Re: [PATCH 7/9] perf tools: Add size to struct perf_record_header_build_id

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Oct 14, 2020 at 08:59:08AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, Oct 13, 2020 at 09:24:39PM +0200, Jiri Olsa escreveu:
> > We do not store size with build ids in perf data,
> > but there's enough space to do it. Adding misc bit
> > PERF_RECORD_MISC_BUILD_ID_SIZE to mark build id event
> > with size.
> > 
> > With this fix the dso with md5 build id will have correct
> > build id data and will be usable for debuginfod processing
> > if needed (coming in following patches).
> > 
> > Acked-by: Ian Rogers <irogers@google.com>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  tools/lib/perf/include/perf/event.h | 12 +++++++++++-
> >  tools/perf/util/build-id.c          |  8 +++++---
> >  tools/perf/util/header.c            | 10 +++++++---
> >  3 files changed, 23 insertions(+), 7 deletions(-)
> > 
> > diff --git a/tools/lib/perf/include/perf/event.h b/tools/lib/perf/include/perf/event.h
> > index a6dbba6b9073..988c539bedb6 100644
> > --- a/tools/lib/perf/include/perf/event.h
> > +++ b/tools/lib/perf/include/perf/event.h
> > @@ -201,10 +201,20 @@ struct perf_record_header_tracing_data {
> >  	__u32			 size;
> >  };
> >  
> > +#define PERF_RECORD_MISC_BUILD_ID_SIZE (1 << 15)
> > +
> >  struct perf_record_header_build_id {
> >  	struct perf_event_header header;
> >  	pid_t			 pid;
> > -	__u8			 build_id[24];
> > +	union {
> > +		__u8		 build_id[24];
> > +		struct {
> > +			__u8	 data[20];
> > +			__u8	 size;
> > +			__u8	 reserved1__;
> > +			__u16	 reserved2__;
> > +		};
> > +	};
> >  	char			 filename[];
> >  };
> 
> Hey, shouldn't we just append the extra info at the end, i.e. keep it
> like:
> 
>  struct perf_record_header_build_id {
>  	struct perf_event_header header;
>  	pid_t			 pid;
> 	__u8			 build_id[24];
>  	char			 filename[];
> 	__u8			 size;
>  };
> 
> 
> No need for PERF_RECORD_MISC_BUILD_ID_SIZE, older tools will continue
> working with new perf data files.

hum, then how would we tell if the last byte (size) is present or not?

> 
> OTOH BUILD_ID_SIZE is 20 and the space on this header is 24, so the last
> 4 bytes were not being used, so older tools don't look into it, they
> should continue working, have you tested this case? I.e. getting the
> perf binary in, say, fedora and check that it works with this new
> perf_record_header_build_id layout?

yes, that still works (tested), because we copied only 20 bytes
of the build_id[24] and did not care about the rest

jirka