Re: [PATCH] perf probe: read DWARF files from the correct CU

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 11 Jul 2023 09:57:45 -0300
Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

> Em Thu, Jun 15, 2023 at 10:01:37PM +0200, Georg Müller escreveu:
> > After switching from dwarf_decl_file() to die_get_decl_file(), it is not
> > possible to add probes for certain functions of certain binaries:
> > 
> >  $ perf probe -x /usr/lib/systemd/systemd-logind match_unit_removed
> >  A function DIE doesn't have decl_line. Maybe broken DWARF?
> >  A function DIE doesn't have decl_line. Maybe broken DWARF?
> >  Probe point 'match_unit_removed' not found.
> >     Error: Failed to add events.
> > 
> > The problem is that die_get_decl_file() uses the wrong CU to search for
> > the file. elfutils commit e1db5cdc9f has some good explanation for this:
> > 
> >     dwarf_decl_file uses dwarf_attr_integrate to get the DW_AT_decl_file
> >     attribute. This means the attribute might come from a different DIE
> >     in a different CU. If so, we need to use the CU associated with the
> >     attribute, not the original DIE, to resolve the file name.
> > 
> > This patch uses the same source of information as elfutils: use attribute
> > DW_AT_decl_file and use this CU to search for the file.
> 
> Thanks, applied to the perf-tools branch, that will be submitted for
> Linux v6.5.

Thanks Arnaldo and Georg,

It sounds perfect reason why that happened. I didn't expect the case that
the attribute comes from another CU...

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

> 
> - Arnaldo
>  
> > Fixes: dc9a5d2ccd5c ("perf probe: Fix to get declared file name from clang DWARF5")
> > Signed-off-by: Georg Müller <georgmueller@gmx.net>
> > Link: https://lore.kernel.org/r/5a00d5a5-7be7-ef8a-4044-9a16249fff25@gmx.net/
> > ---
> >  tools/perf/util/dwarf-aux.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> > index b07414409771..137b3ed9897b 100644
> > --- a/tools/perf/util/dwarf-aux.c
> > +++ b/tools/perf/util/dwarf-aux.c
> > @@ -478,8 +478,10 @@ static const char *die_get_file_name(Dwarf_Die *dw_die, int idx)
> >  {
> >  	Dwarf_Die cu_die;
> >  	Dwarf_Files *files;
> > +	Dwarf_Attribute attr_mem;
> > 
> > -	if (idx < 0 || !dwarf_diecu(dw_die, &cu_die, NULL, NULL) ||
> > +	if (idx < 0 || !dwarf_attr_integrate(dw_die, DW_AT_decl_file, &attr_mem) ||
> > +	    !dwarf_cu_die(attr_mem.cu, &cu_die, NULL, NULL, NULL, NULL, NULL, NULL) ||
> >  	    dwarf_getsrcfiles(&cu_die, &files, NULL) != 0)
> >  		return NULL;
> > 
> > --
> > 2.41.0
> > 
> 
> -- 
> 
> - Arnaldo


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>