------------+
In reality, the layout of a typical ELF executable binary on a disk file is like this:
+-------------------------------+
| ELF File Header |
+-------------------------------+
| Program Header for segment #1 |
+-------------------------------+
| Program Header for segment #2 |
+-------------------------------+
| ... |
+-------------------------------+
| Contents (Byte Stream) |
| ... |
+-------------------------------+
| Section Header for section #1 |
+-------------------------------+
| Section Header for section #2 |
+-------------------------------+
| ... |
+-------------------------------+
| ".shstrtab" section |
+-------------------------------+
| ".symtab" section |
+-------------------------------+
| ".strtab" section |
+-------------------------------+
The ELF File Header contains the file offsets of the first Program Header,
the first Section Header, and .shstrtab section which contains
the section names (a series of NULL-terminated strings)
The ELF File Header also contains the number of Program Headers
and the number of Section Headers.
Each Program Header describes a "segment": It contains the permissions (Readable, Writeable, or Executable)
, offset of the "segment" (which is just a byte stream) into the file, and the size of the
"segment". The following table shows the purposes of special segments.
Some information
can be found in GNU Binutil's source file include/elf/common.h:
| ELF Segment |
Purpose |
| DYNAMIC |
For dynamic binaries, this segment hold dynamic linking information and is usually
the same as .dynamic section in ELF's linking view. See paragraph below.
|
| GNU_EH_FRAME |
Frame unwind information (EH = Exception Handling). This segment is usually the same as .eh_frame_hdr section in ELF's linking view.
|
| GNU_RELRO |
This segment indicates the memory region which should be made Read-Only after relocation is done.
This segment usually appears in a dynamic link library and it
contains .ctors, .dtors, .dynamic, .got
sections. See paragraph below.
|
| GNU_STACK |
The permission flag of this segment indicates whether the
stack is executable or not.
This segment does not have any content; it is just an indicator.
|
| INTERP |
For dynamic binaries, this holds the full pathname of runtime linker ld.so
This segement is the same as .interp section in ELF's linking view.
|
| LOAD |
Loadable program segment. Only segments of this type are loaded into memory during execution. |
| NOTE |
Auxiliary information. For core dumps, this segment contains the status of the process (when the core dump is created),
such as the signal (the process received and caused it to dump core), pending & held signals,
process ID, parent process ID, user ID, nice value,
cumulative user & system time, values of registers (including the program counter!) For more info, see
struct elf_prstatus and struct elf_prpsinfo in Linux kernel source file
include/linux/elfcore.h
and struct user_regs_struct in
arch/x86/include/asm/user_64.h |
| TLS |
Thread-Local Storage |
Likewise, each Section Header contains the file offset of its corresponding "content"
and the size of the "content".
The following table shows the purposes of some special sections. Most information
here comes from LSB specification.
Some information can be found in GNU Binutil's source file
bfd/elf.c (look for
bfd_elf_special_section)
and bfd/elflink.c (look for
double-quoted section names such as ".got.plt")
| ELF Section |
Purpose |
| .bss |
Uninitialized global data ("Block Started by Symbol").
Depending on the compilers, uninitialized global variables could
be stored in a nameness section called COMMON (named after
Fortran 77's "common blocks".) To wit, consider
the following code:
int globalVar;
static int globalStaticVar;
void dummy() {
static int localStaticVar;
}
Compile with gcc -c, then on x86_64, the resulting object file has the
following structure:
$ objdump -t foo.o
SYMBOL TABLE:
....
0000000000000000 l O .bss 0000000000000004 globalStaticVar
0000000000000004 l O .bss 0000000000000004 localStaticVar.1619
....
0000000000000004 O *COM* 0000000000000004 globalVar
so only the file-scope and local-scope global variables are in
the .bss section.
If one wants globalVar to reside in the .bss section,
use the -fno-common
compiler command-line option. Using -fno-common
is encouraged, as the following example shows:
$ cat foo.c
int globalVar;
$ cat bar.c
double globalVar;
int main(){}
$ gcc foo.c bar.c
Not only there is no error message about redefinition of the same symbol
in both source files (notice we did not use the extern keyword here),
there is no complaint about their different data
types and sizes either. However, if one uses -fno-common,
the compiler will complain:
/tmp/ccM71JR7.o:(.bss+0x0): multiple definition of `globalVar'
/tmp/ccIbS5MO.o:(.bss+0x0): first defined here
ld: Warning: size of symbol `globalVar' changed from 8 in /tmp/ccIbS5MO.o to 4 in /tmp/ccM71JR7.o
|
| .comment |
A series of NULL-terminated strings containing compiler information. |
| .ctors |
Pointers to functions which are marked as
__attribute__ ((constructor)) as well as static C++ objects' constructors.
They will be used by __libc_global_ctors function.
See paragraphs below.
|
| .data |
Initialized data. |
| .data.rel.ro |
Similar to .data section, but this section
should be made Read-Only after relocation is done.
|
| .debug_XXX |
Debugging information (for the programs which are compiled with -g option)
which is in the DWARF 2.0 format.
See here for DWARF debugging format.
|
| .dtors |
Pointers to functions which are marked as
__attribute__ ((destructor)) as well as static C++ objects' destructors.
See paragraphs below.
|
| .dynamic |
For dynamic binaries, this section holds dynamic linking information used by ld.so.
See paragraphs below. |
| .dynstr |
NULL-terminated strings of names of symbols in .dynsym section.
One can use commands such as readelf -p .dynstr a.out to see these strings.
|
| .dynsym |
Runtime/Dynamic symbol table. For dynamic binaries, this section is the symbol table of
globally visible symbols. For example, if a dynamic link library wants to export
its symbols, these symbols will be stored here. On the other hand, if
a dynamic executable binary uses symbols from a dynamic link library,
then these symbols are stored here too.
The symbol names (as NULL-terminated strings) are stored in .dynstr section.
|
.eh_frame
.eh_frame_hdr |
Frame unwind information (EH = Exception Handling).
See here
for details.
To see the content of .eh_frame section, use
readelf --debug-dump=frames-interp a.out
|
| .fini |
Code which will be executed when program exits normally. See paragraphs below. |
| .fini_array |
Pointers to functions which will be executed when program exits normally. See paragraphs below. |
| .GCC.command.line |
A series of NULL-terminated strings containing
GCC command-line (that is used to compile the code) options. This feature is supported since GCC 4.5
and the program must be compiled with -frecord-gcc-switches option.
|
| .gnu.hash |
GNU's extension to hash table for symbols.
See here for its structure and the hash algorithm.
The link editor ld calls bfd_elf_gnu_hash in
in GNU Binutil's source file bfd/elf.c
to compute the hash value.
The runtime linker ld.so calls do_lookup_x in
elf/dl-lookup.c
to do the symbol look-up. The hash computing function here is dl_new_hash.
|
| .gnu.linkonceXXX |
GNU's extension. It means only a single copy of the section will be used in linking.
This is used to by g++. g++ will emit each template expansion in its own section.
The symbols will be defined as weak, so that multiple definitions
are permitted.
|
| .gnu.version |
Versions of symbols.
See here,
here,
here,
and
here
for details of symbol versioning.
|
| .gnu.version_d |
Version definitions of symbols. |
| .gnu.version_r |
Version references (version needs) of symbols. |
| .got |
For dynamic binaries, this Global Offset Table holds the addresses of variables which are
relocated upon loading. See paragraphs below.
|
| .got.plt |
For dynamic binaries, this Global Offset Table holds the addresses of functions in dynamic libraries.
They are used by trampoline code in .plt section.
If .got.plt section is present, it contains at least three entries, which
have special meanings. See paragraphs below.
|
| .hash |
Hash table for symbols.
See here for its structure and the hash algorithm.
The link editor ld calls bfd_elf_hash in
in GNU Binutil's source file bfd/elf.c
to compute the hash value.
The runtime linker ld.so calls do_lookup_x in
elf/dl-lookup.c
to do the symbol look-up. The hash computing function here is _dl_elf_hash.
|
| .init |
Code which will be executed when program initializes. See paragraphs below. |
| .init_array |
Pointers to functions which will be executed when program starts. See paragraphs below. |
| .interp |
For dynamic binaries, this holds the full pathname of runtime linker ld.so |
| .jcr |
Java class registration information.
Like .ctors section, it contains a list of addresses
which will be used by _Jv_RegisterClasses function
in CRT (C Runtime) startup files (see gcc/crtstuff.c
in GCC's source tree)
|
| .note.ABI-tag |
This Linux-specific section is structured as a note
section in ELF specification. Its content is mandated
here.
|
| .note.gnu.build-id |
A unique build ID. See here and
here
|
| .note.GNU-stack |
See here
|
| .nvFatBinSegment |
This segment contains information of nVidia's CUDA fat binary container. Its format
is described by struct __cudaFatCudaBinaryRec in __cudaFatFormat.h
|
| .plt |
For dynamic binaries, this Procedure Linkage Table holds the trampoline/linkage code. See paragraphs below. |
| .preinit_array |
Similar to .init_array section. See paragraphs below. |
| .rela.dyn |
Runtime/Dynamic relocation table.
For dynamic binaries, this relocation table holds information of variables which
must be relocated upon loading. Each entry in this table is a
struct Elf64_Rela (see /usr/include/elf.h) which
has only three members:
- offset (the variable's [usually position-independent] virtual memory address
which holds the "patched" value during the relocation process)
- info (Index into .dynsym section and Relocation Type)
- addend
See paragraphs below for details about runtime relocation.
|
| .rela.plt |
Runtime/Dynamic relocation table.
This relocation table is similar to the one in .rela.dyn section;
the difference is this one is for functions, not variables.
The relocation type of entries in this table is
R_386_JMP_SLOT or R_X86_64_JUMP_SLOT and
the "offset" refers to memory addresses which are
inside .got.plt section.
Simply put, this table holds information to relocate entries in
.got.plt section.
|
.rel.text
.rela.text |
Compile-time/Static relocation table.
For programs compiled with -c option,
this section provides information to the link editor ld
where and how to "patch" executable code in .text section.
The difference between .rel.text and .rela.text
is entries in the former does not have addend member.
(Compare struct Elf64_Rel with struct Elf64_Rela in /usr/include/elf.h)
Instead, the addend is taken from the memory location
described by offset member.
Whether to use .rel or .rela is platform-dependent.
For x86_32, it is .rel and for x86_64, .rela
|
.rel.XXX
.rela.XXX |
Compile-time/Static relocation table for other sections. For example,
.rela.init_array is the relocation table for .init_array
section.
|
| .rodata |
Read-only data. |
| .shstrtab |
NULL-terminated strings of section names.
One can use commands such as readelf -p .shstrtab a.out to see these strings.
|
| .strtab |
NULL-terminated strings of names of symbols in .symtab section.
One can use commands such as readelf -p .strtab a.out to see these strings.
|
| .symtab |
Compile-time/Static symbol table.
This is the main symbol table used in compile-time linking
or runtime debugging.
The symbol names (as NULL-terminated strings) are stored in .strtab section.
Both .symtab and .symtab can be stripped away by the strip
command.
|
| .tbss |
Similar to .bss section, but for Thread-Local data. See paragraphs below. |
| .tdata |
Similar to .data section, but for Thread-Local data. See paragraphs below. |
| .text |
User's executable code |
How is an executable binary in Linux being executed ?
First, the operating system must recognize executable binaries. For example,
zcat /proc/config.gz | grep CONFIG_BINFMT_ELF can show whether the Linux kernel is compiled
to support ELF executable binary format (if
/proc/config.gz does not exist, try
/lib/modules/`uname -r`/build/.config)
When the shell makes an execvc system call to run an executable binary, the Linux kernel responds as
follows (see here and
here for more details) in sequence:
- sys_execve function (in arch/x86/kernel/process.c) handles the execvc system call
from user space. It calls do_execve function.
- do_execve function (in fs/exec.c) opens the executable binary file and does some preparation.
It calls search_binary_handler function.
- search_binary_handler function (in fs/exec.c) finds out the type of executable binary
and calls the corresponding handler, which in our case, is load_elf_binary function.
- load_elf_binary (in fs/binfmt_elf.c) loads the user's executable binary file into memory.
It allocates memory segments and zeros out the BSS section by calling the padzero function.
load_elf_binary also examines
whether the user's executable binary contains an INTERP segment or not.
- If the executable binary is dynamically linked, then the compiler will usually creates an
INTERP segment (which is usually the same as .interp section in
ELF's "linking view"), which contains the full pathname of an "interpreter", usually
is the Glibc runtime linker ld.so.
To see this, use command readelf -p .interp a.out
According to AMD64 System V Application Binary Interface,
the only valid interpreter for programs conforming to AMD64 ABI is /lib/ld64.so.1
and on Linux, GCC usually uses /lib64/ld-linux-x86-64.so.2
or /lib/ld-linux-x86-64.so.2 instead:
$ gcc -dumpspecs
....
*link:
...
%{!m32:%{!dynamic-linker:-dynamic-linker %{muclibc:%{mglibc:%e-mglibc and -muclibc used
together}/lib/ld64-uClibc.so.0;:/lib/ld-linux-x86-64.so.2}}}}
...
To change the runtime linker, compile the program using something like
gcc foo.c -Wl,-I/my/own/ld.so
The System V Application Binary Interface
specifies, the operating system, instead of running the user's executable binary, should run this
"interpreter". This interpreter should complete the binding of user's executable binary
to its dependencies.
- Thus, if the ELF executable binary file contains an INTERP segment, load_elf_binary will
call load_elf_interp function to load the image of this interpreter as well.
- Finally, load_elf_binary calls start_thread (in arch/x86/kernel/process_64.c)
and passes control to either the interpreter or the user program.
What about ld.so ?
ld.so is the runtime linker/loader (the compile-time linker
ld is formally called "link editor")
for dynamic executables. It provides the
following services:
- Analyzes the user's executable binary's DYNAMIC segment and determines what
dependencies are required. (See below)
- Locates and loads these dependencies, analyzes their DYNAMIC segments
to determine if more dependencies are required.
- Performs any necessary relocations to bind these objects.
- Calls any initialization functions (see below) provided by these dependencies.
- Passes control to user's executable binary.
Compile your own ld.so
The internal working of
ld.so is complex, so you might want to compile and experiment your
own
ld.so.
The source code of
ld.so can be found in
Glibc. The main files are
elf/rtld.c,
elf/dl-reloc.c, and
sysdeps/x86_64/dl-machine.h.
This link
provides general tips for building Glibc. Glibc's own
INSTALL and
FAQ documents
are useful too.
To compile Glibc (ld.so cannot be compiled independently) download and unpack Glibc source tarball.
- Make sure the version of Glibc you downloaded is the same as the system's current one.
- Make sure the environmental variable LD_RUN_PATH is not set.
- Read the INSTALL and make sure all necessary tool chains (Make, Binutils, etc)
are up-to-date.
- Make sure the file system you are doing the compilation is case sensitive, or
you will see weird errors like
/scratch/elf/librtld.os: In function `process_envvars':
/tmp/glibc-2.x.y/elf/rtld.c:2718: undefined reference to `__open'
...
- ld.so should be compiled with the optimization flag on
(-O2 is the default). Failing to do so will end up with weird errors (see Question 1.23 in
FAQ)
- Suppose Glibc is unpacked at
/tmp/glibc-2.x.y/
Then edit /tmp/glibc-2.x.y/Makefile.in: Un-comment the line # PARALLELMFLAGS = -j 4
and
change 4 to an appropriate number.
- Since we are only interested in ld.so and not the whole Glibc,
we only want to build the essential source files needed by ld.so.
To do so, edit /tmp/glibc-2.x.y/Makeconfig: Find the line started with
all-subdirs = csu assert ctype locale intl catgets math setjmp signal \
...
and change it to
all-subdirs = csu elf gmon io misc posix setjmp signal stdlib string time
- Find a scratch directory, say /scratch. Then
$ cd /scratch
$ /tmp/glibc-2.x.y/configure --prefix=/scratch --disable-profile
$ gmake
- Since we are not building the entire Glibc, when the gmake
stops (probably with some errors), check if /scratch/elf/ld.so exists
or not.
- ld.so is a static binary, which means it has its own
implementation of standard C routines (e.g. memcpy, strcmp, etc)
It has its own printf-like routine called _dl_debug_printf.
_dl_debug_printf
is not the full-blown printf and has very limited capabilities.
For example, to print the address, one would need to use
_dl_debug_printf("0x%0*lx\n", (int)sizeof (void*)*2, &foo);
How does ld.so work ?
ld.so, by its nature, cannot be a dynamic executable itself. The
entry point of
ld.so is
_start defined in
the macro
RTLD_START (in
sysdeps/x86_64/dl-machine.h).
_start is placed at the beginning of
.text section, and
the default
ld script specifies
"Entry point address" (in ELF header, use
readelf -h ld.so|grep Entry command to see)
to be the address of
_start (use
ld -verbose | grep ENTRY command to see). One
can set the entry point to a different address at compile time
by
-e option)
so
ld.so is executed from here. The very first thing it does is to call
_dl_start of
elf/rtld.c. To see this, run gdb on some ELF executable binary, and do
(gdb) break _dl_start
Function "_dl_start" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (_dl_start) pending.
(gdb) run
Starting program: a.out
Breakpoint 1, 0x0000003433e00fa0 in _dl_start () from /lib64/ld-linux-x86-64.so.2
(gdb) bt
#0 0x0000003433e00fa0 in _dl_start () from /lib64/ld-linux-x86-64.so.2
#1 0x0000003433e00a78 in _start () from /lib64/ld-linux-x86-64.so.2
#2 0x0000000000000001 in ?? ()
#3 0x00007fffffffe4f2 in ?? ()
#4 0x0000000000000000 in ?? ()
...
(gdb) x/10i $pc
0x3433e00a70 <_start>: mov %rsp,%rdi
0x3433e00a73 <_start+3>: callq 0x3433e00fa0 <_dl_start>
0x3433e00a78 <_dl_start_user>: mov %rax,%r12
0x3433e00a7b <_dl_start_user+3>: mov 0x21b30b(%rip),%eax # 0x343401bd8c <_dl_skip_args>
...
At this breakpoint, we can use
pmap to see the memory map of a.out, which would
look like this:
0000000000400000 8K r-x-- a.out
0000000000601000 4K rw--- a.out
0000003433e00000 112K r-x-- /lib64/ld-2.5.so
000000343401b000 8K rw--- /lib64/ld-2.5.so
00007ffffffea000 84K rw--- [ stack ]
ffffffffff600000 8192K ----- [ anon ]
total 8408K
The memory segment of
/lib64/ld-2.5.so indeed starts at 3433e00000 (page aligned) and
this can be verified by running
readelf -t /lib64/ld-2.5.so.
If we put another breakpoint at main and continue, then when it stops, the memory
map would change to this:
0000000000400000 8K r-x-- a.out
0000000000601000 4K rw--- a.out
0000003433e00000 112K r-x-- /lib64/ld-2.5.so
000000343401b000 4K r---- /lib64/ld-2.5.so
000000343401c000 4K rw--- /lib64/ld-2.5.so
0000003434200000 1336K r-x-- /lib64/libc-2.5.so <-- The first "LOAD" segment, which contains .text and .rodata sections
000000343434e000 2044K ----- /lib64/libc-2.5.so <-- "Hole"
000000343454d000 16K r---- /lib64/libc-2.5.so <-- Relocation (GNU_RELRO) info -+---- The second "LOAD" segment
0000003434551000 4K rw--- /lib64/libc-2.5.so <-- .got.plt .data sections -+
0000003434552000 20K rw--- [ anon ] <-- The remaining zero-filled sections (e.g. .bss)
0000003434e00000 88K r-x-- /lib64/libpthread-2.5.so <-- The first "LOAD" segment, which contains .text and .rodata sections
0000003434e16000 2044K ----- /lib64/libpthread-2.5.so <-- "Hole"
0000003435015000 4K r---- /lib64/libpthread-2.5.so <-- Relocation (GNU_RELRO) info -+---- The second "LOAD" segment
0000003435016000 4K rw--- /lib64/libpthread-2.5.so <-- .got.plt .data sections -+
0000003435017000 16K rw--- [ anon ] <-- The remaining zero-filled sections (e.g. .bss)
00002aaaaaaab000 4K rw--- [ anon ]
00002aaaaaac6000 12K rw--- [ anon ]
00007ffffffea000 84K rw--- [ stack ]
ffffffffff600000 8192K ----- [ anon ]
total 14000K
Indeed,
ld.so has brought in all the required dynamic libraries.
Note that there
are two memory regions of 2044KB with null permissions.
As mentioned earlier, the ELF's 'execution view' is concerned with how to load an executable
binary into memory. When ld.so brings in the dynamic libraries, it looks at the segments labelled
as LOAD (look at "Program Headers" and "Section to Segment mapping"
from readelf -a xxx.so command.) Usually there are two LOAD segments, and
there is a "hole" between the two segments (look at the VirtAddr and MemSiz of these
two segments), so ld.so will
make this hole inaccessible deliberately: Look for the PROT_NONE symbol in
_dl_map_object_from_fd in elf/dl-load.c
Also note that each of
libc-2.5.so and libpthread-2.5.so has a read-only memory region
(at 0x343454d000 and 0x3435015000, respectively). This is a for
security reasons.
These are relocation info (run readelf -l xxx.so |grep GNU_RELRO
command to see) which should be made Read-Only after the relocation is done
and this achieved by _dl_protect_relro function in elf/dl-reloc.c.
The GNU_RELRO segment is contained in the the second LOAD segment, which
contains the following sections (look at "Program Headers" and "Section to Segment mapping"
from readelf -l xxx.so command):
.tdata, .fini_array, .ctors, .dtors, __libc_subfreeres,
__libc_atexit, __libc_thread_subfreeres, .data.rel.ro, .dynamic,
.got, .got.plt, .data, and .bss. Except for
.got.plt, .data, and .bss, all sections in the the second LOAD segment
are also in the GNU_RELRO segment, and they are thus made read-only.
The two [anon] memory segments at 0x3434552000 and 0x3435017000 are for sections which do not take space in the ELF
binary files. For example, readelf -t xxx.so will show that .bss section
has NOBITS flag, which means that section takes no disk space. When segments
containing NOBITS sections are mapped into memory, ld.so allocates
extra memory pages to accomodate these NOBITS sections. A LOAD
segment is usually structured as a series of contiguous sections, and if
a segment contains NOBITS sections, these NOBITS sections will
be grouped together and placed at the tail of the segment.
So what does _dl_start do ?
- Allocate the initial TLS block and initialize the Thread Pointer if needed (these are for ld.so's own, not for the user program)
- Call _dl_sysdep_start, which will call dl_main
- dl_main does the majority of the hard work, for example:
It calls process_envvars to handle these LD_ prefix environmental
variables such as LD_PRELOAD, LD_LIBRARY_PATH.
It examines the NEEDED field(s) in the user executable binary's DYNAMIC segment
section (see below) to determine the dependencies.
It calls _dl_init_paths (in elf/dl-load.c)
to initialize the dynamic libraries search paths.
According to ld.so man page
and this page,
the dynamic libraries are searched in the following order:
- The RPATH in the DYNAMIC segment if there is no
RUNPATH in the DYNAMIC segment.
RPATH can be specified when
the code is compiled with gcc -Wl,-rpath=...
Use of RPATH is deprecated
because it has an obvious drawback: There is no way to override
it except using LD_PRELOAD environmental variable
or removing it from the DYNAMIC segment.
Both RPATH and RUNPATH can
contain $ORIGIN
(or equivalently ${ORIGIN}), which will be
expanded to the value of environmental variable LD_ORIGIN_PATH
or the full path of the loaded object
(unless the programs use setuid or setgid)
- The LD_LIBRARY_PATH environmental variable (unless
the programs use setuid or setgid)
- The RUNPATH in the DYNAMIC segment.
RUNPATH can be specified when
the code is compiled with gcc -Wl,-rpath=...,--enable-new-dtags
One can use chrpath
tool to manipulate RPATH and RUNPATH settings.
- /etc/ld.so.cache
- /lib
- /usr/lib
It calls _dl_map_object_from_fd (in elf/dl-load.c)
to load the dynamic libraries, sets up the right read/write/execute permissions for the memory segments,
(within _dl_map_object_from_fd, look at calls to mmap, mprotect and symbols such as
PROT_READ, PROT_WRITE, PROT_EXEC, PROT_NONE),
zeroes out BSS sections of dynamic libraries (inside _dl_map_object_from_fd function, look at calls to memset),
updates the link map, and performs relocations.
It calls _dl_relocate_object (in elf/dl-reloc.c) to perform runtime relocations (see details below).
- When _dl_start returns, it continues to execute
code in _dl_start_user (see sysdeps/x86_64/dl-machine.h)
- _dl_start_user will call _dl_init_internal, which will call call_init
to invoke initialization function of each dynamic library loaded.
Note that _dl_init_internal is defined in elf/dl-init.c as:
void
internal_function
_dl_init (struct link_map *main_map, int argc, char **argv, char **env)
call_init is also in elf/dl-init.c
- The initialization function of a dynamic library, say libfoo.so, is located at the
address marked with type "INIT" in the output of readelf -d libfoo.so
For Glibc, its initialization function is named _init (not to be confused with the _init
inside the user's executable binary) and its source code is in
sysdeps/unix/sysv/linux/x86_64/init-first.c.
_init will do the following things:
- At the end of _dl_start_user, the control transfers to user program's entry point address (use readelf -h a.out|grep Entry to see)
which is usually the initial address of .text section and contains
the entry of a function named _start, and in the control transfer, the finalizer function
_dl_fini is passed as an argument,
and the stack frames are completely clobbered, as if the user program
is run without any ld.so intervention. The latter is done by manipulating the stack (see the
on-stack auxiliary vector adjustment
code and HAVE_AUX_VECTOR in dl_main)
Here is the call graph,
which is worth a thousand words
and see here
on how it is generated.
To see ld.so in action, set the environmental
variable LD_DEBUG to all and then run a user program.
The above debugging information does not show mmap and mprotect calls.
However, we can use strace. If we run the user program again with
strace -e trace=mmap,mprotect,munmap,open a.out
we should see something like the
following:
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2ae62c0d1000
.... (a lot of failed attempts to open 'libpthread.so.0' using LD_LIBRARY_PATH)
open("/etc/ld.so.cache", O_RDONLY) = 3
mmap(NULL, 104801, PROT_READ, MAP_PRIVATE, 3, 0) = 0x2ae62c0d2000
open("/lib64/libpthread.so.0", O_RDONLY) = 3
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2ae62c0ec000
mmap(0x3434e00000, 2204528, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3434e00000 <-- Bring in the first "LOAD" segment
mprotect(0x3434e16000, 2093056, PROT_NONE) = 0 <-- Make the "hole" inaccessible
mmap(0x3435015000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x15000) = 0x3435015000 <-- Bring in the second "LOAD" segment
mmap(0x3435017000, 13168, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x3435017000
(note: 0x3435017000 is the [anon] part which follows immediately after libpthread-2.5.so)
...
.... (a lot of failed attempts to open 'libc.so.6' using LD_LIBRARY_PATH)
open("/lib64/libc.so.6", O_RDONLY) = 3
mmap(0x3434200000, 3498328, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3434200000 <-- Bring in the first "LOAD" segment
mprotect(0x343434e000, 2093056, PROT_NONE) = 0 <-- Make the "hole" inaccessible
mmap(0x343454d000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x14d000) = 0x343454d000 <-- Bring in the second "LOAD" segment
mmap(0x3434552000, 16728, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x3434552000
(note: 0x3434552000 is the [anon] part which follows immediately after libc-2.5.so)
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2ae62c0ed000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2ae62c0ee000
mprotect(0x343454d000, 16384, PROT_READ) = 0 <-- Make the GNU_RELRO segment read-only
mprotect(0x3435015000, 4096, PROT_READ) = 0 <-- Make the GNU_RELRO segment read-only
mprotect(0x343401b000, 4096, PROT_READ) = 0
munmap(0x2ae62c0d2000, 104801)= 0
mmap(NULL, 10489856, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_32BIT, -1, 0) = 0x40dc7000
mprotect(0x40dc7000, 4096, PROT_NONE) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2aaaaaaab000
.plt section
This section contains trampolines for functions defined in dynamic libraries.
A sample disassembly (run the command
objdump -M intel -dj .plt a.out) will show the following:
4003c0 <printf@plt-0x10>:
4003c0: push QWORD PTR [RIP+0x2004d2] # 600898 <_GLOBAL_OFFSET_TABLE_+0x8>
4003c6: jmp QWORD PTR [RIP+0x2004d4] # 6008a0 <_GLOBAL_OFFSET_TABLE_+0x10>
4003cc: nop DWORD PTR [RAX+0x0]
4003d0 <printf@plt>:
4003d0: jmp QWORD PTR [RIP+0x2004d2] # 6008a8 <_GLOBAL_OFFSET_TABLE_+0x18>
4003d6: push 0
4003db: jmp 4003c0 <printf@plt-0x10>
4003e0 <__libc_start_main@plt>:
4003e0: jmp QWORD PTR [RIP+0x2004ca] # 6008b0 <_GLOBAL_OFFSET_TABLE_+0x20>
4003e6: push 1
4003eb: jmp 4003c0 <printf@plt-0x10>
The
_GLOBAL_OFFSET_TABLE_ (labeled as
R_X86_64_JUMP_SLOT and starts at address 0x600890) is located in
.got.plt section (to see this, run the command
objdump -h a.out |grep -A 1 600890
or the command
readelf -r a.out)
The data in
.got.plt section look like the following
during runtime
(use gdb to see them)
(gdb) b *0x4003d0
(gdb) run
(gdb) x/6a 0x600890
0x600890: 0x6006e8 <_DYNAMIC> 0x32696159a8
0x6008a0: 0x326950aa20 <_dl_runtime_resolve> 0x4003d6 <printf@plt+6>
0x6008b0: 0x326971c3f0 <__libc_start_main> 0x0
When
printf is called the first time in the user program, the
jump at 4003d0 will jump to
4003d6, which is just the next instruction (
push 0)
The it jumps to 4003c0, which does not have a function name (so it is
shown as
<printf@plt-0x10>). At 4003c6, it will jumps
to
_dl_runtime_resolve. This function (in Glibc's source file
sysdeps/x86_64/dl-trampoline.S)
is a trampoline to
_dl_fixup (in Glibc's source file
elf/dl-runtime.c).
_dl_fixup again, is part of Glibc runtime linker
ld.so. In particular,
it will change
the address stored at 6008a8 to the actual
address of printf in libc.so.6. To see this, set up a
hardware watchpoint
(gdb) watch *0x6008a8
(gdb) cont
Continuing.
Hardware watchpoint 2: *0x6008a8
Old value = 4195286
New value = 1769244016
0x000000326950abc2 in fixup () from /lib64/ld-linux-x86-64.so.2
If we continue execution,
printf will be called, as
expected. When
printf is called again in the user program, the
jump at 4003d0 will bounce directly to
printf:
(gdb) x/6a 0x600890
0x600890: 0x6006e8 <_DYNAMIC> 0x32696159a8
0x6008a0: 0x326950aa20 <_dl_runtime_resolve> 0x3269748570 <printf>
0x6008b0: 0x326971c3f0 <__libc_start_main> 0x0
.init, .fini, .preinit_array, .init_array and .fini_array sections
.init and
.fini sections contain code to do
initialization and termination, as
specified by the
System V Application Binary Interface.
If the code is compiled by GCC, then one will see the following code in
.init and
.fini sections, respectively:
4003a8 <_init>:
4003a8: sub RSP, 8
4003ac: call call_gmon_start
4003b1: call frame_dummy
4003b6: call __do_global_ctors_aux
4003bb: add RSP, 8
4003bf: ret
400618 <_fini>:
400618: sub RSP, 8
40061c: call __do_global_dtors_aux
400621: add RSP, 8
400625: ret
There is only one function:
_init, in
.init section, and
likewise, only one function:
_fini in
.fini section.
Both
_init and
_fini are
synthesized at compile time
by the compiler/linker. Glibc
provides its own prolog and epilog for
_init and
_fini, but
the compiler is free to choose how to use them and add more code into
_init
and
_fini.
In Glibc, the source file sysdeps/generic/initfini.c
(and some system dependent ones, such as sysdeps/x86_64/elf/initfini.c)
is compiled into two files: /usr/lib64/crti.o for prolog
and /usr/lib64/crtn.o for epilog.
For the compiler part, GCC uses different prolog and epilog files, depending
on the compiler command-line options. To see them, execute gcc -dumpspec,
and one can see
...
*endfile:
%{ffast-math|funsafe-math-optimizations:crtfastmath.o%s}
%{mpc32:crtprec32.o%s}
%{mpc64:crtprec64.o%s}
%{mpc80:crtprec80.o%s}
%{shared|pie:crtendS.o%s;:crtend.o%s}
crtn.o%s
...
*startfile:
%{!shared: %{pg|p|profile:gcrt1.o%s;pie:Scrt1.o%s;:crt1.o%s}}
crti.o%s
%{static:crtbeginT.o%s;shared|pie:crtbeginS.o%s;:crtbegin.o%s}
...
The detailed explanation of GCC spec file is
here.
For above snippet, it means, for example, if compiler command-line
option
-ffast-math is used, include GCC's
crtfastmath.o
file (this file can be found under
/usr/lib/gcc/<arch>/<version>/)
at the end of the linking process. Glibc's
crtn.o is always
included at the end of linking. The
%s means this preceding file is a startup file. (GCC allows
to skip startup files during linking using
-nostartfiles compiler option)
Similarly, if -shared compiler command-line option is not used,
then always include Glibc's crt1.o at the start of the linking process.
crt1.o contains the function _start in .text section (not .init section!)
_start is the function that is executed before anything else... see below.
Next, include Glibc's crti.o in the linking. Finally, include either
crtbeginT.o, crtbeginS.o, or crtbegin.o (both are part of GCC, of course), depending on
whether -static or -shared (or neither) is used.
So, for example, if a program is compiled using dynamic linking (which is default), no profiling, no fast
math optimizations, then the linking will include the following files in the following order:
- crt1.o (part of Glibc)
- crti.o (part of Glibc and contributes the code at 4003a8, 4003ac, 400618, and the body of call_gmon_start)
- crtbegin.o (part of GCC and contributes the code at 4003b1 and 40061c, and the body of frame_dummy and __do_global_dtors_aux)
- user's code
- crtend.o (part of GCC and contributes the code at 4003b6 and the body of __do_global_ctors_aux)
- crtn.o (part of Glibc and contributes the code at 4003bb, 4003bf, 400621, 400625)
Why
__do_global_ctors_aux is in
crtend*.o and
__do_global_dtors_aux
is in
crtbegin*.o ? Recall the order of invocation of destructors should be the reverse order
of invocation of constructors. Therefore, GCC doing so will ensure
__do_global_ctors_aux is called
as late as possible in
.init section and
__do_global_dtors_aux is called
as early as possible in
.fini section.
Now back to the 4003a8 <_init>.
call_gmon_start is part of the Glibc prolog /usr/lib64/crti.o.
It initializes gprof related
data structures.
frame_dummy is in GCC code gcc/crtstuff.c and it
is used to set up excepion handling and Java class registration (JCR) information.
The most interesting code is __do_global_ctors_aux (in
GCC's gcc/crtstuff.c and
gcc/gbl-ctors.h) What it does
is to call functions which are marked as
__attribute__ ((constructor)) (and static C++ objects' constructors) one by one:
__SIZE_TYPE__ nptrs = (__SIZE_TYPE__) __CTOR_LIST__[0];
unsigned i;
if (nptrs == (__SIZE_TYPE__)-1)
for (nptrs = 0; __CTOR_LIST__[nptrs + 1] != 0; nptrs++);
for (i = nptrs; i >= 1; i--)
__CTOR_LIST__[i] ();
The array
__CTOR_LIST__ is stored in a special section called
.ctors.
Suppose a function called
foo is marked as
__attribute__ ((constructor)),
then the runtime call stack trace would be
(gdb) break foo
(gdb) run
(gdb) bt
#0 0x00000000004004d8 in foo ()
#1 0x0000000000400606 in __do_global_ctors_aux ()
#2 0x00000000004003bb in _init ()
#3 0x00000000004005a0 in ?? ()
#4 0x0000000000400561 in __libc_csu_init ()
#5 0x000000326971c46f in __libc_start_main ()
#6 0x000000000040041a in _start ()
Similarly, the
__do_global_dtors_aux in
_fini function
will invoke all functions which are marked as
__attribute__ ((destructor)).
__do_global_dtors_aux code is also
in GCC's source tree at
gcc/crtstuff.c. If
a function called
foo is marked as
__attribute__ ((destructor))
(and static C++ objects' destructors), then the runtime call stack trace would be
(gdb) bt
#0 0x0000000000400518 in foo ()
#1 0x00000000004004ca in __do_global_dtors_aux ()
#2 0x0000000000400641 in _fini ()
#3 0x00000032699367e8 in ?? () from /lib64/tls/libc.so.6
#4 0x0000003269730c95 in exit () from /lib64/tls/libc.so.6
#5 0x000000326971c4d2 in __libc_start_main () from /lib64/tls/libc.so.6
#6 0x000000000040045a in _start ()
The array
__DTOR_LIST__ contains the addresses of these destructors
and it is stored in a special section called
.dtors.
What user functions will be executed before main and at program exit?
As above call strack trace shows,
_init is NOT the only function to be called before
main.
It is
__libc_csu_init function (in Glibc's source file
csu/elf-init.c)
that determines what functions to be run before
main
and the order of running them. Its code is like this
void __libc_csu_init (int argc, char **argv, char **envp)
{
#ifndef LIBC_NONSHARED
{
const size_t size = __preinit_array_end - __preinit_array_start;
size_t i;
for (i = 0; i < size; i++)
(*__preinit_array_start [i]) (argc, argv, envp);
}
#endif
_init ();
const size_t size = __init_array_end - __init_array_start;
for (size_t i = 0; i < size; i++)
(*__init_array_start [i]) (argc, argv, envp);
}
(Symbols such as
__preinit_array_start,
__preinit_array_end,
__init_array_start,
__init_array_end are defined by the default
ld script;
look for
PROVIDE
and PROVIDE_HIDDEN keywords in the output of
ld -verbose command.)
The __libc_csu_fini function has similar code, but what
functions to be executed at program exit are actually determined by exit:
void __libc_csu_fini (void)
{
#ifndef LIBC_NONSHARED
size_t i = __fini_array_end - __fini_array_start;
while (i-- > 0)
(*__fini_array_start [i]) ();
_fini ();
#endif
}
To see what's going on, consider the following C code example:
#include <stdio.h>
#include <stdlib.h>
void preinit(int argc, char **argv, char **envp) {
printf("%s\n", __FUNCTION__);
}
void init(int argc, char **argv, char **envp) {
printf("%s\n", __FUNCTION__);
}
void fini() {
printf("%s\n", __FUNCTION__);
}
__attribute__((section(".init_array"))) typeof(init) *__init = init;
__attribute__((section(".preinit_array"))) typeof(preinit) *__preinit = preinit;
__attribute__((section(".fini_array"))) typeof(fini) *__fini = fini;
void __attribute__ ((constructor)) constructor() {
printf("%s\n", __FUNCTION__);
}
void __attribute__ ((destructor)) destructor() {
printf("%s\n", __FUNCTION__);
}
void my_atexit() {
printf("%s\n", __FUNCTION__);
}
void my_atexit2() {
printf("%s\n", __FUNCTION__);
}
int main() {
atexit(my_atexit);
atexit(my_atexit2);
}
The output will be
preinit
constructor
init
my_atexit2
my_atexit
fini
destructor
The
.preinit_array and
.init_array sections must contain
function pointers (NOT code!) The prototype of these functions must be
void func(int argc,char** argv,char** envp)
__libc_csu_init execute them in the following order:
- Function pointers in .preinit_array section
- Functions marked as __attribute__ ((constructor)), via _init
- Function pointers in .init_array section
The
.fini_array section must also contain
function pointers
and the prototype is like the destructor, i.e. taking no arguments and returning void. If the program exits
normally, then
the
exit function (Glibc source file
stdlib/exit.c) is called and it
will do the following:
- In reverse order, functions registered via atexit or on_exit
- Function pointers in .fini_array section, via __libc_csu_fini
- Functions marked as __attribute__ ((destructor)), via __libc_csu_fini (which calls _fini after Step 2)
- stdio cleanup functions
It is not advisable to put a code in .init section, e.g.
void __attribute__((section(".init"))) foo() {
...
}
because doing so will cause
__do_global_ctors_aux NOT to be called. The
.init
section will now look like this:
4003a0 <_init>:
4003a0: sub RSP, 8
4003a4: call call_gmon_start
4003a9: call frame_dummy
4003ae <foo>:
4003ae: push RBP
4003af: mov RBP, RSP
.... (foo's body)
4003b2: leave
4003b3: ret
4003b4: call __do_global_ctors_aux
4003b9: add RSP, 8
4003bd: ret
Now .init section contains more than one function, but the
epilog of _init is distorted by the insertion of foo
Similarly, it is not advisable to put a code in .fini section,
because otherwise the code will look like this:
4006d8 <_fini>:
4006d8: sub RSP, 8
4006dc: call __do_global_dtors_aux
4006e1 <foo>:
4006e1: push RBP
4006e2: mov RBP, RSP
.... (foo's body)
4006ef: leave
4006f0: ret
4006f1: add RSP, 8
4006f5: ret
Now the epilog of
_fini is distorted by the insertion of
foo, so
the stack frame pointer will not be adjusted (
add RSP, 8 is not executed),
causing segmentation fault.
What do _start and __libc_start_main do?
The above call stack traces show that
_start calls
__libc_start_main, which runs
all of the code before
main.
_start is part of Glibc code, as in sysdeps/x86_64/elf/start.S.
As mentioned earlier, it is compiled as /usr/lib64/crt1.o and is statically linked to
user's executable binary during compilation. To see this, run gcc with -v command, and
the last line would be something like:
.../collect2 ... /usr/lib64/crt1.o /usr/lib64/crti.o ... /usr/lib64/crtn.o
_start is always placed
at the beginning of .text section, and
the default ld script specifies
"Entry point address" (in ELF header, use readelf -h ld.so|grep Entry command to see)
to be the address of _start (use ld -verbose | grep ENTRY command to see), so
_start is guaranteed to
be run before anything else. (This is changeable, however, at compile time
one can specify a different initial address
by
-e option)
_start does only one thing: It sets up the arguments needed by
__libc_start_main and then call it.
__libc_start_main's source code is csu/libc-start.c
and its function prototype is:
__libc_start_main (int (*main) (int, char **, char **),
int argc,
char *argv,
int (*init) (int, char **, char **),
void (*fini) (void),
void (*rtld_fini) (void),
void *stack_end)
)
__libc_start_main does quite a lot of work in
addition to kicking off
__libc_csu_init:
- Set up argv and envp
- Initialize the thread local storage by calling __pthread_initialize_minimal (which
only calls __libc_setup_tls).
__libc_setup_tls will initialize Thread Control Block
and Dynamic Thread Vector.
- Set up the thread stack guard
- Register the destructor (i.e. the rtld_fini argument passed to __libc_start_main)
of the dynamic linker (by calling __cxa_atexit) if there is any
- Initialize Glibc inself by calling __libc_init_first
- Register __libc_csu_fini (i.e. the fini argument passed to __libc_start_main)
using __cxa_atexit
- Call __libc_csu_init (i.e. the init argument
passed to __libc_start_main)
- Call function pointers in .preinit_array section
- Execute the code in .init section, which is usually _init function.
What _init function does is compiler-specific.
For GCC, _init executes user functions marked as __attribute__ ((constructor))
(in __do_global_dtors_aux)
- Call function pointers in .init_array section
- Set up data structures needed for thread unwinding/cancellation
- Call main of user's program.
- Call exit
So if the last line of user program's
main is
return XX,
then the
XX will be passed to
exit at Step #11 above. If
the last line is not
return XX or is simply
return, then
the value passed to
exit would be undefined.
Of course, if
the user program calls exit or abort, then exit
will gets called.
Here is the call graph,
which is worth a thousand words
and see here
on how it is generated.
If one tries to build a program which does not contain main, then one should see the following error:
/usr/lib/crt1.o: In function `_start': (.text+0x20): undefined reference to `main'
collect2: ld returned 1 exit status
As mentioned earlier,
crt1.o (part of Glibc) contains the function
_start, which will call
__libc_start_main and pass
main (a function pointer) as one of the arguments.
If one uses
nm -u /usr/lib/crt1.o
then it will show
main is a undefined symbol in
crt1.o. Now let's disassemble
crt1.o:
$ objdump -M intel -dj .text /usr/lib/crt1.o
crt1.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <_start>:
0: 31 ed xor ebp,ebp
2: 49 89 d1 mov r9,rdx
5: 5e pop rsi
6: 48 89 e2 mov rdx,rsp
9: 48 83 e4 f0 and rsp,0xfffffffffffffff0
d: 50 push rax
e: 54 push rsp
f: 49 c7 c0 00 00 00 00 mov r8,0x0
16: 48 c7 c1 00 00 00 00 mov rcx,0x0
1d: 48 c7 c7 00 00 00 00 mov rdi,0x0
24: e8 00 00 00 00 call 29 <_start+0x29>
29: f4 hlt
...
Above shows
.text+0x20 refers to
the 4 bytes of an
mov instruction. This means during the
linking, the address of
main should be resolved
and then inserted at the right memory location: .text+0x20. Now let's cross reference
the relocation table:
$ readelf -p /usr/lib/crt1.o
Relocation section '.rela.text' at offset 0x410 contains 4 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000000012 00090000000b R_X86_64_32S 0000000000000000 __libc_csu_fini + 0
000000000019 000b0000000b R_X86_64_32S 0000000000000000 __libc_csu_init + 0
000000000020 000c0000000b R_X86_64_32S 0000000000000000 main + 0
000000000025 000f00000002 R_X86_64_PC32 0000000000000000 __libc_start_main - 4
Above shows where
0x20 comes from.
How to find the address of main of an executable binary ?
When an ELF executable binary is stripped off symbolic information, it
is not clear where the
main is located.
From above analysis, it's possible to find out the address of main (which is
NOT the "Entry point address" seen from the output of readelf -h a.out | grep Entry
command. "Entry point address" is the address of _start)
Since the address of main is the first argument to the call
to __libc_start_main, we can extract the value of the first
argument as follows.
On 64-bit x86, the calling convention
requires that the first argument
goes to RDI register, so the
address can be extracted by
objdump -j .text -d a.out | grep -B5 'call.*__libc_start_main' | awk '/mov.*%rdi/ { print $NF }'
On
32-bit x86, the C calling
convention ("
cdecl") is that the first argument
is the
last item to be pushed onto the stack
before the call, so the
address can be extracted by
objdump -j .text -d a.out | grep -B2 'call.*__libc_start_main' | awk '/push.*0x/ { print $NF }'
PIC, TLS, and AMD64 code models
Relocation is the process of connecting symbolic references with symbolic definitions.
The runtime relocation is done by
ld.so, as in
elf_machine_rela function
in Glibc's source file
sysdeps/x86_64/dl-machine.h.
The link-time relocation is done by the link-editor
ld, which uses the relocation
table in the object file (
.rela.text section). Each symbolic reference has an entry
in the relocation table, and
each entry contains three fields: offset, info (relocation type, symbol table index), and addend.
The relocation types are:
| Relocation type |
Meaning |
Used when |
| R_X86_64_16 |
Direct 16 bit zero extended |
|
| R_X86_64_32 |
Direct 32 bit zero extended |
|
| R_X86_64_32S |
Direct 32 bit
sign extended |
|
| R_X86_64_64 |
Direct 64 bit |
Large code model |
| R_X86_64_8 |
Direct 8 bit sign extended |
|
| R_X86_64_COPY |
Copy symbol at runtime |
|
| R_X86_64_DTPMOD64 |
ID of module containing symbol |
TLS |
| R_X86_64_DTPOFF32 |
Offset in TLS block |
TLS |
| R_X86_64_DTPOFF64 |
Offset in module's TLS block |
TLS |
| R_X86_64_GLOB_DAT |
.got section, which contains addresses to the actual functions in DLL |
|
| R_X86_64_GOT32 |
32 bit GOT entry |
|
| R_X86_64_GOT64 |
64-bit GOT entry offset |
PIC & Large code model |
| R_X86_64_GOTOFF64 |
64-bit GOT offset |
PIC & Large code model |
| R_X86_64_GOTPC32 |
32-bit PC relative offset to GOT |
|
| R_X86_64_GOTPC32_TLSDESC |
32-bit PC relative to TLS descriptor in GOT |
TLS |
| R_X86_64_GOTPC64 |
64-bit PC relative offset to GOT |
PIC & Large code model |
| R_X86_64_GOTPCREL |
32 bit signed PC relative offset to GOT |
PIC |
| R_X86_64_GOTPCREL64 |
64-bit PC relative offset to GOT entry |
PIC & Large code model |
| R_X86_64_GOTPLT64 |
Like GOT64, indicates that PLT entry needed |
PIC & Large code model |
| R_X86_64_GOTTPOFF |
32 bit signed PC relative offset to GOT entry for IE symbol |
TLS |
| R_X86_64_JUMP_SLOT |
.got.plt section, which contains addresses to the actual functions in DLL |
DLL |
| R_X86_64_PC16 |
16 bit sign extended PC relative |
|
| R_X86_64_PC32 |
PC relative 32 bit signed |
|
| R_X86_64_PC64 |
64-bit PC relative |
Large code model |
| R_X86_64_PC8 |
8 bit sign extended PC relative |
|
| R_X86_64_PLT32 |
32 bit PLT address |
|
| R_X86_64_PLTOFF64 |
64-bit GOT relative offset to PLT entry |
PIC & Large code model |
| R_X86_64_RELATIVE |
Adjust by program base |
|
| R_X86_64_SIZE32 |
|
|
| R_X86_64_SIZE64 |
|
|
| R_X86_64_TLSDESC |
2 by 64-bit TLS descriptor |
TLS |
| R_X86_64_TLSDESC_CALL |
Relaxable call through TLS descriptor |
TLS |
| R_X86_64_TLSGD |
32 bit signed PC relative offset to two GOT entries for GD symbol |
TLS & PIC |
| R_X86_64_TLSLD |
32 bit signed PC relative offset to two GOT entries for LD symbol |
TLS |
| R_X86_64_TPOFF32 |
Offset in initial TLS block |
TLS |
| R_X86_64_TPOFF64 |
Offset in initial TLS block |
TLS & Large code model |
According to Chapter 3.5 of AMD64 System V Application Binary Interface,
there are four code models and they differ in addressing modes (absolute versus relative):
Now consider the following C code
extern int esrc[100];
int gsrc[100];
static int ssrc[100];
void foo() {
int k;
k = esrc[5];
k = gsrc[5];
k = ssrc[5];
}
- Small code model, no PIC (i.e. compiled with just gcc -c):
k = esrc[5]; mov EAX, DWORD PTR[RIP+0x0]
mov DWORD PTR[RBP-0x4], EAX
k = gsrc[5]; mov EAX, DWORD PTR[RIP+0x0]
mov DWORD PTR[RBP-0x4], EAX
k = ssrc[5]; mov EAX, DWORD PTR[RIP+0x0]
mov DWORD PTR[RBP-0x4], EAX
and the relocation table is (use readelf -r foo.o command)
type Sym. Name + Addend
R_X86_64_PC32 esrc + 10
R_X86_64_PC32 gsrc + 10
R_X86_64_PC32 .bss + 10
All of the 0x0's in the generated assembly will be filled at link-time
with their relative offsets in respective sections, as indicated by the relocation table.
- Large code model, no PIC (i.e. compiled with gcc -c -mcmodel=large)
k = esrc[5]; mov RAX, 0x0
mov EAX, DWORD PTR[RAX+0x10]
mov DWORD PTR[RBP-0x4], EAX
k = gsrc[5]; mov RAX, 0x0
mov EAX, DWORD PTR[RAX+0x10]
mov DWORD PTR[RBP-0x4], EAX
k = ssrc[5]; mov RAX, 0x0
mov EAX, DWORD PTR[RAX+0x10]
mov DWORD PTR[RBP-0x4], EAX
and the relocation table is:
type Sym. Name + Addend
R_X86_64_64 esrc + 0
R_X86_64_64 gsrc + 0
R_X86_64_64 .bss + 0
All of the 0x0's in the generated assembly will be filled at link-time
with their (64-bit) absolute addresses.
- Small code model, PIC (i.e. compiled with gcc -c -fPIC. Note that adding -shared or not will not affect the generated code)
k = esrc[5]; mov RAX, QWORD PTR[RIP+0x0]
mov EAX, DWORD PTR[RAX+0x10]
mov DWORD PTR[RBP-0x4], EAX
k = gsrc[5]; mov RAX, QWORD PTR[RIP+0x0]
mov EAX, DWORD PTR[RAX+0x10]
mov DWORD PTR[RBP-0x4], EAX
k = ssrc[5]; mov EAX, DWORD PTR[RIP+0x0]
mov DWORD PTR[RBP-0x4], EAX
and the relocation table is:
type Sym. Name + Addend
R_X86_64_GOTPCREL esrc - 4
R_X86_64_GOTPCREL gsrc - 4
R_X86_64_PC32 .bss + 10
The first two 0x0's in the generated assembly will be filled with the relative
offset of _GLOBAL_OFFSET_TABLE_ (i.e. the .got.plt section)
- Large code model, PIC (i.e. compiled with gcc -c -fPIC -mcmodel=large)
lea RBX, [RIP-0x7]
mov R11, 0x0
add RBX, R11
k = esrc[5]; mov RAX, 0x0
mov RAX, QWORD PTR[RBX+RAX*1]
mov EAX, DWORD PTR[RAX+0x10]
mov DWORD PTR[RBP-0x4], EAX
k = gsrc[5]; mov RAX, 0x0
mov RAX, QWORD PTR[RBX+RAX*1]
mov EAX, DWORD PTR[RAX+0x10]
mov DWORD PTR[RBP-0x4], EAX
k = ssrc[5]; mov RAX, 0x0
mov RAX, QWORD PTR[RBX+RAX*1]
mov EAX, DWORD PTR[RAX+0x10]
mov DWORD PTR[RBP-0x4], EAX
The first 0x0 is in the generated assembly will be filled with the absolute
address of _GLOBAL_OFFSET_TABLE_
_GLOBAL_OFFSET_TABLE_, .got.plt section, and DYNAMIC segment
Earlier we see that the
_GLOBAL_OFFSET_TABLE_ is located in
.got.plt section:
(gdb) b *0x4003d0
(gdb) run
(gdb) x/6a 0x600890
0x600890: 0x6006e8 <_DYNAMIC> 0x32696159a8
0x6008a0: 0x326950aa20 <_dl_runtime_resolve> 0x4003d6 <printf@plt+6>
0x6008b0: 0x326971c3f0 <__libc_start_main> 0x0
According to Chapter 5.2 of
AMD64 System V Application Binary Interface,
the first 3 entries of this table are reserved for special purposes.
The first entry is set up during compilation by the link editor
ld.
The second and third entries are set up during runtime by the runtime linker
ld.so
(see function
_dl_relocate_object in Glibc source file
elf/dl-reloc.c
and in particular, notice the
ELF_DYNAMIC_RELOCATE macro,
which calls function
elf_machine_runtime_setup in
sysdeps/x86_64/dl-machine.h)
The first entry _DYNAMIC has value 6006e8, and this is exactly
the starting address of .dynamic section (or DYNAMIC segment, in ELF's "execution view".)
The runtime linker ld.so uses this section to find the all necessary
information needed for runtime relocation and dynamic linking.
To see DYNAMIC segment's content, use readelf -d a.out command, or
objdump -x a.out, or just use x/50a 0x6006e8 in gdb.
The readelf -d a.out command will show something like this:
Dynamic section at offset 0x6e8 contains 21 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libc.so.6] <-- dependent dynamic library name
0x000000000000000c (INIT) 0x4003a8 <-- address of .init section
0x000000000000000d (FINI) 0x400618 <-- address of .fini section
0x0000000000000004 (HASH) 0x400240 <-- address of .hash section
0x000000006ffffef5 (GNU_HASH) 0x400268 <-- address of .gnu.hash section
0x0000000000000005 (STRTAB) 0x4002e8 <-- address of .strtab section
0x0000000000000006 (SYMTAB) 0x400288 <-- address of .symtab section
0x000000000000000a (STRSZ) 63 (bytes) <-- size of .strtab section
0x000000000000000b (SYMENT) 24 (bytes) <-- size of an entry in .symtab section
0x0000000000000015 (DEBUG) 0x0 <-- see below
0x0000000000000003 (PLTGOT) 0x600860 <-- address of .got.plt section
0x0000000000000002 (PLTRELSZ) 48 (bytes) <-- total size of .rela.plt section
0x0000000000000014 (PLTREL) RELA <-- RELA or REL ?
0x0000000000000017 (JMPREL) 0x400368 <-- address of .rela.plt section
0x0000000000000007 (RELA) 0x400350 <-- address of .rela.dyn section
0x0000000000000008 (RELASZ) 24 (bytes) <-- total size of .rela.dyn section
0x0000000000000009 (RELAENT) 24 (bytes) <-- size of an entry in .rela.dyn section
0x000000006ffffffe (VERNEED) 0x400330 <-- address of .gnu.version_r section
0x000000006fffffff (VERNEEDNUM) 1 <-- number of needed versions
0x000000006ffffff0 (VERSYM) 0x400328 <-- address of .gnu.version section
0x0000000000000000 (NULL) 0x0 <-- marks the end of .dynamic section
Each entry in
DYNAMIC segment is a struct of only two members:
"tag" and "value". The
NEEDED,
INIT ... above
are "tags" (see
/usr/include/elf.h)
Other tags of interest are:
BIND_NOW The same as BIND_NOW in FLAGS. This has been superseded by
BIND_NOW in FLAGS
CHECKSUM The checksum value used by prelink.
DEBUG At runtime ld.so will fill its value with the runtime
address of r_debug structure (see elf/rtld.c)
and this info is used by GDB (see elf_locate_base function
in GDB's source tree).
FINI Address of .fini section
FINI_ARRAY Address of .fini_array section
FINI_ARRAYSZ Size of .fini_array section
FLAGS Additional flags, such as BIND_NOW, STATIC_TLS, TEXTREL..
FLAGS_1 Additional flags used by Solaris, such as NOW (the same as BIND_NOW), INTERPOSE..
GNU_PRELINKED The timestamp string when the binary object is last prelinked.
INIT Address of .init section
INIT_ARRAY Address of .init_array section
INIT_ARRAYSZ Size of .init_array section
INTERP Address of .interp section
PREINIT_ARRAY Address of .preinit_array section
PREINIT_ARRAYSZ Size of .preinit_array section
RELACOUNT Number of R_X86_64_RELATIVE entries in RELA segment (.rela.dyn
section)
RPATH Dynamic library search path, which has higher precendence than
LD_LIBRARY_PATH. RPATH is ignored if RUNPATH is present.
Use of RPATH is deprecated.
When one uses "gcc -Wl,-rpath=... " to build binaries, the info
is stored here.
RUNPATH Dynamic library search path, which has lower precendence than
LD_LIBRARY_PATH.
When one uses "gcc -Wl,-rpath=...,--enable-new-dtags"
to build binaries, the info is stored here.
(See here for details.)
One can use chrpath
tool to manipulate RPATH and RUNPATH settings.
SONAME Shared object (i.e. dynamic library) name. When one uses
"gcc -Wl,-soname=... " to build binaries, the info is
stored here.
TEXTREL Relocation might modify .text section.
VERDEF Address of .gnu.version_d section
VERDEFNUM Number of version definitions.
Runtime Relocation
After exploring
DYNAMIC segment, we can explain how
ld.so performs
runtime relocation.
First, before ld.so loads all dependent libraries of a dynamic executable,
it needs to run its own relocation! Even if ld.so is a statically-linked binary,
it also has a DYNAMIC segment and thus PLTREL (.rela.dyn section)
and JMPREL (.rela.plt section) tags:
$ readelf -a `readelf -p .interp /bin/sh | awk '/ld/ {print $3}'`
....
Dynamic section at offset 0x14e18 contains 22 entries:
Tag Type Name/Value
0x000000000000000e (SONAME) Library soname: [ld-linux-x86-64.so.2]
0x0000000000000004 (HASH) 0x3269500190
0x0000000000000005 (STRTAB) 0x3269500578
0x0000000000000006 (SYMTAB) 0x3269500260
0x000000000000000a (STRSZ) 388 (bytes)
0x000000000000000b (SYMENT) 24 (bytes)
0x0000000000000003 (PLTGOT) 0x3269614f98
0x0000000000000002 (PLTRELSZ) 120 (bytes)
0x0000000000000014 (PLTREL) RELA
0x0000000000000017 (JMPREL) 0x32695009a0
0x0000000000000007 (RELA) 0x32695007c0
0x0000000000000008 (RELASZ) 480 (bytes)
0x0000000000000009 (RELAENT) 24 (bytes)
0x000000006ffffffc (VERDEF) 0x3269500740
0x000000006ffffffd (VERDEFNUM) 4
0x0000000000000018 (BIND_NOW)
0x000000006ffffffb (FLAGS_1) Flags: NOW
0x000000006ffffff0 (VERSYM) 0x32695006fc
0x000000006ffffff9 (RELACOUNT) 19
0x000000006ffffdf8 (CHECKSUM) 0x4c4e099e
0x000000006ffffdf5 (GNU_PRELINKED) 2010-08-26T08:13:28
0x0000000000000000 (NULL) 0x0
Relocation section '.rela.dyn' at offset 0x7c0 contains 20 entries:
Offset Info Type Sym. Value Sym. Name + Addend
003269614cf0 000000000008 R_X86_64_RELATIVE 000000326950dd80
....
003269615820 000000000008 R_X86_64_RELATIVE 0000003269501140
003269614fe0 001e00000006 R_X86_64_GLOB_DAT 0000003269615980 _r_debug + 0
Relocation section '.rela.plt' at offset 0x9a0 contains 5 entries:
Offset Info Type Sym. Value Sym. Name + Addend
003269614fb0 000b00000007 R_X86_64_JUMP_SLO 000000326950f1b0 __libc_memalign + 0
003269614fb8 000c00000007 R_X86_64_JUMP_SLO 000000326950f2b0 malloc + 0
003269614fc0 001200000007 R_X86_64_JUMP_SLO 000000326950f2c0 calloc + 0
003269614fc8 001800000007 R_X86_64_JUMP_SLO 000000326950f340 realloc + 0
003269614fd0 002000000007 R_X86_64_JUMP_SLO 000000326950f300 free + 0
Note that the
ld.so is
prelinked. On Fedora and Red Hat Enterprise Linux
(RHEL) systems,
prelink is run every two weeks.
To see if your Linux has similar setup, check
/etc/sysconfig/prelink
and
/etc/prelink.conf
What does this prelink do? It changes the base address of a dynamic library
to the actual address in the user program's address space when it is loaded into memory.
Of course, ld.so recognizes GNU_PRELINKED
tag and will load a dynamic library to its this base address (recall the first argument of
mmap is the preferred address; of course, this is subject to the operating system.)
Normally, a dynamic library
is built as position independent code,
i.e. the -fPIC compiler command-line option, and thus the base address is
0. For example, a normal libc.so has ELF program header as follows (readelf -l command):
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000179058 0x0000000000179058 R E 200000
LOAD 0x0000000000179730 0x0000000000379730 0x0000000000379730
0x0000000000004668 0x00000000000090f8 RW 200000
....
And when calling
mmap with address 0 (i.e. NULL)
the operating system can choose any address it feels appropriate.
A prelinked one, on the other hand, has its ELF program header as follows:
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
LOAD 0x0000000000000000 0x0000003433e00000 0x0000003433e00000
0x000000000001bb80 0x000000000001bb80 R E 200000
LOAD 0x000000000001bb90 0x000000343401bb90 0x000000343401bb90
0x0000000000000f58 0x00000000000010f8 RW 200000
What is the advantage of prelinking?
ld.so will not process
R_X86_64_RELATIVE relocation types
since they are already in the "right" place in user program's address space.
The extra benefit of this is the memory regions which
ld.so would have written to (if
R_X86_64_RELATIVE needs
processing) will not incur any Copy-On-Writes and thus can be made Read-Only.
According to this post, for GUI
programs, which tend to link against dozens of dynamic libraries and use lengthy
C++ demangled names, the speed up can be an order of magnitude.
How to disable prelinking at runtime?
Run the user program with LD_USE_LOAD_BIAS environmental
variable set to 0.
How does ld.so process its own relocation?
The relocation is done by _dl_relocate_object function
in Glibc's elf/dl-reloc.c, which will call
elf_machine_rela function in sysdeps/x86_64/dl-machine.h
to do the majority of work.
First to be processed is the .rela.dyn relocation table,
which contains a bunch of R_X86_64_RELATIVE types
and one R_X86_64_GLOB_DAT type (the variable _r_debug)
If prelink is used, i.e. ld.so is indeed loaded
to the desired address, then R_X86_64_RELATIVE
relocation types will be ignored. If not,
then the address calculation for R_X86_64_RELATIVE types
is
Base Address + Value Stored at [Base Address + Offset]
For example, in
ld.so's case, its base address
is 2a95556000 (can be obtained from
pmap command; inside
ld.so,
it calls
elf_machine_load_address function to get this value)
0000400000 4K r-x-- /tmp/a.out
0000500000 4K rw--- /tmp/a.out
2a95556000 92K r-x-- /lib64/ld.so
2a9556d000 8K rw--- [ anon ]
2a95599000 4K rw--- [ anon ]
2a9566c000 4K r---- /lib64/ld.so
2a9566d000 4K rw--- /lib64/ld.so
3269700000 1216K r-x-- /lib64/libc-2.3.4.so
...
And
ld.so's
.rela.dyn relocation table is (
no prelinked!
If
ld.so is prelinked, the offset will be in a much higher address)
Relocation section '.rela.dyn' at offset 0x7c0 contains 20 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000116d50 000000000008 R_X86_64_RELATIVE 000000000000e250
...
so the relocation for 000000116d50 is processed as
0x2a95556000 + *(0x116d50+0x2a95556000)
and this new value is stored at 0x2a9566cd50 (=0x116d50+0x2a95556000)
As R_X86_64_RELATIVE types do not require symbol lookups,
they are handled in a tight loop in
elf_machine_rela_relative function in
sysdeps/x86_64/dl-machine.h
Any relocation types other than R_X86_64_RELATIVE need to go
through symbol resolution first.
So what about R_X86_64_GLOB_DAT relocation type in ld.so ?
First, RESOLVE_MAP (a macro defined within elf/dl-reloc.c)
is called (with r_type = R_X86_64_GLOB_DAT)
to find out which ELF binary (could be the user's program or its dependent
dynamic libraries)
contains this symbol. Then
R_X86_64_GLOB_DAT relocation type is calculated as
Base Address + Symbol Value + Addend
where
Base Address is the base address
of ELF binary which contains the symbol, and
Symbol Value is the symbol value from
the symbol table of ELF binary which contains the symbol.
So for ld.so,
Relocation section '.rela.dyn' at offset 0x7c0 contains 20 entries:
Offset Info Type Sym. Value Sym. Name + Addend
....
000000116fe0 001e00000006 R_X86_64_GLOB_DAT 00000000001179c0 _r_debug + 0
The relocation for 000000116fe0 is processed as
0x2a95556000 + 0x1179c0 + 0
because
ld.so determines
_r_debug
can be found from itself. The calculated value is stored at 0x2a9566cfe0 (=0x116fe0+0x2a95556000).
The next to be processed by ld.so
is its own .rela.plt relocation table,
which contains a bunch of R_X86_64_JUMP_SLOT types.
This reloction type is handled exactly the same way as R_X86_64_GLOB_DAT.
After ld.so finishes its own relocation, it loads user program's
dependent libraries and process their relocations one by one.
First, ld.so handles libc.so's relocation.
libc.so has two relocation types we have not covered so far:
R_X86_64_64 and R_X86_64_TPOFF64.
R_X86_64_64 relocation type is processed by first looking
up the symbol's runtime absolute address, and then
calculating
Absolute Address + Addend
And the
R_X86_64_TPOFF64 relocation type is calculated as
Symbol Value + Addend - TLS Offset
which usually results in a negative value.
R_X86_64_COPY relocation type
R_X86_64_COPY relocation type is used when a dynamic binary refers
to an
initialized global variable (not a function!) defined in a dynamic link library. Unlike
functions,
for variables, there is no lazy binding, and
the trampoline trick used in .plt section
does not work. Instead, the global variable will actually be allocated
in dynamic binary's
.bss section.
To see how R_X86_64_COPY relocation type works, consider the following two code:
foo.c
int foo=4;
void foo_access() {
foo=5;
}
bar.c
#include <stdio.h>
extern int foo;
int main() {
printf("foo=%d\n",foo);
}
Now compile them as follows:
$ gcc -shared -fPIC -Wl,-soname=libfoo.so foo.c -o /tmp/libfoo.so
$ gcc bar.c -o bar -L/tmp -lfoo
And run them as
$ LD_PRELOAD=/tmp/libfoo.so ./bar
Before explaining what happened during runtime, we need to examine
the binaries first.
The foo_access in libfoo.so is like this:
69c <foo_access>:
69c: push rbp
69d: mov rbp,rsp
6a0: mov rax,QWORD PTR [rip+0x100269] # 100910 <_DYNAMIC+0x198>
6a7: mov DWORD PTR [rax],0x5
6ad: leave
6ae: ret
So for
libfoo.so, the
address of variable
foo is
in its
.got section, not
.data section:
$ readelf -a /tmp/libfoo.so
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
...
[18] .got PROGBITS 0000000000100908 00000908
0000000000000020 0000000000000008 WA 0 0 8
[19] .got.plt PROGBITS 0000000000100928 00000928
0000000000000020 0000000000000008 WA 0 0 8
...
[20] .data PROGBITS 0000000000100948 00000948
0000000000000014 0000000000000000 WA 0 0 8
...
Relocation section '.rela.dyn' at offset 0x520 contains 6 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000100948 000000000008 R_X86_64_RELATIVE 0000000000100948
000000100950 000000000008 R_X86_64_RELATIVE 0000000000100768
000000100908 000f00000006 R_X86_64_GLOB_DAT 0000000000000000 __cxa_finalize + 0
000000100910 001100000006 R_X86_64_GLOB_DAT 0000000000100958 foo + 0
....
But what about the address
0x100958 ? This address
is in
libfoo.so's
.data section! Well,
0x100958
has the initial value of
foo (in our example, 4) At runtime,
ld.so
will copy this value to
bar's
.bss section:
$ objdump -sj .data libfoo.so
libfoo.so: file format elf64-x86-64
Contents of section .data:
100948 48091000 00000000 68071000 00000000 H.......h.......
100958 04000000 ....
Next, disassemble the main function of bar:
4005f8 <main>:
4005f8: push rbp
4005f9: mov rbp,rsp
4005fc: mov esi,DWORD PTR [rip+0x1003de] # 5009e0 <__bss_start>
400602: mov edi,0x40070c
400607: mov eax,0x0
40060c: call 400528 <printf@plt>
400611: leave
400612: ret
So the variable
foo is indeed located in
bar's
.bss section. Let's double check with
nm:
$ nm -n bar | grep 5009e0
00000000005009e0 A __bss_start
00000000005009e0 A _edata
00000000005009e0 B foo
(Symbols such as
__bss_start and
_edata are defined by the default
ld script;
one can search them in the output of
ld -verbose command.)
The dynamic relocation table of bar is:
Relocation section '.rela.dyn' at offset 0x490 contains 2 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000500998 000c00000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
0000005009e0 000700000005 R_X86_64_COPY 00000000005009e0 foo + 0
Now what happens during runtime is this: After
ld.so loads all dependent
dynamic libraries, it starts processing their relocations.
When it sees
foo of
libfoo.so, it
calls
RESOLVE_MAP with r_type =
R_X86_64_GLOB_DAT to get
the Base Address, which is 0, and Symbol Value, which is
5009e0. Next it
sees
foo of
libfoo.so has
R_X86_64_GLOB_DAT relocation type,
so it calculates the new address as 5009e0 = 0 + 5009e0 + 0 (addend)
and stores the result somewhere inside
.got section.
After ld.so has processed relocations of all
dynamic libraries, it starts processing the relocation table
of bar. When it sees foo of bar, it
calls RESOLVE_MAP again, but with r_type = R_X86_64_COPY. This time, the address returned is
the runtime address of foo in libfoo.so's
.data section. As mentioned earlier, this
address holds the initial value of foo.
Next it sees foo of bar has R_X86_64_COPY
relocation type, so it uses memcpy
to copy data to 5009e0
(see the Sym. Value of .rela.dyn section of bar above)
from the runtime address of foo in libfoo.so's
.data section (see Glibc source file sysdeps/x86_64/dl-machine.h)
The above example also illustrates the difference
between .got section and .got.plt section.
For the runtime linker ld.so, all it knows is
entries in PLTREL segment, i.e. .rela.dyn section,
(which corresponds to .got section)
must be resolved/relocated immediately, while entries in
JMPREL segment, i.e. .rela.plt section,
(which corresponds to .got.plt section) can use
lazy binding. For x86_64 architecture, the relocation is actually not
needed for R_X86_64_JUMP_SLOT relocation types (albeit the
symbol resolution is still needed)
PIC or no PIC
When building a dynamic library, we are told to
always compile the code with
-fPIC
option.
What's the difference then ?
Consider the following simple code:
#include <stdio.h>
int bar;
void foo() {
printf("%d\n",bar);
}
Compile the above code in 32-bit mode with and without
-fPIC:
$ gcc -shared -m32 foo.c -o nopic.so
$ gcc -shared -m32 -fPIC foo.c -o pic.so
(If you try to compile the above in 64-bit mode,
GCC will
stop and insist you should compile with -fPIC option, i.e. you are going to
see error message such as
relocation R_X86_64_PC32 against symbol `XXXYYY' can not be used when making a shared object; recompile with -fPIC)
The sections and relocation tables of
nopic.so
and
pic.so
are shown at left and right hand side, respectively:
Section Headers: Section Headers:
[Nr] Name Type Addr [Nr] Name Type Addr
[ 0] NULL 0000 [ 0] NULL 0000
... ...
[ 8] .init PROGBITS 02f8 [ 8] .init PROGBITS 02f0
[ 9] .plt PROGBITS 0310 [ 9] .plt PROGBITS 0308
[10] .text PROGBITS 0340 [10] .text PROGBITS 0350
[11] .fini PROGBITS 0488 [11] .fini PROGBITS 04a8
[12] .rodata PROGBITS 04a4 [12] .rodata PROGBITS 04c4
... ...
[17] .dynamic DYNAMIC 14c0 [17] .dynamic DYNAMIC 14e0
[18] .got PROGBITS 1590 [18] .got PROGBITS 15a8
[19] .got.plt PROGBITS 159c [19] .got.plt PROGBITS 15b8
[20] .data PROGBITS 15b0 [20] .data PROGBITS 15d0
... ...
Relocation section '.rel.dyn' at offset 0x2b0 Relocation section '.rel.dyn' at offset 0x2b0
contains 7 entries: contains 5 entries:
Offset Info Type Sym.Value Sym. Name Offset Info Type Sym.Value Sym. Name
00000439 00000008 R_386_RELATIVE 000015d0 00000008 R_386_RELATIVE
000015b0 00000008 R_386_RELATIVE 000015a8 00000106 R_386_GLOB_DAT 000015dc bar
00000434 00000101 R_386_32 000015bc bar ...
00000445 00000602 R_386_PC32 00000000 printf
...
Relocation section '.rel.plt' at offset 0x2e8: Relocation section '.rel.plt' at offset 0x2d8
contains 2 entries: contains 3 entries:
Offset Info Type Sym.Value Sym. Name Offset Info Type Sym.Value Sym. Name
000015a8 00000207 R_386_JUMP_SLOT 00000000 __gmon_start__ 000015c4 00000207 R_386_JUMP_SLOT 00000000 __gmon_start__
000015ac 00000a07 R_386_JUMP_SLOT 00000000 __cxa_finalize 000015c8 00000607 R_386_JUMP_SLOT 00000000 printf
...
When we compile with
-fPIC we can see the variable
bar
has the right relocation type (
R_386_GLOB_DAT)
and the relocation takes place in the right section (
.got) The same for
printf.
Without -fPIC, the relocations of the format string "\n", bar
and printf all take place inside the .text section!
But we know .text section is in a Read-Only LOAD
segment, so what ld.so would do ?
As expected, ld.so will make .text section
writeable, patch the bytes, and make it Read-Only again. Since the
relocation of both bar and printf are
in .rel.dyn, their relocations are performed immediately
(no lazy binding), so this approach is feasible.
So how does ld.so handle
R_386_RELATIVE,
R_386_32
and R_386_PC32 relocation types ?
Let's look at the disassembly:
0000042c <foo>:
42c: 55 push ebp
42d: 89 e5 mov ebp,esp
42f: 83 ec 18 sub esp,0x18
432: 8b 15 00 00 00 00 mov edx,DWORD PTR ds:0x0 <-- reference to bar
438: b8 a4 04 00 00 mov eax,0x4a4 <-- reference to "%d\n" format string in .rodata
43d: 89 54 24 04 mov DWORD PTR [esp+0x4],edx
441: 89 04 24 mov DWORD PTR [esp],eax
444: e8 fc ff ff ff call 445 <foo+0x19> <-- reference to printf
449: c9 leave
44a: c3 ret
How would the 4 bytes starting at 445 (
R_386_PC32 type)
be patched ? Suppose at runtime, our
nopic.so is loaded
into memory with base address 8000, and the 4 bytes
to be patched are now at 8000 + 445 = 8445.
Furthermore, suppose
ld.so has determined
the entry address of
printf to be 10000, then
ld.so calculates the
relative offset as follows:
10000 - 8445 + fffffffc = 7bb7
(fffffffc is -4) so
ld.so replaces
fc ff ff ff
with
b7 7b 00 00
To patch the 4 bytes starting at 434 (R_386_32 type) is simpler.
ld.so will simply overwrite the 4 bytes with the runtime absolute
address of bar.
To patch the 4 bytes starting at 439 (R_386_RELATIVE type)
ld.so calculates the address as
10000 + 4a4 = 104a4
so
ld.so replaces
a4 04 00 00
with
a4 04 01 00
Finally, what about the R_386_RELATIVE relocation at 15b0 ?
15b0 is the starting address of .data section, and the first 4 bytes
of .data section stores its own address, 15b0. So it has to be
relocated and patched as 115b0.
In conclusion, R_386_RELATIVE means "32-bit relative to base address",
R_386_PC32 means the "32-bit IP-relative offset"
and R_386_32 means the "32-bit absolute."
Troubleshooting ld.so
What is "error while loading shared libraries: requires glibc 2.5 or later dynamic linker" ?
The cause of this error is the dynamic binary (or one of its dependent shared libraries)
you want to run only has
.gnu.hash section, but the
ld.so on the target machine
is too old to recognize
.gnu.hash; it only recognizes the old-school
.hash section.
This usually happens when the dynamic binary in question is built using newer version of GCC.
The solution is to recompile the code with either -static compiler command-line option
(to create a static binary), or the following option:
-Wl,--hash-style=both
This tells the link editor
ld to create both
.gnu.hash and
.hash sections.
According to ld documentation here,
the old-school .hash section is the default, but the compiler can override it. For example,
the GCC (which is version 4.1.2) on RHEL (Red Hat Enterprise Linux) Server release 5.5 has
this line:
$ gcc -dumpspecs
....
*link:
%{!static:--eh-frame-hdr} %{!m32:-m elf_x86_64} %{m32:-m elf_i386} --hash-style=gnu %{shared:-shared} ....
...
For more information, see here.
What is "Floating point exception" ?
The cause of this error is the same as the previous question. On certain systems, e.g. RHEL, the old version
ld.so
is
backported to emit "error while loading shared libraries: requires glibc 2.5 or later dynamic linker", but
this is not always the case, and you will see this error instead.
What is ".../libc.so.6: version `GLIBC_2.4' not found " ?
As the error message says, some of the symbols need Glibc version 2.4 or higher. This can also be
seen by
$ objdump -x foo | grep 'Version References' -A10
Version References:
required from libc.so.6:
0x0d696914 0x00 03 GLIBC_2.4
0x09691a75 0x00 02 GLIBC_2.2.5
...
The fix is to recompile the code with
-static compiler command-line option.
What is "FATAL: kernel too old" ?
Even if you recompile the code with
-static compiler command-line option to avoid
any dependency on the dynamic Glibc library, you could still encounter the error
in question, and your code will exit with Segmentation Fault error.
This kernel version check is done by DL_SYSDEP_OSCHECK macro in Glibc's
sysdeps/unix/sysv/linux/dl-osinfo.h
It calls _dl_discover_osversion to get current kernel's version.
To wit, run your code (suppose it is not stripped) inside gdb,
(gdb) run
Starting program: foo
FATAL: kernel too old
Program received signal SIGSEGV, Segmentation fault.
0x00000000004324a9 in ptmalloc_init ()
(gdb) call _dl_discover_osversion()
$1 = 132617
(gdb) p/x $1
$2 = 0x20609
(gdb)
Here
0x20609 means the current kernel version is 2.6.9.
The fix (or hack) is to add the following function in your code:
int _dl_discover_osversion() { return 0xffffff; }
and compile your code with
-static compiler command-line option.
Exploring Glibc's pthread_t
When one creates a thread using the Pthread API, one will get a
pthread_t object as a handle.
In Glibc,
pthread_t is actually a pointer pointing to a
pthread
struct, which is opaque. Its definition can be found in Glibc's source tree at
nptl/descr.h. The first member of
pthread struct is yet
another struct called
tcbhead_t defined in
system-dependent header files such as
nptl/sysdeps/x86_64/tls.h. It holds TLS related
information. It contains at least an integer member called
multiple_threads which
indicates if the process is running in multi-thread mode.
The second member of pthread struct is also
a struct called list_t defined in
nptl/sysdeps/pthread/list.h.
The third and fourth members of pthread struct are thread ID and thread
group ID (both are of pid_t type).
Other members of pthread struct which are of interest: int cancelhandling for
cancellation information, int flags for thread attributes,
start_routine for start position of the code to be executed for the thread,
void *arg for the argument to start_routine
void *stackblock and size_t stackblock_size for thread-specific
stack information.
Since pthread struct is opaque, how can one obtain the above information,
or more precisely, how can one obtain the offsets of these members within the
pthread struct ? We can use the known information and search
for the memory region pointed by pthread_t, as in this code snippet.
