powerpc/perf: Fix crashes with generic_compat_pmu & BHRB

The bhrb_filter_map ("The Branch History Rolling Buffer") callback is
only defined in raw CPUs' power_pmu structs. The "architected" CPUs
use generic_compat_pmu, which does not have this callback, and crashes
occur if a user tries to enable branch stack for an event.

This add a NULL pointer check for bhrb_filter_map() which behaves as
if the callback returned an error.

This does not add the same check for config_bhrb() as the only caller
checks for cpuhw->bhrb_users which remains zero if bhrb_filter_map==0.

Fixes: be80e758d0c2 ("powerpc/perf: Add generic compat mode pmu driver")
Cc: [e-mail] # v5.2+
Signed-off-by: Alexey Kardashevskiy <[e-mail]>
Reviewed-by: Madhavan Srinivasan <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/perf/core-book3s.c

···
 	ret = 0;
  out:
 	if (has_branch_stack(event)) {
-		power_pmu_bhrb_enable(event);
-		cpuhw->bhrb_filter = ppmu->bhrb_filter_map(
-					event->attr.branch_sample_type);
+		u64 bhrb_filter = -1;
+
+		if (ppmu->bhrb_filter_map)
+			bhrb_filter = ppmu->bhrb_filter_map(
+				event->attr.branch_sample_type);
+
+		if (bhrb_filter != -1) {
+			cpuhw->bhrb_filter = bhrb_filter;
+			power_pmu_bhrb_enable(event);
+		}
 	}
 
 	perf_pmu_enable(event->pmu);
···
 	int n;
 	int err;
 	struct cpu_hw_events *cpuhw;
-	u64 bhrb_filter;
 
 	if (!ppmu)
 		return -ENOENT;
···
 	err = power_check_constraints(cpuhw, events, cflags, n + 1);
 
 	if (has_branch_stack(event)) {
-		bhrb_filter = ppmu->bhrb_filter_map(
+		u64 bhrb_filter = -1;
+
+		if (ppmu->bhrb_filter_map)
+			bhrb_filter = ppmu->bhrb_filter_map(
 					event->attr.branch_sample_type);
 
 		if (bhrb_filter == -1) {