Re: [RFC]confusion about syscall

[王哲 <wangzhe5004@xxxxxxxxx>]

2012/7/15 Peter Teoh <htmldeveloper@xxxxxxxxx>

Hi Mulyadi and WangZhe,

Nice to write to you again....:-).

On Sun, Jul 15, 2012 at 1:49 PM, Mulyadi Santosa <mulyadi.santosa@xxxxxxxxx> wrote:

Hi...

On Sun, Jul 15, 2012 at 9:28 AM, 王哲 <wangzhe5004@xxxxxxxxx> wrote:
> and the second program:
>
> #include <stdio.h>
> #include <unistd.h>
>
> int main(void)
> {
>     unsigned long value = 0;
>     value = getpid();
>     return 0;
> }
>
> and disassembling it:( objdump -d a.out)
> ...
> 08048300 <getpid@plt>:
>  8048300:    ff 25 00 a0 04 08        jmp    *0x804a000
>  8048306:    68 00 00 00 00           push   $0x0
>  804830b:    e9 e0 ff ff ff           jmp    80482f0 <_init+0x3c>

Looks like jumping into vsyscall page to me...

after I start the process, and doing a gdb -p <pid>:

(gdb) disassemble main 

Dump of assembler code for function main:

   0x0000000000400564 <+0>: push   %rbp

   0x0000000000400565 <+1>: mov    %rsp,%rbp

   0x0000000000400568 <+4>: sub    $0x10,%rsp

   0x000000000040056c <+8>: movq   $0x0,-0x8(%rbp)

   0x0000000000400574 <+16>: mov    $0x0,%eax

   0x0000000000400579 <+21>: callq  0x400460 <getpid@plt>

   0x000000000040057e <+26>: cltq   

   0x0000000000400580 <+28>: mov    %rax,-0x8(%rbp)

   0x0000000000400584 <+32>: movabs $0x9184e72a000,%rdi

   0x000000000040058e <+42>: mov    $0x0,%eax

   0x0000000000400593 <+47>: callq  0x400470 <sleep@plt>

   0x0000000000400598 <+52>: mov    $0x0,%eax

   0x000000000040059d <+57>: leaveq 

   0x000000000040059e <+58>: retq   

End of assembler dump.

(gdb) disassemble getpid

Dump of assembler code for function getpid:

   0x00007f19ae558530 <+0>: mov    %fs:0x2d4,%edx

   0x00007f19ae558538 <+8>: cmp    $0x0,%edx

   0x00007f19ae55853b <+11>: jle    0x7f19ae558540 <getpid+16>

   0x00007f19ae55853d <+13>: mov    %edx,%eax

   0x00007f19ae55853f <+15>: retq   

   0x00007f19ae558540 <+16>: jne    0x7f19ae558554 <getpid+36>

   0x00007f19ae558542 <+18>: mov    %fs:0x2d0,%eax

   0x00007f19ae55854a <+26>: test   %eax,%eax

   0x00007f19ae55854c <+28>: nopl   0x0(%rax)

   0x00007f19ae558550 <+32>: je     0x7f19ae558554 <getpid+36>

   0x00007f19ae558552 <+34>: repz retq 

   0x00007f19ae558554 <+36>: mov    $0x27,%eax

   0x00007f19ae558559 <+41>: syscall 

   0x00007f19ae55855b <+43>: test   %edx,%edx

   0x7f19ae55855d <getpid+45>: jne    0x7f19ae558552 <getpid+34>

   0x7f19ae55855f <getpid+47>: mov    %eax,%fs:0x2d0

   0x7f19ae558567 <getpid+55>: retq   

   Hi peter:
       question1: why your system is "0x00007f19ae558554 <+36>: mov    $0x27,%eax",
getpid syscall  number is 0x14

       question2: i use gdb disassemble getpid just like you and the result:

    (gdb) disassemble getpid
 Dump of assembler code for function getpid:
   0xb7771a40 <+0>:    mov    %gs:0x6c,%edx
   0xb7771a47 <+7>:    cmp    $0x0,%edx
   0xb7771a4a <+10>:    jle    0xb7771a50 <getpid+16>
   0xb7771a4c <+12>:    mov    %edx,%eax
   0xb7771a4e <+14>:    repz ret
   0xb7771a50 <+16>:    jne    0xb7771a62 <getpid+34>
   0xb7771a52 <+18>:    mov    %gs:0x68,%eax
   0xb7771a58 <+24>:    test   %eax,%eax
   0xb7771a5a <+26>:    lea    0x0(%esi),%esi
   0xb7771a60 <+32>:    jne    0xb7771a4e <getpid+14>
   0xb7771a62 <+34>:    mov    $0x14,%eax
   0xb7771a67 <+39>:    call   *%gs:0x10

  can you explain the meaning of "call   *%gs:0x10"?
 
  Thanks!


 

And to check the address space:

(gdb) info sharedlibrary 

From                To                  Syms Read   Shared Object Library

0x00007f19ae4cb8c0  0x00007f19ae5dec60  Yes (*)     /lib/libc.so.6

0x00007f19ae830af0  0x00007f19ae849704  Yes (*)     /lib64/ld-linux-x86-64.so.2

(*): Shared library is missing debugging information.

and if u want:

cat /proc/2282/maps 

7f19ae82a000-7f19ae82b000 rw-p 0017d000 08:05 9922                       /lib/libc-2.11.1.so

7f19ae830000-7f19ae850000 r-xp 00000000 08:05 8824                       /lib/ld-2.11.1.so

7ffff2031000-7ffff2052000 rw-p 00000000 00:00 0                          [stack]

7ffff21af000-7ffff21b0000 r-xp 00000000 00:00 0                          [vdso]

ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]

noticed also that static analysis tools like "objdump -d" is generally avoided, if u want to understand dynamic addresses.   From above, we can conclude that the "sysenter" (this is intel syntax, or "syscall", in AMD syntax as used by gdb disassembly above) is used for the transition to the kernel - as embedded inside the libc.so.6.

 

--
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

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--
Regards,
Peter Teoh

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