gettimeofday syscall如何工作? [英] How does the gettimeofday syscall wor‍k?

问题描述

gettimeofday是根据此页面的x86-86的系统调用(只需搜索gettimeofday在框中):

gettimeofday is a syscall of x86-86 according to this page(just search gettimeofday in the box):

int gettimeofday(struct timeval *tv, struct timezone *tz);

我认为反汇编应该很容易，只需准备两个指针并调用相关的syscall，但是反汇编的作用还很多:

I thought the disassembly should be easy enough, just prepare the two pointers and call the related syscall, but its disassembly is doing much more:

(gdb) disas gettimeofday
Dump of assembler code for function gettimeofday:
0x00000034f408c2d0 <gettimeofday+0>: sub    $0x8,%rsp
0x00000034f408c2d4 <gettimeofday+4>: mov    $0xffffffffff600000,%rax
0x00000034f408c2db <gettimeofday+11>: callq  *%rax
0x00000034f408c2dd <gettimeofday+13>: cmp    $0xfffff001,%eax
0x00000034f408c2e2 <gettimeofday+18>: jae    0x34f408c2e9 <gettimeofday+25>
0x00000034f408c2e4 <gettimeofday+20>: add    $0x8,%rsp
0x00000034f408c2e8 <gettimeofday+24>: retq   
0x00000034f408c2e9 <gettimeofday+25>: mov    0x2c4cb8(%rip),%rcx        # 0x34f4350fa8 <free+3356736>
0x00000034f408c2f0 <gettimeofday+32>: xor    %edx,%edx
0x00000034f408c2f2 <gettimeofday+34>: sub    %rax,%rdx
0x00000034f408c2f5 <gettimeofday+37>: mov    %edx,%fs:(%rcx)
0x00000034f408c2f8 <gettimeofday+40>: or     $0xffffffffffffffff,%rax
0x00000034f408c2fc <gettimeofday+44>: jmp    0x34f408c2e4 <gettimeofday+20>
End of assembler dump. 

我完全看不到syscall.

任何人都可以解释它的工作原理吗?

Can anyone explain how it works?

推荐答案

gettimeofday()在Linux上称为vsyscall和/或vdso.因此，您会看到两行:

gettimeofday() on Linux is what's called a vsyscall and/or vdso. Hence you see the two lines:

0x00000034f408c2d4 : mov    $0xffffffffff600000,%rax
0x00000034f408c2db : callq  *%rax

在您的反汇编中.地址0xffffffffff600000是vsyscall页面(在x86_64上).

in your disassembly. The address 0xffffffffff600000 is the vsyscall page (on x86_64).

该机制将特定的内核创建的代码页映射到用户内存中，因此可以进行一些系统调用"，而不会增加用户/内核上下文切换的开销，而是普通"函数调用.实际的实现是

The mechanism maps a specific kernel-created code page into user memory, so that a few "syscalls" can be made without the overhead of a user/kernel context switch, but rather as "ordinary" function call. The actual implementation is right here.

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