Re: bfd_find_inliner_info reports different (wrong) information compared to GDB - off-by-one?

[Mark Wielaard <mjw at redhat dot com>]

Hi Milian,

On Wed, 2016-10-05 at 17:45 +0200, Milian Wolff wrote:
> I'm trying to improve Linux perf report by adding inliner information to the 
> callstacks it reports, to make the output more easily interpretable when 
> looking at complicated C++ applications.

You might want to take a look at eu-stack (src/stack.c) in elfutils
which does this too. If you are using the elfutils libraries (libdw.so).

> I have a proof of concept locally using bfd_find_inliner_info

I don't know what bfd_find_inliner_info is.
It sound like something from binutils?

> , but have 
> noticed the following seemingly broken behavior:
> 
> Input file:
> ~~~~~~~~~test.cpp~~~~~~~
> #include <cstdlib>
> #include <iostream>
> #include <complex>
> 
> int main(int argc, char** argv)
> {
>     double a = 0;
>     double b = 0;
>     if (argc > 1)
>         a = atof(argv[1]);
>     if (argc > 2)
>         b = atof(argv[2]);
>     auto c = std::complex<double>(a, b);
>     std::cout << std::abs(c) << std::endl;
> }
> ~~~~~~~~~~~~~~~~~~~~~~~~
> 
> I build it with:
> 
> ~~~~~~~~~building~~~~~~~
> $ g++ -g -O2 -o test test.cpp
> ~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Then I get a "good" backtrace using GDB:
> 
> ~~~~~~~~~gdb~~~~~~~~~~~~
> $ gdb ./test
> (gdb) break hypot
> Function "hypot" not defined.
> Make breakpoint pending on future shared library load? (y or [n]) y
> Breakpoint 1 (hypot) pending.
> (gdb) run
> Starting program: /tmp/test 
> 
> Breakpoint 1, 0x00007ffff776e910 in hypot () from /usr/lib/libm.so.6
> (gdb) bt
> #0  0x00007ffff776e910 in hypot () from /usr/lib/libm.so.6
> #1  0x00000000004007f4 in std::__complex_abs (__z=<optimized out>) at /usr/
> include/c++/6.2.1/complex:589
> #2  std::abs<double> (__z=<synthetic pointer>) at /usr/include/c++/6.2.1/
> complex:597
> #3  main (argc=<optimized out>, argv=<optimized out>) at test.cpp:14
> ~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Note what inliners it reports for the address 0x00000000004007f4. Let's try 
> bfd_find_inliner_info via eu-addr2line -i:
> 
> ~~~~~~~~~eu-addr2line~~~
> eu-addr2line -i -a 0x00000000004007f4 -e ./test
> 0x00000000004007f4
> /usr/include/c++/6.2.1/ostream:221
> /tmp/test.cpp:14
> ~~~~~~~~~~~~~~~~~~~~~~~~
> 
> This is wrong and seems to be off-by-one:
> 
> ~~~~~~~~~eu-addr2line~~~
> eu-addr2line -i -a 0x00000000004007f3 -e ./test
> 0x00000000004007f3
> /usr/include/c++/6.2.1/complex:589
> /usr/include/c++/6.2.1/complex:597
> /tmp/test.cpp:14
> ~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Can someone with more knowledge in this area sched some light on what is going 
> on here please? Is it a bug, or is this fuzzy behavior and GDB happens to be 
> better at guessing the "right" thing?

I don't know what gdb exactly displays as address in that backtrace.
It might display the return address, in which case subtracting 1 is the
right thing to do on x86_64 (that would be an approximation of the call
address, which is what you want).

And in fact that is what eu-stack does:

(gdb) gcore test.core
Saved corefile test.core

$ eu-stack -v --core test.core --exec test
PID 32623 - core
TID 32623:
#0  0x00007ffff77f8cf0     __hypot - libm.so.6
    /usr/src/debug/glibc-2.17-c758a686/math/w_hypot.c:23
#1  0x0000000000400824 - 1 __complex_abs - test
    /usr/include/c++/4.8.2/complex:587
#2  0x0000000000400824 - 1 abs<double> - test
    /usr/include/c++/4.8.2/complex:595
#3  0x0000000000400824 - 1 main - test
    /tmp/test.cpp:14
#4  0x00007ffff721db35 - 1 __libc_start_main - libc.so.6
    ../csu/libc-start.c:274
#5  0x000000000040088f - 1 _start - test

Cheers,

Mark