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Posted Jan 17, 2002
Authored by Mayhem | Site devhell.org

Reversing the ELF - Stepping with GDB during PLT uses and .GOT fixup. This is a GDB tutorial about runtime process fixup using the Procedure Linkage Table section (.plt) and the Global Offset Table section (.got) by the dynamic linker ld-linux.so. ASM knowledge will be helpful. More info on ELF here.

tags | paper
systems | linux, unix
SHA-256 | d827aaba5feb045e90dea774ade60c84ce956eb244b90457391bfb60f6d84432


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Reversing the ELF

Stepping with GDB during PLT uses and .GOT fixup

mayhem (mayhem@hert.org)


This text is a GDB tutorial about runtime process fixup using the Procedure
Linkage Table section (.plt) and the Global Offset Table section (.got) .
If you dont know what is ELF, you should read the ELF ultimate documentation
you can find here :


Some basic ASM knowledge may be requested .

This text has not been written for ELF specialists . This tutorial is an
alternative , interactive way to understand the PLT mechanisms .

When the executable is mapped into memory, the core code segment _start
fonction the executable file is called as soon as the _dl_start function
of the dynamic linker has returned . I will go step by step in the
process structures initialization mechanims using objdump, gdb and gcc ;

bash-2.03$ cat test.c
int called()

int main()
bash-2.03$ cc test.c && objdump -D ./a.out | less


08048318 <_start>:
8048318: 31 ed xor %ebp,%ebp
804831a: 5e pop %esi
804831b: 89 e1 mov %esp,%ecx
804831d: 83 e4 f8 and $0xfffffff8,%esp
8048320: 50 push %eax
8048321: 54 push %esp
8048322: 52 push %edx
8048323: 68 04 84 04 08 push $0x8048404
8048328: 68 98 82 04 08 push $0x8048298
804832d: 51 push %ecx
804832e: 56 push %esi
804832f: 68 c8 83 04 08 push $0x80483c8
8048334: e8 cf ff ff ff call 8048308 <_init+0x70> <===
8048339: f4 hlt
804833a: 90 nop
804833b: 90 nop


Even if the dynamic linker creates some basic stuffs before everything,
he first core function called is _start .

We can see that this function initialize the stack . Some things we have
to notice :

$0x8048404 : _fini() function offset in the .fini section
$0x8048298 : _init() function offset in the .init section
$0x80483c8 : main() function offset in the .text section

This information is pushed because it's used in the libc in the
__libc_start_main function . This libc function has to :

- call the constructors .
- call the main .
- call the destructors .

The call to the offset 0x8048308 points in the Procedure Linkage Table
(.plt section) . This section provides a way to transfert inter-object
calls . Remember we're trying to call the __libc_start_main function .

The PLT code for this entry is :

080482c8 <.plt>:
80482c8: ff 35 54 94 04 08 pushl 0x8049454
80482ce: ff 25 58 94 04 08 jmp *0x8049458
80482d4: 00 00 add %al,(%eax)
80482d6: 00 00 add %al,(%eax)
80482d8: ff 25 5c 94 04 08 jmp *0x804945c
80482de: 68 00 00 00 00 push $0x0
80482e3: e9 e0 ff ff ff jmp 80482c8 <_init+0x30>
80482e8: ff 25 60 94 04 08 jmp *0x8049460
80482ee: 68 08 00 00 00 push $0x8
80482f3: e9 d0 ff ff ff jmp 80482c8 <_init+0x30>
80482f8: ff 25 64 94 04 08 jmp *0x8049464
80482fe: 68 10 00 00 00 push $0x10
8048303: e9 c0 ff ff ff jmp 80482c8 <_init+0x30>
8048308: ff 25 68 94 04 08 jmp *0x8049468 *YOU ARE HERE*
804830e: 68 18 00 00 00 push $0x18
8048313: e9 b0 ff ff ff jmp 80482c8 <_init+0x30>

We can see that our call in the PLT is followed by a JMP :

jmp *0x8049468

The 0x8049468 offset is in the Global Offset Table (.got section) . The
stars means (for non x86 att syntax experts readers) that we are using
the four byte pointer at 0x8049468 as an offset . This offset in actually
retreivable from the Global Offset Table (GOT) .

In the beginning, the GOT offsets points on the following push (offset
0x804830e : look at the objdump trace above) . At the moment , the GOT
in our process is said to be "empty", and is going to be filled as long
as the process calls remote functions (one entry is updated each time the
program calls this remote function *for the first time*) .

.got :

[00] 0x8049470 [01] (nil)
[02] (nil) [03] 0x80482de
[04] 0x80482ee [05] 0x80482fe
[06] 0x804830e [07] (nil)

The GOT is empty : every offsets point on a push instruction in
the procedure linkage table (.plt) .

We can see that the third first entries in the GOT have special values :

- The [0] entry contains the offset for the .dynamic section of
this object, it's used by the dynamic linker to know some
preferences and some very useful informations . Look at the
ELF reference for further details . Some stuff are also explained
in the dynamic linking related chapter of this tfile .

- The [1] one is the link_map structure offset associated with
this object, it's an internal structure in ld.so describing
a lot of interresting stuffs, I wont go in depth with it in this
paper .

- The [2] entry contains the runtime process fixup function offset
(pointing in the dynamic linking code zone). This pointer is used by
the first entry of the plt which is called when you want to launch
a remote (undefined) function for the first time .

The 2nd and 3rd entries are set to NULL at the beginning and are filled by
the dynamic linker before the process code segment starting function takes
control . These are filled in elf_machine_runtime_setup() in
sysdeps/i386/dl-machine.h .

With that mechanism, we have to execute three call instructions (if the
corresponding GOT entry has not been updated yet), or two call instructions
(if the GOT has been filled) . Remember that the GOT has been filled during
the first call on the corresponding function . Only external functions need
that mechanism, since only in-core code segment functions offsets are known
before the process start (the executable object base address is known) .

Get back to our code, we can see that this entry jumps to the 3rd offset of
the GOT entry, it means that the code calls the dynamic linker's resolution
function dl-resolve() . Some other papers have been describing useful
information gathering with it (check Nergal's phrack 58 or grugq's subversive
dynamic linking paper)

We can note that these 2 offsets are pushed each time a call is done via
the PLT :

- The first is an offset (in octets) in the .rel.plt section of the
program binary . It's used to identify the corresponding symbol for
the function we are trying to get the real absolute address .
- The second one is the offset contained in the 2nd GOT entry
(it's 0x8049454 in our code) .

This information allows us to identify the symbol for which we want to do
a relocation . Let's discover the runtime .got fixup offset with gdb and
the procfs :

bash-2.03$ gdb a.out

(gdb) b called
Breakpoint 1 at 0x80483b7

(gdb) r
Starting program: /home/mayhem/a.out
Breakpoint 1, 0x80483b7 in called ()

(gdb) disassemble called
Dump of assembler code for function called:
0x80483b4 <called>: push %ebp
0x80483b5 <called+1>: mov %esp,%ebp
0x80483b7 <called+3>: push $0x8048428
0x80483bc <called+8>: call 0x80482e8 <puts> <======== LOOK HERE !
0x80483c1 <called+13>: add $0x4,%esp
0x80483c4 <called+16>: leave
0x80483c5 <called+17>: ret
0x80483c6 <called+18>: mov %esi,%esi
End of assembler dump.

What is this routine ?

(gdb) x/3i 0x80482e8
0x80482e8 <puts>: jmp *0x8049460
0x80482ee <puts+6>: push $0x8
0x80482f3 <puts+11>: jmp 0x80482c8 <_init+48>

It's the procedure linkage table entry for this function, it uses offset
in the GOT . As the GOT is not yet filled, the offset is the 32 bits address
of the following push ("push $0x8") . This entry is going to be modified
by the dynamic linker as soon as the symbol resolution is done ( see
chapter 2) .

(gdb) x/1x 0x8049460
0x8049460 <_GLOBAL_OFFSET_TABLE_+16>: 0x080482ee

Each first time you call a remote function, the PLT first entry code
is executed :

(gdb) x/3i 0x80482c8
0x80482c8 <_init+48>: pushl 0x8049454
0x80482ce <_init+54>: jmp *0x8049458
0x80482d4 <_init+60>: add %al,(%eax)

This first entry uses the third entry of the GOT , then the dynamic
linker takes control :

(gdb) x/1x 0x8049458
0x8049458 <_GLOBAL_OFFSET_TABLE_+8>: 0x40009a10


Let's see what is the library containing this function at the offset
0x40009a10 .

bash-2.04$ pidof a.out
bash-2.04$ cat /proc/7905/maps
08048000-08049000 r-xp 00000000 03:01 135375 /home/mayhem/a.out
08049000-0804a000 rw-p 00000000 03:01 135375 /home/mayhem/a.out
40000000-40012000 r-xp 00000000 03:01 229408 /lib/ld-2.1.2.so <== *GOOD*
40012000-40013000 rw-p 00011000 03:01 229408 /lib/ld-2.1.2.so
40013000-40014000 rw-p 00000000 00:00 0
4001a000-400fb000 r-xp 00000000 03:01 229410 /lib/libc-2.1.2.so
400fb000-400ff000 rw-p 000e0000 03:01 229410 /lib/libc-2.1.2.so
400ff000-40102000 rw-p 00000000 00:00 0
bfffe000-c0000000 rwxp fffff000 00:00 0

The wanted function is in the dynamic linker code segment, more precisely
in the _dl_runtime_fixup() function, in the sysdeps/i386/dl-machine.h
file . To be honest, i had difficulties to find it , because I could not
deduce the symbol giving its virtual address, and this for 2 reasons :

- The libc and the dynamic linker are stripped (the symbol table
and the debug information has been removed) . As a consequence,
I could get the function name from the symbol table .

- In a shared library, offsets are relative (actually it's
calculated from the beginning of the file, so you can't retreive
the absolute function addresses from the ld.so symbol table) .
You can do a "objdump -D /lib/libc.so.6" to see it by yourself .

Even if I had a symbol table, I could not have got the good offset .
I could have compare the first bytes of the function in the debugged
process and the library (ld.so) code segment to find some hexadecimal
sequences matching, but I prefered to read the sources . ;)

The solution is to calculate the relocation ourself, most of the
time we have : base_addr + symbol_value = runtime address . You
can get the base address from the procfs (if you have read access,
this is okay by default) and you can check the symbol's value
from the library's .dynsym section (list of exported symbols).
Note that you can retreive it from the symbol table (.symtab)
but this section may be removed from the shared library using
known basic tools like strip(3) .

Shoutouts to silvio and grugq !


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