Messages in this thread | | | Date | Thu, 31 Mar 2022 15:20:00 +0100 | Subject | Re: [PATCH] perf session: Remap buf if there is no space for event | From | James Clark <> |
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On 30/03/2022 04:11, Denis Nikitin wrote: > If a perf event doesn't fit into remaining buffer space return NULL to > remap buf and fetch the event again. > Keep the logic to error out on inadequate input from fuzzing. > > This fixes perf failing on ChromeOS (with 32b userspace): > > $ perf report -v -i perf.data > ... > prefetch_event: head=0x1fffff8 event->header_size=0x30, mmap_size=0x2000000: fuzzed or compressed perf.data? > Error: > failed to process sample > > Fixes: 57fc032ad643 ("perf session: Avoid infinite loop when seeing invalid header.size") > Signed-off-by: Denis Nikitin <denik@chromium.org> > --- > tools/perf/util/session.c | 15 ++++++++++++--- > 1 file changed, 12 insertions(+), 3 deletions(-) > > diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c > index 3b8dfe603e50..45a30040ec8d 100644 > --- a/tools/perf/util/session.c > +++ b/tools/perf/util/session.c > @@ -2095,6 +2095,7 @@ prefetch_event(char *buf, u64 head, size_t mmap_size, > bool needs_swap, union perf_event *error) > { > union perf_event *event; > + u16 event_size; > > /* > * Ensure we have enough space remaining to read > @@ -2107,15 +2108,23 @@ prefetch_event(char *buf, u64 head, size_t mmap_size, > if (needs_swap) > perf_event_header__bswap(&event->header); > > - if (head + event->header.size <= mmap_size) > + event_size = event->header.size; > + if (head + event_size <= mmap_size) > return event; > > /* We're not fetching the event so swap back again */ > if (needs_swap) > perf_event_header__bswap(&event->header); > > - pr_debug("%s: head=%#" PRIx64 " event->header_size=%#x, mmap_size=%#zx:" > - " fuzzed or compressed perf.data?\n",__func__, head, event->header.size, mmap_size); > + /* Check if the event fits into the next mmapped buf. */ > + if (event_size <= mmap_size - head % page_size) { > + /* Remap buf and fetch again. */ > + return NULL;
Hi Denis,
I tested this and it does fix the issue with a 32bit build. One concern is that the calculation to see if it will fit in the next map is dependent on the implementation of reader__mmap(). I think it would be possible for that to change slightly and then you could still get an infinite loop.
But I can't really see a better way to do it, and it's unlikely for reader__mmap() to be modified to map data in a way to waste part of the buffer so it's probably fine.
Maybe you could extract a function to calculate where the new offset would be in the buffer and share it between here and reader__mmap(). That would also make it more obvious what the 'head % page_size' bit is for.
Either way:
Reviewed-by: James Clark <james.clark@arm.com>
> + } > + > + /* Invalid input. Event size should never exceed mmap_size. */ > + pr_debug("%s: head=%#" PRIx64 " event->header.size=%#x, mmap_size=%#zx:" > + " fuzzed or compressed perf.data?\n", __func__, head, event_size, mmap_size); > > return error; > }
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