Re: [PATCH v2 1/3] Revert "perf session: Fix decompression of PERF_RECORD_COMPRESSED records"

Petr Malat
2020-11-24T18:23:49Z

Hi!
On Tue, Nov 24, 2020 at 03:36:45PM +0100, Jiri Olsa wrote:
> On Tue, Nov 24, 2020 at 11:29:15AM +0100, Petr Malat wrote:
> > Both mmapped and compressed events can be split by the buffer boundary,
> > it doesn't make sense to handle them differently.
> I'm going to need more than this, if there's a problem
> with current code please share more details, what's
> broken and how it shows
It's easy to trigger the problem - make a perf recording larger than
MMAP_SIZE (32MB on 32-bit platform) and run perf report on it. There
is a small chance recorded events will be aligned on the 32 MB
boundary and in that case just repeat the test.

The problem was introduced by "perf session: Avoid infinite loop when
seeing invalid header.size", which instead of aborting the execution
when there is a truncated event at the end of the file just terminated
execution whenever there is a split event. Later then the problem has
been noticed for compressed events and fixed by "perf session: Fix
decompression of PERF_RECORD_COMPRESSED records" by effectively
reverting "perf session: Avoid infinite loop when seeing invalid
header.size" for compressed events, which left uncompressed events
broken.

I think the best is to revert these 2 changes and fix the original
problem by aborting when there is no actual shift during remapping - as
long as we shift, it's clear we must approach the end of the file so
such an algorithm can't loop forever.
BR,
Petr