Messages in this thread | | | From | Ian Rogers <> | Date | Mon, 27 Nov 2023 11:28:24 -0800 | Subject | Re: [PATCH v4 03/53] libperf: Lazily allocate mmap event copy |
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On Sun, Nov 5, 2023 at 10:12 AM Namhyung Kim <namhyung@kernel.org> wrote: > > On Fri, Nov 3, 2023 at 8:49 AM Ian Rogers <irogers@google.com> wrote: > > > > 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. > > Acked-by: Namhyung Kim <namhyung@kernel.org> > > > But I have another concern (not related to this change). > > > > > > > > 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; > > I'm not sure if it's ok to read less than the actual size, IOW > it seems to assume 'size' is smaller than sizeof(*event). > I guess it's true for most cases as union perf_event has > perf_record_mmap2 (among others) which contains a > filename array of size PATH_MAX. > > But the SAMPLE record can be larger than that when it has > PERF_SAMPLE_AUX IIRC. It'd happen only if it crossed the mmap > boundary and I'm afraid it'd corrupt the data.
Thanks, I was thinking this would just be a drop in change but I think given this feedback it would be better to switch from allocating once a PERF_SAMPLE_MAX_SIZE buffer to allocating or reallocating one based on size. This potentially saves memory when size is less than PERF_SAMPLE_MAX_SIZE and by removing the min calculation for the amount copied (len) we can potentially exceed it and fix a potential bug. I'll add this in v5.
Thanks, Ian
> Thanks, > Namhyung > > > > > > 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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