[Bug gprof/21189] gprof doesn't work with code built with PIE

[nickc at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=21189

Nick Clifton <nickc at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |nickc at redhat dot com

--- Comment #1 from Nick Clifton <nickc at redhat dot com> ---
Hi Matthias,

> Since --enabled-default-pie was enabled in Debian for GCC 6, gprof no longer
> works.

I think that this might be a glibc bug.  Specifically in their implementation
of mcount() for PIE environments.

Running "gprof -d infloop | grep main" shows that main starts at address 0x860:

  [get_src_info] 0x860 -> infloop.c:1 (main)
  [core_create_function_syms] 15 main 0x860

And that it covers 0x2f bytes:

  [symtab_finalize] 0x860-0x88f main

But, the data in the gmon.out file us using much higher addresses:

  [hist_read_rec] n_lowpc 0x4004e0 n_highpc 0x400718 ncnt 144

  [assign_samples] bin_low_pc=0x40063a, bin_high_pc=0x40063e, bin_count=18
  [assign_samples] bin_low_pc=0x40063e, bin_high_pc=0x400642, bin_count=38
  [assign_samples] bin_low_pc=0x400646, bin_high_pc=0x40064a, bin_count=82
  [assign_samples] total_time 138.000000

Of course I may be wrong - I am not an expert on gprof - but it does look to me
like the contents of the gmon.out file are wrong.

Alternatively maybe gprof needs a way compute a starting address bias for PIE
executables.

Cheers
  Nick

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