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

[Jiri Olsa]

On Mon, Nov 12, 2018 at 01:26:37AM +0100, Peter Zijlstra wrote:
> 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...

I thought there's no callchain anyway, because we use zero-ed regs

> 
> 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?

sounds ok, will post it

thanks,
jirka