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

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Date:   Tue, 11 Jul 2023 09:57:45 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Georg Müller <georgmueller@....net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        regressions@...ts.linux.dev,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf probe: read DWARF files from the correct CU

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.

- Arnaldo
 
> Fixes: dc9a5d2ccd5c ("perf probe: Fix to get declared file name from clang DWARF5")
> Signed-off-by: Georg Müller <georgmueller@....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