linux-kernel - Re: [RFC] perf session: Fix compression processing

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Date:   Mon, 11 Nov 2019 18:53:35 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] perf session: Fix compression processing

On 11.11.2019 18:46, Jiri Olsa wrote:
> On Mon, Nov 11, 2019 at 06:41:47PM +0300, Alexey Budankov wrote:
>> On 11.11.2019 17:56, Jiri Olsa wrote:
>>> On Mon, Nov 11, 2019 at 05:38:49PM +0300, Alexey Budankov wrote:
>>>>
>>>> On 04.11.2019 1:24, Jiri Olsa wrote:
>>>>> hi,
>>>> <SNIP>
>>>>> ---
>>>>> The compressed data processing occasionally fails with:
>>>>>   $ perf report --stdio -vv
>>>>>   decomp (B): 44519 to 163000
>>>>>   decomp (B): 48119 to 174800
>>>>>   decomp (B): 65527 to 131072
>>>>>   fetch_mmaped_event: head=0x1ffe0 event->header_size=0x28, mmap_size=0x20000: fuzzed perf.data?
>>>>>   Error:
>>>>>   failed to process sample
>>>>>   ...
>>>>>
>>>>> It's caused by recent fuzzer fix that does not take into account
>>>>> that compressed data do not need to by fully present in the buffer,
>>>>> so it's ok to just return NULL and not to fail.
>>>>>
>>>>> Fixes: 57fc032ad643 ("perf session: Avoid infinite loop when seeing invalid header.size")
>>>>> Link: http://lkml.kernel.org/n/tip-q1biqscs4stcmc9bs1iokfro@git.kernel.org
>>>>> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
>>>>> ---
>>>>>  tools/perf/util/session.c | 8 +++++---
>>>>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
>>>>> index f07b8ecb91bc..3589ed14a629 100644
>>>>> --- a/tools/perf/util/session.c
>>>>> +++ b/tools/perf/util/session.c
>>>>> @@ -1959,7 +1959,7 @@ static int __perf_session__process_pipe_events(struct perf_session *session)
>>>>>  
>>>>>  static union perf_event *
>>>>>  fetch_mmaped_event(struct perf_session *session,
>>>>> -		   u64 head, size_t mmap_size, char *buf)
>>>>> +		   u64 head, size_t mmap_size, char *buf, bool decomp)
>>>>
>>>> bools in interface make code less transparent.
>>>>
>>>>>  {
>>>>>  	union perf_event *event;
>>>>>  
>>>>> @@ -1979,6 +1979,8 @@ fetch_mmaped_event(struct perf_session *session,
>>>>>  		/* We're not fetching the event so swap back again */
>>>>>  		if (session->header.needs_swap)
>>>>>  			perf_event_header__bswap(&event->header);
>>>>> +		if (decomp)
>>>>> +			return NULL;
>>>>>  		pr_debug("%s: head=%#" PRIx64 " event->header_size=%#x, mmap_size=%#zx: fuzzed perf.data?\n",
>>>>>  			 __func__, head, event->header.size, mmap_size);
>>>>>  		return ERR_PTR(-EINVAL);
>>>>> @@ -1997,7 +1999,7 @@ static int __perf_session__process_decomp_events(struct perf_session *session)
>>>>>  		return 0;
>>>>>  
>>>>>  	while (decomp->head < decomp->size && !session_done()) {
>>>>> -		union perf_event *event = fetch_mmaped_event(session, decomp->head, decomp->size, decomp->data);
>>>>> +		union perf_event *event = fetch_mmaped_event(session, decomp->head, decomp->size, decomp->data, true);
>>>>
>>>> It looks like this call can be skipped, at all, in this case.
>>>
>>> not sure what you mean, we are in decomp code no?
>>
>> Ok, it is inside "not fetching" branch. 
>> NULL return value means to proceed getting further over the trace.
>> Checking record type == COMPRESSED at the higher level could 
>> probably be cleaner fix and also work faster.
> 
> any chance you could post the fix? the patch I did was a
> quick fix to get the feature working for presentation ;-)
> you're probably thinking of the proper approach

Please share the exact reproducing steps 
so I could come up with something.

~Alexey

> 
> thanks,
> jirka
> 
>