Re: [Libunwind-devel] Backtrace on linux_x86_64 having a weird behavior with binary compiled with gcc -O2 flag

[Dave Watson]

On 08/16/17 09:50 AM, hai huang wrote:
> Actually, this problem can be reproduced with an even simpler program:
> 
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/time.h>
> 
> int main() {
>         struct timeval tv;
>         gettimeofday(&tv, NULL);
> }
> 
> 
> If I compile this with `gcc -O2 -o test_O2 test.c` and run
> `libunwind/tests/test-ptrace -s -v ./test_O2` to trace it, I will get the
> same behavior as above with `main+0x1e` showing up twice in the backtrace.
> However, the weird thing is if I replace gettimeofday() call with a
> sleep(1) call, even compiling with the `-O2` flag, backtrace looks fine.
> Maybe there's a strange interaction between libunwind and gettimeofday
> syscall?

So my first guess would be that it's because you are trying to ptrace
and backtrace syscalls, but gettimeofday is no longer a syscall on
modern linux systems, it's a vdso call: 

https://github.com/lattera/glibc/blob/master/sysdeps/unix/sysv/linux/x86_64/gettimeofday.c#L34

And this is why sleep works fine - it's stil a normal syscall