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

[Namhyung Kim <namhyung@kernel.org>]

On Fri, Feb 2, 2024 at 3:41 PM Ian Rogers <irogers@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@arm.com>
> Closes: https://lore.kernel.org/lkml/2cd09e7c-eb88-6726-6169-647dcd0a8101@arm.com/
> Signed-off-by: Ian Rogers <irogers@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.

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
>