[PATCH v3 2/8] libperf cpumap: Ensure empty cpumap is NULL from alloc

[Ian Rogers]

On Fri, Feb 16, 2024 at 4:25 PM Namhyung Kim <namhyung at kernel.org> wrote:
>
> On Fri, Feb 2, 2024 at 3:41 PM Ian Rogers <irogers at google.com> wrote:
> >
> > Potential corner cases could cause a cpumap to be allocated with size
> > 0, but an empty cpumap should be represented as NULL. Add a path in
> > perf_cpu_map__alloc to ensure this.
> >
> > Suggested-by: James Clark <james.clark at arm.com>
> > Closes: https://lore.kernel.org/lkml/2cd09e7c-eb88-6726-6169-647dcd0a8101@arm.com/
> > Signed-off-by: Ian Rogers <irogers at google.com>
> > ---
> >  tools/lib/perf/cpumap.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/lib/perf/cpumap.c b/tools/lib/perf/cpumap.c
> > index ba49552952c5..cae799ad44e1 100644
> > --- a/tools/lib/perf/cpumap.c
> > +++ b/tools/lib/perf/cpumap.c
> > @@ -18,9 +18,13 @@ void perf_cpu_map__set_nr(struct perf_cpu_map *map, int nr_cpus)
> >
> >  struct perf_cpu_map *perf_cpu_map__alloc(int nr_cpus)
> >  {
> > -       RC_STRUCT(perf_cpu_map) *cpus = malloc(sizeof(*cpus) + sizeof(struct perf_cpu) * nr_cpus);
> > +       RC_STRUCT(perf_cpu_map) *cpus;
> >         struct perf_cpu_map *result;
> >
> > +       if (nr_cpus == 0)
> > +               return NULL;
>
> But allocation failure also returns NULL.  Then callers should check
> what's the expected result.

Right, we don't have a habit of just aborting on memory allocation
errors. In the case that NULL is returned it is assumed that an empty
CPU map is appropriate. Adding checks throughout the code base that an
empty CPU map is only returned when 0 is given is beyond the scope of
this patch set.

Thanks,
Ian

> Thanks,
> Namhyung
>
> > +
> > +       cpus = malloc(sizeof(*cpus) + sizeof(struct perf_cpu) * nr_cpus);
> >         if (ADD_RC_CHK(result, cpus)) {
> >                 cpus->nr = nr_cpus;
> >                 refcount_set(&cpus->refcnt, 1);
> > --
> > 2.43.0.594.gd9cf4e227d-goog
> >