Question

当我注意到很多线程都有类似这样的跟踪时，我正在查看chrome的线程堆栈：

0, wow64cpu.dll!TurboDispatchJumpAddressEnd+0x6c0
1, wow64cpu.dll!TurboDispatchJumpAddressEnd+0x4a8
2, wow64.dll!Wow64SystemServiceEx+0x1ce
3, wow64.dll!Wow64LdrpInitialize+0x429
4, ntdll.dll!RtlIsDosDeviceName_U+0x24c87
5, ntdll.dll!LdrInitializeThunk+0xe
6, ntdll.dll!ZwWaitForSingleObject+0x15
7, kernel32.dll!WaitForSingleObjectEx+0x43
8, kernel32.dll!WaitForSingleObject+0x12
9, chrome.dll!ovly_debug_event+0x16574
10, chrome.dll!ovly_debug_event+0x14904
11, chrome.dll!ovly_debug_event+0x14826
12, chrome.dll!ovly_debug_event+0x16d19
13, chrome.dll!ovly_debug_event+0x1bea1b
14, chrome.dll!ovly_debug_event+0xe8ff4
15, chrome.dll!ovly_debug_event+0x16b50
16, chrome.dll!ovly_debug_event+0x16ab2
17, kernel32.dll!BaseThreadInitThunk+0x12
18, ntdll.dll!RtlInitializeExceptionChain+0x63
19, ntdll.dll!RtlInitializeExceptionChain+0x36

铬源在sel_ldr.c中有以下代码，似乎将ovly_debug_event声明为几乎为空的函数：

void _ovly_debug_event (void) {
#ifdef __GNUC__
  /*
   * The asm volatile is here as instructed by the GCC docs.
   * It's not enough to declare a function noinline.
   * GCC will still look inside the function to see if it's worth calling.
   */
  __asm__ volatile ("");
#elif NACL_WINDOWS
  /*
   * Visual Studio inlines empty functions even with noinline attribute,
   * so we need a compile memory barrier to make this function not to be
   * inlined. Also, it guarantees that nacl_global_xlate_base initialization
   * is not reordered. This is important for gdb since it sets breakpoint on
   * this function and reads nacl_global_xlate_base value.
   */
  _ReadWriteBarrier();
#endif
}

static void StopForDebuggerInit (uintptr_t mem_start) {
  /* Put xlate_base in a place where gdb can find it.  */
  nacl_global_xlate_base = mem_start;

  NaClSandboxMemoryStartForValgrind(mem_start);

  _ovly_debug_event();
}

这引出了一个问题：为什么chrome似乎花了这么多时间在一个仅用于调试的函数中，并且几乎是空的？

Answer 1

注意大量偏移，例如0x16574进入此功能。您似乎没有chrome.dll的私有符号，因此调试器正在查找最接近（最好，最接近的上一个）公开导出的符号。

换句话说，您不在_ovly_debug_event中。

，这是一个在可执行文件中布局的函数，但不是公开导出的。

要尝试解决此问题，如果您想查看实际发生的情况，可以添加 http://chromium-browser-symsrv.commondatastorage.googleapis.com 到你的符号路径。在windbg中，命令将是

.sympath + SRV * C：\ tmp * http://chromium-browser-symsrv.commondatastorage.googleapis.com

Answer 2

此外，该函数实际上是GDB的辅助函数，可以帮助调试叠加。请参阅https://sourceware.org/gdb/onlinedocs/gdb/Automatic-Overlay-Debugging.html。

ovly_debug_event在chrome中做了什么？

2 个答案: