Re: [PATCH v4 03/53] libperf: Lazily allocate mmap event copy

[Guilherme Amadio]

Hi, 

On Thu, Nov 02, 2023 at 10:56:45AM -0700, Ian Rogers wrote:
> The event copy in the mmap is used to have storage to a read
> event. Not all users of mmaps read the events, such as perf record, so
> switch the allocation to being on first read rather than being
> embedded within the perf_mmap.
> 
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
>  tools/lib/perf/include/internal/mmap.h | 2 +-
>  tools/lib/perf/mmap.c                  | 9 +++++++++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/perf/include/internal/mmap.h b/tools/lib/perf/include/internal/mmap.h
> index 5a062af8e9d8..b11aaf5ed645 100644
> --- a/tools/lib/perf/include/internal/mmap.h
> +++ b/tools/lib/perf/include/internal/mmap.h
> @@ -33,7 +33,7 @@ struct perf_mmap {
>  	bool			 overwrite;
>  	u64			 flush;
>  	libperf_unmap_cb_t	 unmap_cb;
> -	char			 event_copy[PERF_SAMPLE_MAX_SIZE] __aligned(8);
> +	void			*event_copy;
>  	struct perf_mmap	*next;
>  };
>  
> diff --git a/tools/lib/perf/mmap.c b/tools/lib/perf/mmap.c
> index 2184814b37dd..91ae46aac378 100644
> --- a/tools/lib/perf/mmap.c
> +++ b/tools/lib/perf/mmap.c
> @@ -51,6 +51,8 @@ int perf_mmap__mmap(struct perf_mmap *map, struct perf_mmap_param *mp,
>  
>  void perf_mmap__munmap(struct perf_mmap *map)
>  {
> +	free(map->event_copy);
> +	map->event_copy = NULL;
>  	if (map && map->base != NULL) {

If map can be NULL as the if statement above suggests, then there is a
potential a null pointer dereference bug here. Suggestion:

    if (!map)
        return;

    free(map->event_copy);
    map->event_copy = NULL;
    if (map->base != NULL) {

    ...

Cheers,
-Guilherme

>  		munmap(map->base, perf_mmap__mmap_len(map));
>  		map->base = NULL;
> @@ -226,6 +228,13 @@ static union perf_event *perf_mmap__read(struct perf_mmap *map,
>  			unsigned int len = min(sizeof(*event), size), cpy;
>  			void *dst = map->event_copy;
>  
> +			if (!dst) {
> +				dst = malloc(PERF_SAMPLE_MAX_SIZE);
> +				if (!dst)
> +					return NULL;
> +				map->event_copy = dst;
> +			}
> +
>  			do {
>  				cpy = min(map->mask + 1 - (offset & map->mask), len);
>  				memcpy(dst, &data[offset & map->mask], cpy);
> -- 
> 2.42.0.869.gea05f2083d-goog
> 
>