Re: [PATCH] perf session: Remap buf if there is no space for event

[Denis Nikitin]

Hi James,

Thanks for your review.

On Thu, Mar 31, 2022 at 7:20 AM James Clark <james.clark@xxxxxxx> wrote:
>
>
>
> 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@xxxxxxxxxxxx>
>
> 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.

Good point. I will send a separate patch to handle this.

Thanks,
Denis

>
> Either way:
>
> Reviewed-by: James Clark <james.clark@xxxxxxx>
>