Re: [PATCH v5 02/50] libperf: Lazily allocate/size mmap event copy

[Namhyung Kim]

On Mon, Nov 27, 2023 at 2:09 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> The event copy in the mmap is used to have storage to read an
> event. Not all users of mmaps read the events, such as perf
> record. The amount of buffer was also statically set to
> PERF_SAMPLE_MAX_SIZE rather than the amount necessary from the
> header's event size. Switch to a model where the event_copy is
> reallocated if too small to the event's size. This adds the potential
> for the event to move, so if a copy of the event pointer were stored
> it could be broken. All the current users do:
>
>   while(event = perf_mmap__read_event()) { ... }
>
> and so they would be broken due to the event being overwritten if they
> had stored the pointer. Manual inspection and address sanitizer
> testing also shows the event pointer not being stored.
>
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>

Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Thanks,
Namhyung


> ---
>  tools/lib/perf/include/internal/mmap.h |  3 ++-
>  tools/lib/perf/mmap.c                  | 21 ++++++++++++++++++---
>  2 files changed, 20 insertions(+), 4 deletions(-)
>
> diff --git a/tools/lib/perf/include/internal/mmap.h b/tools/lib/perf/include/internal/mmap.h
> index 5a062af8e9d8..5f08cab61ece 100644
> --- a/tools/lib/perf/include/internal/mmap.h
> +++ b/tools/lib/perf/include/internal/mmap.h
> @@ -33,7 +33,8 @@ 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;
> +       size_t                   event_copy_sz;
>         struct perf_mmap        *next;
>  };
>
> diff --git a/tools/lib/perf/mmap.c b/tools/lib/perf/mmap.c
> index 2184814b37dd..c829db7bf1fa 100644
> --- a/tools/lib/perf/mmap.c
> +++ b/tools/lib/perf/mmap.c
> @@ -19,6 +19,7 @@
>  void perf_mmap__init(struct perf_mmap *map, struct perf_mmap *prev,
>                      bool overwrite, libperf_unmap_cb_t unmap_cb)
>  {
> +       /* Assume fields were zero initialized. */
>         map->fd = -1;
>         map->overwrite = overwrite;
>         map->unmap_cb  = unmap_cb;
> @@ -51,13 +52,19 @@ int perf_mmap__mmap(struct perf_mmap *map, struct perf_mmap_param *mp,
>
>  void perf_mmap__munmap(struct perf_mmap *map)
>  {
> -       if (map && map->base != NULL) {
> +       if (!map)
> +               return;
> +
> +       free(map->event_copy);
> +       map->event_copy = NULL;
> +       map->event_copy_sz = 0;
> +       if (map->base) {
>                 munmap(map->base, perf_mmap__mmap_len(map));
>                 map->base = NULL;
>                 map->fd = -1;
>                 refcount_set(&map->refcnt, 0);
>         }
> -       if (map && map->unmap_cb)
> +       if (map->unmap_cb)
>                 map->unmap_cb(map);
>  }
>
> @@ -223,9 +230,17 @@ static union perf_event *perf_mmap__read(struct perf_mmap *map,
>                  */
>                 if ((*startp & map->mask) + size != ((*startp + size) & map->mask)) {
>                         unsigned int offset = *startp;
> -                       unsigned int len = min(sizeof(*event), size), cpy;
> +                       unsigned int len = size, cpy;
>                         void *dst = map->event_copy;
>
> +                       if (size > map->event_copy_sz) {
> +                               dst = realloc(map->event_copy, size);
> +                               if (!dst)
> +                                       return NULL;
> +                               map->event_copy = dst;
> +                               map->event_copy_sz = size;
> +                       }
> +
>                         do {
>                                 cpy = min(map->mask + 1 - (offset & map->mask), len);
>                                 memcpy(dst, &data[offset & map->mask], cpy);
> --
> 2.43.0.rc1.413.gea7ed67945-goog
>