Re: [PATCH] perf/x86/intel: Init early callchain for bts event

[Peter Zijlstra]

On Sun, Nov 11, 2018 at 07:16:50PM +0100, Jiri Olsa wrote:
> Vince reported crash in bts flush code when touching the
> callchain data, which was supposed to be initialized
> as an 'early' callchain data.
> 
>   BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
>   ...

> It was triggered by fuzzer by can be easilt reproduced by:
>   # perf record -e cpu/branch-instructions/p -g -c 1
> 
> The problem is that bts drain code does not initialize sample's
> early callchain data and calls perf_prepare_sample with NULL
> sample->callchain, even if it's expected to exist via
> __PERF_SAMPLE_CALLCHAIN_EARLY sample type bit.

Not sure that is the actual problem, nor that this:

> @@ -612,6 +614,9 @@ int intel_pmu_drain_bts_buffer(void)
>  
>  	perf_sample_data_init(&data, 0, event->hw.last_period);
>  
> +	if (event->attr.sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY)
> +		data.callchain = &__empty_callchain;
> +
>  	/*
>  	 * BTS leaks kernel addresses in branches across the cpl boundary,
>  	 * such as traps or system calls, so unless the user is asking for

is the right fix.

If you look at commit:

  6cbc304f2f36 ("perf/x86/intel: Fix unwind errors from PEBS entries (mk-II)")

Then the right fix would be to do perf_callchain() from the BTS drain
code -- if '/p'.

Because prior to that commit, we would do a perf_callchain() in
intel_pmu_drain_bts_buffer()'s call to perf_prepare_sample(), which
would do an actual stack unwind for a branch entry.

With your patch, we get an empty stack for every entry.

Which is a change in behaviour...

Now arguably, this is really stupid behaviour. Who in his right mind
wants callchain output on BTS entries. And even if they do, BTS +
precise_ip is nonsensical.

So in my mind disallowing precise_ip on BTS would be the simplest fix.

Hmm?