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    SubjectRe: [PATCH v4 03/53] libperf: Lazily allocate mmap event copy
    On Fri, Nov 3, 2023 at 1:33 AM Guilherme Amadio <amadio@gentoo.org> wrote:
    >
    > 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@google.com>
    > > ---
    > > 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) {
    >
    > ...

    Makes sense, will fix in v5. Waiting to get additional feedback to
    avoid too much email.

    Thanks,
    Ian

    > 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
    > >
    > >

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