[RFC]confusion about syscall

[RFC]confusion about syscall

[王哲]

Hi all:

 i write two simple program to invoke syscall getpid.

the first  program (getpid1.c) is as followed:
#include <stdio.h>
#include <unistd.h>

int main(void)
{
    unsigned long value = 0;
    __asm__("int $0x80":"=a"(value):"0"(20));
    return 0;
}

and use objdump -d a.out disassembling it :
...
 80483c2:    b8 14 00 00 00       mov    $0x14,%eax
 80483c7:    89 c3                    mov    %eax,%ebx
 80483c9:    89 d8                    mov    %ebx,%eax
 80483cb:    cd 80                    int    $0x80
...


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>
...
080483e4 <main>:
 80483e4:    55                                      push   %ebp
 80483e5:    89 e5                                  mov    %esp,%ebp
 80483e7:    83 e4 f0                              and    $0xfffffff0,%esp
 80483ea:    83 ec 10                             sub    $0x10,%esp
 80483ed:    c7 44 24 0c 00 00 00           movl   $0x0,0xc(%esp)
 80483f4:    00
 80483f5:    e8 06 ff ff ff                         call   8048300
<getpid@plt>
 80483fa:    89 44 24 0c                         mov    %eax,0xc(%esp)
 80483fe:    b8 00 00 00 00                    mov    $0x0,%eax
...

question:
      why i can't find the "movl $0x14,%eax"  "int 0x80" in the second
program after disassembling?

any advice will be help!
thanks in advance!
wanny
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[RFC]confusion about syscall

[Peter Teoh]

int 80, i think, is hardly used nowadays:

http://wiki.osdev.org/System_Calls
http://articles.manugarg.com/systemcallinlinux2_6.html
http://semipublic.comp-arch.net/wiki/SYSENTER/SYSEXIT_vs._SYSCALL/SYSRET

and even for SYSENTER instruction, it will not be compiled into your
program, but exists inside some external libraries, unless u compile it as
static.

On Sun, Jul 15, 2012 at 10:28 AM, ?? <wangzhe5004@gmail.com> wrote:

> Hi all:
>
>  i write two simple program to invoke syscall getpid.
>
> the first  program (getpid1.c) is as followed:
> #include <stdio.h>
> #include <unistd.h>
>
> int main(void)
> {
>     unsigned long value = 0;
>     __asm__("int $0x80":"=a"(value):"0"(20));
>     return 0;
> }
>
> and use objdump -d a.out disassembling it :
> ...
>  80483c2:    b8 14 00 00 00       mov    $0x14,%eax
>  80483c7:    89 c3                    mov    %eax,%ebx
>  80483c9:    89 d8                    mov    %ebx,%eax
>  80483cb:    cd 80                    int    $0x80
> ...
>
>
> 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>
> ...
> 080483e4 <main>:
>  80483e4:    55                                      push   %ebp
>  80483e5:    89 e5                                  mov    %esp,%ebp
>  80483e7:    83 e4 f0                              and    $0xfffffff0,%esp
>  80483ea:    83 ec 10                             sub    $0x10,%esp
>  80483ed:    c7 44 24 0c 00 00 00           movl   $0x0,0xc(%esp)
>  80483f4:    00
>  80483f5:    e8 06 ff ff ff                         call   8048300
> <getpid@plt>
>  80483fa:    89 44 24 0c                         mov    %eax,0xc(%esp)
>  80483fe:    b8 00 00 00 00                    mov    $0x0,%eax
> ...
>
> question:
>       why i can't find the "movl $0x14,%eax"  "int 0x80" in the second
> program after disassembling?
>
> any advice will be help!
> thanks in advance!
> wanny
>
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
>


-- 
Regards,
Peter Teoh
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[RFC]confusion about syscall

[Mulyadi Santosa]

Hi...

On Sun, Jul 15, 2012 at 9:28 AM, ?? <wangzhe5004@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@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...

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

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

[RFC]confusion about syscall

[王哲]

2012/7/15 Mulyadi Santosa <mulyadi.santosa@gmail.com>

> Hi...
>
> On Sun, Jul 15, 2012 at 9:28 AM, ?? <wangzhe5004@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@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...
>

   thanks for reply.
   and i wander why the address of vsyscall page is not 0xffffe000 in my
system?
...
b7806000-b7807000 r-xp 00000000 00:00 0          [vdso]
b7807000-b7827000 r-xp 00000000 08:08 2491275    /lib/i386-linux-gnu/
ld-2.15.so
b7827000-b7828000 r--p 0001f000 08:08 2491275    /lib/i386-linux-gnu/
ld-2.15.so
b7828000-b7829000 rw-p 00020000 08:08 2491275    /lib/i386-linux-gnu/
ld-2.15.so
 ...

root at glinuxi-G31M-ES2C:~# ldd /bin/ls
    linux-gate.so.1 =>  (0xb786b000)
    libselinux.so.1 => /lib/i386-linux-gnu/libselinux.so.1 (0xb7839000)
    librt.so.1 => /lib/i386-linux-gnu/librt.so.1 (0xb7830000)
    libacl.so.1 => /lib/i386-linux-gnu/libacl.so.1 (0xb7826000)
...

>
> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer and consultant
>
> blog: the-hydra.blogspot.com
> training: mulyaditraining.blogspot.com
>
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[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@gmail.com>wrote:

> Hi...
>
> On Sun, Jul 15, 2012 at 9:28 AM, ?? <wangzhe5004@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@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

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
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>



-- 
Regards,
Peter Teoh
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[RFC]confusion about syscall

[王哲]

2012/7/15 Peter Teoh <htmldeveloper@gmail.com>

> 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, ?? <wangzhe5004@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@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
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>
>
>
> --
> Regards,
> Peter Teoh
>
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[RFC]confusion about syscall

[Peter Teoh]

just sharing my analysis, correct me if wrong:

On Sun, Jul 15, 2012 at 8:36 PM, ?? <wangzhe5004@gmail.com> wrote:

>
>
> 2012/7/15 Peter Teoh <htmldeveloper@gmail.com>
>
>> 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, ?? <wangzhe5004@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@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
>
> yes u are right - for 32-bit kernel:

In arch/x86/kernel>
grep getpid *.S
syscall_table_32.S: .long sys_getpid /* 20 */

but my linux kernel is 64-bit.



>        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
>
>

See the comment for gs in entry_32.S:

/*
 * User gs save/restore
 *
 * %gs is used for userland TLS and kernel only uses it for stack
 * canary which is required to be at %gs:20 by gcc.  Read the comment
 * at the top of stackprotector.h for more info.
 *
 * Local labels 98 and 99 are used.
 */
#ifdef CONFIG_X86_32_LAZY_GS

And inside stackprotector.h, content of which is still beyond my completely
understanding at the moment, I copied it here:

/*
 * GCC stack protector support.
 *
 * Stack protector works by putting predefined pattern at the start of
 * the stack frame and verifying that it hasn't been overwritten when
 * returning from the function.  The pattern is called stack canary
 * and unfortunately gcc requires it to be at a fixed offset from %gs.
 * On x86_64, the offset is 40 bytes and on x86_32 20 bytes.  x86_64
 * and x86_32 use segment registers differently and thus handles this
 * requirement differently.
 *
 * On x86_64, %gs is shared by percpu area and stack canary.  All
 * percpu symbols are zero based and %gs points to the base of percpu
 * area.  The first occupant of the percpu area is always
 * irq_stack_union which contains stack_canary at offset 40.  Userland
 * %gs is always saved and restored on kernel entry and exit using
 * swapgs, so stack protector doesn't add any complexity there.
 *
 * On x86_32, it's slightly more complicated.  As in x86_64, %gs is
 * used for userland TLS.  Unfortunately, some processors are much
 * slower at loading segment registers with different value when
 * entering and leaving the kernel, so the kernel uses %fs for percpu
 * area and manages %gs lazily so that %gs is switched only when
 * necessary, usually during task switch.
 *
 * As gcc requires the stack canary at %gs:20, %gs can't be managed
 * lazily if stack protector is enabled, so the kernel saves and
 * restores userland %gs on kernel entry and exit.  This behavior is
* controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
 * system.h to hide the details.
 */

Yes, gs register is valid for userspace TLS and thus is per-process, and
for more info:

http://www.akkadia.org/drepper/tls.pdf

http://www.ibm.com/developerworks/linux/library/l-user-space-apps/index.html

http://stackoverflow.com/questions/6021273/how-to-allocate-thread-local-storage

(and lots of relevant links besides it).



  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
>>>
>>> _______________________________________________
>>> Kernelnewbies mailing list
>>> Kernelnewbies at kernelnewbies.org
>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>>
>>
>>
>>
>> --
>> Regards,
>> Peter Teoh
>>
>
>


-- 
Regards,
Peter Teoh
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[RFC]confusion about syscall

[Peter Teoh]

I think this is useful:

http://stackoverflow.com/questions/9355097/looking-for-system-calls-implementation-on-linux-kernel

On Sun, Jul 15, 2012 at 11:24 PM, Peter Teoh <htmldeveloper@gmail.com>wrote:

> just sharing my analysis, correct me if wrong:
>
> On Sun, Jul 15, 2012 at 8:36 PM, ?? <wangzhe5004@gmail.com> wrote:
>
>>
>>
>> 2012/7/15 Peter Teoh <htmldeveloper@gmail.com>
>>
>>> 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, ?? <wangzhe5004@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@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
>>
>> yes u are right - for 32-bit kernel:
>
> In arch/x86/kernel>
> grep getpid *.S
> syscall_table_32.S: .long sys_getpid /* 20 */
>
> but my linux kernel is 64-bit.
>
>
>
>>        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
>>
>>
>
> See the comment for gs in entry_32.S:
>
> /*
>  * User gs save/restore
>  *
>  * %gs is used for userland TLS and kernel only uses it for stack
>  * canary which is required to be at %gs:20 by gcc.  Read the comment
>  * at the top of stackprotector.h for more info.
>  *
>  * Local labels 98 and 99 are used.
>  */
> #ifdef CONFIG_X86_32_LAZY_GS
>
> And inside stackprotector.h, content of which is still beyond my
> completely understanding at the moment, I copied it here:
>
> /*
>  * GCC stack protector support.
>  *
>  * Stack protector works by putting predefined pattern at the start of
>  * the stack frame and verifying that it hasn't been overwritten when
>  * returning from the function.  The pattern is called stack canary
>  * and unfortunately gcc requires it to be at a fixed offset from %gs.
>  * On x86_64, the offset is 40 bytes and on x86_32 20 bytes.  x86_64
>  * and x86_32 use segment registers differently and thus handles this
>  * requirement differently.
>  *
>  * On x86_64, %gs is shared by percpu area and stack canary.  All
>  * percpu symbols are zero based and %gs points to the base of percpu
>  * area.  The first occupant of the percpu area is always
>  * irq_stack_union which contains stack_canary at offset 40.  Userland
>  * %gs is always saved and restored on kernel entry and exit using
>  * swapgs, so stack protector doesn't add any complexity there.
>  *
>  * On x86_32, it's slightly more complicated.  As in x86_64, %gs is
>  * used for userland TLS.  Unfortunately, some processors are much
>  * slower at loading segment registers with different value when
>  * entering and leaving the kernel, so the kernel uses %fs for percpu
>  * area and manages %gs lazily so that %gs is switched only when
>  * necessary, usually during task switch.
>  *
>  * As gcc requires the stack canary at %gs:20, %gs can't be managed
>  * lazily if stack protector is enabled, so the kernel saves and
>  * restores userland %gs on kernel entry and exit.  This behavior is
> * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
>  * system.h to hide the details.
>  */
>
> Yes, gs register is valid for userspace TLS and thus is per-process, and
> for more info:
>
> http://www.akkadia.org/drepper/tls.pdf
>
>
> http://www.ibm.com/developerworks/linux/library/l-user-space-apps/index.html
>
>
> http://stackoverflow.com/questions/6021273/how-to-allocate-thread-local-storage
>
> (and lots of relevant links besides it).
>
>
>
>   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
>>>>
>>>> _______________________________________________
>>>> Kernelnewbies mailing list
>>>> Kernelnewbies at kernelnewbies.org
>>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>>>
>>>
>>>
>>>
>>> --
>>> Regards,
>>> Peter Teoh
>>>
>>
>>
>
>
> --
> Regards,
> Peter Teoh
>



-- 
Regards,
Peter Teoh
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[RFC]confusion about syscall

[Mulyadi Santosa]

Hi...

On Sun, Jul 15, 2012 at 4:18 PM, ?? <wangzhe5004@gmail.com> wrote:
>    thanks for reply.
>    and i wander why the address of vsyscall page is not 0xffffe000 in my
> system?

address space randomization sometimes put the vsyscall page in much
lower address AFAIK. And it varies per invocation...

But wait, i just recall that your getpid() might contain a jump into
unpatched PLT first. It happens for the first time an external
function is called. On subsequent call, getpid() will jump directly to
the needed address in glibc.

Once I wrote about it....try google...

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

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