Re: [PATCH] perf: Adjust perf-inject output data offset for backward compatibility

[Adrian Hunter <adrian.hunter@intel.com>]

On 20/06/22 19:49, Raul Silvera wrote:
> When perf inject creates a new file, it reuses the data offset from the
> input file. If there has been a change on the size of the header, as
> happened in v5.12 -> v5.13, the new offsets will be wrong, resulting in
> a corrupted output file.
> 
> This change adds the function perf_session__data_offset to compute the
> data offset based on the current header size, and uses that instead of
> the offset from the original input file.
> 
> Signed-off-by: Raul Silvera <rsilvera@google.com>
> ---
>  tools/perf/builtin-inject.c |  2 +-
>  tools/perf/util/header.c    | 16 +++++++++++++++-
>  tools/perf/util/header.h    |  2 ++
>  3 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/builtin-inject.c b/tools/perf/builtin-inject.c
> index a75bf11585b5..1dfdcef36607 100644
> --- a/tools/perf/builtin-inject.c
> +++ b/tools/perf/builtin-inject.c
> @@ -916,7 +916,7 @@ static int __cmd_inject(struct perf_inject *inject)
>  		inject->tool.tracing_data = perf_event__repipe_tracing_data;
>  	}
>  
> -	output_data_offset = session->header.data_offset;
> +	output_data_offset = perf_session__data_offset(session->evlist);
>  
>  	if (inject->build_id_all) {
>  		inject->tool.mmap	  = perf_event__repipe_buildid_mmap;
> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> index 53332da100e8..93706ae827f1 100644
> --- a/tools/perf/util/header.c
> +++ b/tools/perf/util/header.c
> @@ -3660,7 +3660,7 @@ static int perf_session__do_write_header(struct perf_session *session,
>  			.size   = evlist->core.nr_entries * sizeof(f_attr),
>  		},
>  		.data = {
> -			.offset = header->data_offset,
> +			.offset = attr_offset + evlist->core.nr_entries * sizeof(f_attr),

This looks too late since, although the header comes before the data,
it also gets written afterwards (at_exit == true).  It would be
better if we could rely on header->data_offset at this point.

>  			.size	= header->data_size,
>  		},
>  		/* event_types is ignored, store zeros */
> @@ -3686,6 +3686,20 @@ int perf_session__write_header(struct perf_session *session,
>  	return perf_session__do_write_header(session, evlist, fd, at_exit, NULL);
>  }
>  
> +size_t perf_session__data_offset(const struct evlist *evlist)
> +{
> +	struct evsel *evsel;
> +	size_t data_offset;
> +
> +	data_offset = sizeof(struct perf_file_header);
> +	evlist__for_each_entry(evlist, evsel) {
> +		data_offset += evsel->core.ids * sizeof(u64);
> +	}
> +	data_offset += evlist->core.nr_entries * sizeof(struct perf_file_attr);
> +
> +	return data_offset;
> +}
> +
>  int perf_session__inject_header(struct perf_session *session,
>  				struct evlist *evlist,
>  				int fd,
> diff --git a/tools/perf/util/header.h b/tools/perf/util/header.h
> index 08563c1f1bff..56916dabce7b 100644
> --- a/tools/perf/util/header.h
> +++ b/tools/perf/util/header.h
> @@ -136,6 +136,8 @@ int perf_session__inject_header(struct perf_session *session,
>  				int fd,
>  				struct feat_copier *fc);
>  
> +size_t perf_session__data_offset(const struct evlist *evlist);
> +
>  void perf_header__set_feat(struct perf_header *header, int feat);
>  void perf_header__clear_feat(struct perf_header *header, int feat);
>  bool perf_header__has_feat(const struct perf_header *header, int feat);