Re: [PATCH] perf tools: Allow build-id with trailing zeros

[Jiri Olsa]

On Fri, Sep 10, 2021 at 09:41:50AM -0700, Namhyung Kim wrote:
> Current perf saves a build-id with size but old versions assumes the
> size of 20.  In case the build-id is less than 20 (like for MD5), it'd
> fill the rest with 0s.
> 
> I saw a problem when old version of perf record saved binary in the
> build-id cache and new version of perf reads the data.  The symbols
> should be read from the build-id cache (as the path no longer has the
> same binary) but it failed due to mismatch in the build-id.
> 
>   symsrc__init: build id mismatch for /home/namhyung/.debug/.build-id/53/e4c2f42a4c61a2d632d92a72afa08f00000000/elf.
> 
> The build-id event in the data has 20 byte build-ids, but it saw a
> different size (16) when it reads the build-id of the elf file in the
> build-id cache.
> 
>   $ readelf -n ~/.debug/.build-id/53/e4c2f42a4c61a2d632d92a72afa08f00000000/elf
> 
>   Displaying notes found in: .note.gnu.build-id
>     Owner                Data size 	Description
>     GNU                  0x00000010	NT_GNU_BUILD_ID (unique build ID bitstring)
>       Build ID: 53e4c2f42a4c61a2d632d92a72afa08f
> 
> Let's fix this by allowing trailing zeros if the size is different.
> 
> Fixes: 39be8d0115b3 ("perf tools: Pass build_id object to dso__build_id_equal()")
> 
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  tools/perf/util/dso.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
> index ee15db2be2f4..0c0dd877d4e9 100644
> --- a/tools/perf/util/dso.c
> +++ b/tools/perf/util/dso.c
> @@ -1349,6 +1349,18 @@ void dso__set_build_id(struct dso *dso, struct build_id *bid)
>  
>  bool dso__build_id_equal(const struct dso *dso, struct build_id *bid)
>  {
> +	static const char zeros[BUILD_ID_SIZE];
> +
> +	if (dso->bid.size > bid->size && dso->bid.size == BUILD_ID_SIZE) {
> +		/*
> +		 * For the backward compatibility, it allows a build-id has
> +		 * trailing zeros.
> +		 */
> +		return !memcmp(dso->bid.data, bid->data, bid->size) &&
> +			!memcmp(&dso->bid.data[bid->size], zeros,
> +				dso->bid.size - bid->size);

we now have memchr_inv in tools, so you could use:
  memchr_inv(&dso->bid.data[bid->size], 0, dso->bid.size - bid->size);

and save 20 bytes in bss ;-) other than that, nice catch

Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>

thanks,
jirka

> +	}
> +
>  	return dso->bid.size == bid->size &&
>  	       memcmp(dso->bid.data, bid->data, dso->bid.size) == 0;
>  }
> -- 
> 2.33.0.309.g3052b89438-goog
>