On Sun, Jul 15, 2012 at 8:36 PM, 王哲 <wangzhe5004@xxxxxxxxx> wrote:
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...
Looks like jumping into vsyscall page to me...
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>
after I start the process, and doing a gdb -p <pid>:(gdb) disassemble mainDump of assembler code for function main:0x0000000000400564 <+0>: push %rbp0x0000000000400565 <+1>: mov %rsp,%rbp0x0000000000400568 <+4>: sub $0x10,%rsp0x000000000040056c <+8>: movq $0x0,-0x8(%rbp)0x0000000000400574 <+16>: mov $0x0,%eax0x0000000000400579 <+21>: callq 0x400460 <getpid@plt>0x000000000040057e <+26>: cltq0x0000000000400580 <+28>: mov %rax,-0x8(%rbp)0x0000000000400584 <+32>: movabs $0x9184e72a000,%rdi0x000000000040058e <+42>: mov $0x0,%eax0x0000000000400593 <+47>: callq 0x400470 <sleep@plt>0x0000000000400598 <+52>: mov $0x0,%eax0x000000000040059d <+57>: leaveq0x000000000040059e <+58>: retqEnd of assembler dump.(gdb) disassemble getpidDump of assembler code for function getpid:0x00007f19ae558530 <+0>: mov %fs:0x2d4,%edx0x00007f19ae558538 <+8>: cmp $0x0,%edx0x00007f19ae55853b <+11>: jle 0x7f19ae558540 <getpid+16>0x00007f19ae55853d <+13>: mov %edx,%eax0x00007f19ae55853f <+15>: retq0x00007f19ae558540 <+16>: jne 0x7f19ae558554 <getpid+36>0x00007f19ae558542 <+18>: mov %fs:0x2d0,%eax0x00007f19ae55854a <+26>: test %eax,%eax0x00007f19ae55854c <+28>: nopl 0x0(%rax)0x00007f19ae558550 <+32>: je 0x7f19ae558554 <getpid+36>0x00007f19ae558552 <+34>: repz retq0x00007f19ae558554 <+36>: mov $0x27,%eax0x00007f19ae558559 <+41>: syscall0x00007f19ae55855b <+43>: test %edx,%edx0x7f19ae55855d <getpid+45>: jne 0x7f19ae558552 <getpid+34>0x7f19ae55855f <getpid+47>: mov %eax,%fs:0x2d00x7f19ae558567 <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:0xb7771a40 <+0>: mov %gs:0x6c,%edx
(gdb) disassemble getpid
Dump of assembler code for function getpid:
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:
(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 sharedlibraryFrom To Syms Read Shared Object Library0x00007f19ae4cb8c0 0x00007f19ae5dec60 Yes (*) /lib/libc.so.60x00007f19ae830af0 0x00007f19ae849704 Yes (*) /lib64/ld-linux-x86-64.so.2(*): Shared library is missing debugging information.and if u want:
cat /proc/2282/maps7f19ae82a000-7f19ae82b000 rw-p 0017d000 08:05 9922 /lib/libc-2.11.1.so7f19ae830000-7f19ae850000 r-xp 00000000 08:05 8824 /lib/ld-2.11.1.so7ffff2031000-7ffff2052000 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
Regards,
Peter Teoh
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