RE: [RFC][PATCH 1/2] Fix event.c misaligned address error

[David Laight <David.Laight@ACULAB.COM>]

From: Ian Rogers
> Sent: 11 September 2019 10:09
> On Wed, Jul 24, 2019 at 3:50 PM Numfor Mbiziwo-Tiapo <nums@google.com> wrote:
> >
> > The ubsan (undefined behavior sanitizer) build of perf throws an
> > error when the synthesize "Synthesize cpu map" function from
> > perf test is run.
> >
> > This can be reproduced by running (from the tip directory):
> > make -C tools/perf USE_CLANG=1 EXTRA_CFLAGS="-fsanitize=undefined"
> >
> > (see cover letter for why perf may not build)
> >
> > then running: tools/perf/perf test 44 -v
> >
> > This bug occurs because the cpu_map_data__synthesize function in
> > event.c calls synthesize_mask, casting the 'data' variable
> > (of type cpu_map_data*) to a cpu_map_mask*. Since struct
> > cpu_map_data is 2 byte aligned and struct cpu_map_mask is 8 byte
> > aligned this causes memory alignment issues.
> >
> > This is fixed by adding 6 bytes of padding to the struct cpu_map_data,
> > however, this will break compatibility between perf data files - a file
> > written by an old perf wouldn't work with a perf with this patch due
> > to event data alignment changing.
> >
> > Comments?
> >
> > Not-Quite-Signed-off-by: Numfor Mbiziwo-Tiapo <nums@google.com>
> > ---
> >  tools/perf/util/event.h | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/tools/perf/util/event.h b/tools/perf/util/event.h
> > index eb95f3384958..82eaf06c2604 100644
> > --- a/tools/perf/util/event.h
> > +++ b/tools/perf/util/event.h
> > @@ -433,6 +433,7 @@ struct cpu_map_mask {
> >
> >  struct cpu_map_data {
> >         u16     type;
> > +       u8 pad[6];
> >         char    data[];
> >  };
> >
> > --
> > 2.22.0.657.g960e92d24f-goog
> >
> An idea here is that, if this breaks backward compatibility, we
> introduce an aligned variant and work to deprecate the unaligned
> variant. I will look into making a patch set.

Adding a pad[6] makes no difference.
You need to add aligened(8) to the structure itself as well.

	David

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