Re: [PATCH v2 00/32] Rewrite the DWARF "partial" reader

[Tom de Vries <tdevries@suse.de>]

On 11/6/21 1:25 PM, Tom de Vries via Gdb-patches wrote:
> On 11/4/21 7:08 PM, Tom Tromey wrote:
>> Here is v2 of my series to rewrit the DWARF partial symbol reader.
>>
>> You can find v1 here:
>>
>>     https://sourceware.org/pipermail/gdb-patches/2021-August/181624.html
>>
>> This update addresses all the review comments.  I believe it fixes all
>> the problems that Tom de Vries found.
>>
>> I regression tested this on x86-64 Fedora 34.
>>
> 
> I regression tested this on openSUSE Leap 15.2 x86_64 and got only known
> FAILs:
> ...
> FAIL: gdb.base/step-over-syscall.exp: clone: displaced=off: single step
> over clone (timeout)
> FAIL: gdb.cp/no-dmgl-verbose.exp: setting breakpoint at 'f(std::string)'
> FAIL: gdb.go/package.exp: setting breakpoint at package2.Foo
> FAIL: gdb.go/package.exp: going to first breakpoint (the program exited)
> ...
> 
> So that looks great :)
> 
> Then I tried an experiment with parsing all .debug files installed on
> the system which happens to be 636 files, totaling at 4.8 GB.
> 
> More concretely, I ran this command in a file loop:
> ...
> $ for f in $(find /usr/lib/debug/ -name "*.debug" | grep -v build-id);
> do echo "Loading $f"; gdb -q -batch $f; done 2>&1 | tee LOG
> ...
> 
> My idea was to compare execution times, but instead I ran into some
> trouble (while using master instead, everything worked fine).
> 
> This triggered a fair amount of times:
> ...
> $ grep -c "has duplicate debug_info_offset 0x0, ignoring .debug_aranges" LOG
> 40
> ...
> as well as:
> ...
> $ grep -c "DW_FORM_GNU_strp_alt used without .debug_str" LOG
> 16
> ...
> 
> I managed to reproduce both with a hello world, like so:
> ...
> $ cat ./hello.c
> #include <stdio.h>
> 
> int
> main (void)
> {
>   printf
> ("hello00000000000000000000000000000000000000000000000000000000000000000000\n");
>   return 0;
> }
> $ gcc ./hello.c -g
> $ cp a.out b.out
> $ dwz -m c.out a.out b.out
> $ gdb -q -batch a.out
> 
> warning: Section .debug_aranges in /home/vries/gdb_versions/devel/a.out
> has duplicate debug_info_offset 0x0, ignoring .debug_aranges.
> DW_FORM_GNU_strp_alt used without .debug_str section [in module
> /home/vries/gdb_versions/devel/c.out]
> ...
> 

OK, I see now what is going wrong.

The compilation unit offsets are:
...
$ readelf --dwarf-depth 0 -wi ./a.out | grep @
  Compilation Unit @ offset 0x0:
  Compilation Unit @ offset 0x1f:
  Compilation Unit @ offset 0x4d:
  Compilation Unit @ offset 0x87:
  Compilation Unit @ offset 0xa9:
  Compilation Unit @ offset 0xf5:
  Compilation Unit @ offset 0x1ce:
$ readelf --dwarf-depth 0 -wi ./c.out | grep @
  Compilation Unit @ offset 0x0:
  Compilation Unit @ offset 0x38:
  Compilation Unit @ offset 0x5e:
  Compilation Unit @ offset 0x2aa:
  Compilation Unit @ offset 0x330:
...
and at the moment of the warning we have:
...
(gdb) p (*per_bfd->all_comp_units[0])->sect_off
$42 = 0
(gdb) p (*per_bfd->all_comp_units[1])->sect_off
$43 = (unknown: 0x1f)
(gdb) p (*per_bfd->all_comp_units[7])->sect_off
$44 = 0
(gdb) p (*per_bfd->all_comp_units[8])->sect_off
$45 = (unknown: 0x38)
...

So, the CUs of a.out and c.out end up in the same vector, and that
triggers the duplicate offset warning.

Thanks,
- Tom