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What does ovly_debug_event do in chrome?

I looked at chrome stacks when I noticed that many threads have a trace similar to this:

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

The chrome source has the following code in sel_ldr.c , which appears to declare ovly_debug_event as an almost empty function:

 void _ovly_debug_event (void) { #ifdef __GNUC__ /* * The asm volatile is here as instructed by the GCC docs. * It not enough to declare a function noinline. * GCC will still look inside the function to see if it 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(); }

The question is: why does chrome seem to spend so much time on a function that is only for debugging and is almost empty in chrome?

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2 answers

Pay attention to mass offsets such as 0x16574 in this function. It seems you do not have private symbols for chrome.dll, so the debugger finds the closest (well, closest previous) publicly exported symbol.

In other words, you are not in _ovly_debug_event. You have a function that was laid out after it in an executable file, but it is not publicly exported.

To try and resolve this, if you want to see what is actually happening, you can add http://chromium-browser-symsrv.commondatastorage.googleapis.com to your symbol path. In windbg the command will be

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

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In addition, this function is actually a helper function for GDB, which helps to debug overlays. See https://sourceware.org/gdb/onlinedocs/gdb/Automatic-Overlay-Debugging.html .

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