perf/x86/intel/pt: Fix crash with stop filters in single-range mode

Add a check for !buf->single before calling pt_buffer_region_size in a
place where a missing check can cause a kernel crash.

Fixes a bug introduced by commit 6706384 ("perf/x86/intel/pt:
Opportunistically use single range output mode"), which added a
support for PT single-range output mode. Since that commit if a PT
stop filter range is hit while tracing, the kernel will crash because
of a null pointer dereference in pt_handle_status due to calling
pt_buffer_region_size without a ToPA configured.

The commit which introduced single-range mode guarded almost all uses of
the ToPA buffer variables with checks of the buf->single variable, but
missed the case where tracing was stopped by the PT hardware, which
happens when execution hits a configured stop filter.

Tested that hitting a stop filter while PT recording successfully
records a trace with this patch but crashes without this patch.

Fixes: 6706384 ("perf/x86/intel/pt: Opportunistically use single range output mode")
Signed-off-by: Tristan Hume <tristan@thume.ca>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@kernel.org
Link: https://lkml.kernel.org/r/20220127220806.73664-1-tristan@thume.ca

Author: trishume

arch/x86/events/intel/pt.c

@@ -897,8 +897,9 @@ static void pt_handle_status(struct pt *pt)
 		 * means we are already losing data; need to let the decoder
 		 * know.
 		 */
-		if (!intel_pt_validate_hw_cap(PT_CAP_topa_multiple_entries) ||
-		    buf->output_off == pt_buffer_region_size(buf)) {
+		if (!buf->single &&
+		    (!intel_pt_validate_hw_cap(PT_CAP_topa_multiple_entries) ||
+		     buf->output_off == pt_buffer_region_size(buf))) {
 			perf_aux_output_flag(&pt->handle,
 			                     PERF_AUX_FLAG_TRUNCATED);
 			advance++;