[RFC]confusion about syscall

[Peter Teoh]

Hi Mulyadi and WangZhe,

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

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

> Hi...
>
> On Sun, Jul 15, 2012 at 9:28 AM, 王哲 <wangzhe5004 at gmail.com> 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 at 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 at 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 at 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

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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