[PATCH v2 9/9] perf pmu: Fix race with concurrent tracepoint creation and removal in perf list

[Ian Rogers <irogers@google.com>]

perf_pmus__print_pmu_events() first counts events via
perf_pmu__num_events() to allocate the aliases array, then populates it
via perf_pmu__for_each_event(). When dynamic tracepoints (such as
kprobes or uprobes during 'perf test' runs) are concurrently created or
removed between the two passes:
- If an event is removed, state.index is smaller than the allocated len,
  leaving trailing zeroed entries in aliases[] with name == NULL and
  pmu == NULL, which causes qsort(cmp_sevent) or the print loop to crash
  with SIGSEGV.
- If a dynamic tracepoint subsystem directory is removed while scanning
  /sys/kernel/tracing/events, tp_pmu__for_each_tp_event() returns
  -ENOENT and aborts enumeration of all remaining tracepoint subsystems.
- If an event is added, perf_pmus__print_pmu_events__callback() aborts
  enumeration when state->index reaches state->aliases_len.

Grow state->aliases dynamically via realloc() when needed, use
state.index as the actual populated length for sorting and printing, and
ignore -ENOENT when a tracepoint subsystem directory disappears during
enumeration.

Fixes: c3245d2093c1 ("perf pmu: Abstract alias/event struct")
Fixes: 45b6e281cb06 ("perf tp_pmu: Add event APIs")
Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/pmus.c   | 21 +++++++++++++++++----
 tools/perf/util/tp_pmu.c |  8 ++++++--
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/tools/perf/util/pmus.c b/tools/perf/util/pmus.c
index e0a4cb2428ca..1355b317f3ed 100644
--- a/tools/perf/util/pmus.c
+++ b/tools/perf/util/pmus.c
@@ -8,6 +8,7 @@
 #include <sys/types.h>
 #include <ctype.h>
 #include <pthread.h>
+#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include "cpumap.h"
@@ -571,7 +572,7 @@ static int cmp_sevent(const void *a, const void *b)
 	}
 
 	/* Order by event name. */
-	return strcmp(as->name, bs->name);
+	return strcmp(as->name ?: "", bs->name ?: "");
 }
 
 static bool pmu_alias_is_duplicate(struct sevent *a, struct sevent *b)
@@ -581,7 +582,7 @@ static bool pmu_alias_is_duplicate(struct sevent *a, struct sevent *b)
 		return false;
 
 	/* Don't remove duplicates for different PMUs */
-	return strcmp(a->pmu_name, b->pmu_name) == 0;
+	return strcmp(a->pmu_name ?: "", b->pmu_name ?: "") == 0;
 }
 
 struct events_callback_state {
@@ -597,8 +598,18 @@ static int perf_pmus__print_pmu_events__callback(void *vstate,
 	struct sevent *s;
 
 	if (state->index >= state->aliases_len) {
-		pr_err("Unexpected event %s/%s/\n", info->pmu->name, info->name);
-		return 1;
+		size_t new_len = max_t(size_t, 16, state->aliases_len * 2);
+		struct sevent *new_aliases;
+
+		new_aliases = realloc(state->aliases, new_len * sizeof(struct sevent));
+		if (!new_aliases) {
+			pr_err("Unexpected event %s/%s/\n", info->pmu->name, info->name);
+			return 1;
+		}
+		memset(&new_aliases[state->aliases_len], 0,
+		       (new_len - state->aliases_len) * sizeof(struct sevent));
+		state->aliases = new_aliases;
+		state->aliases_len = new_len;
 	}
 	assert(info->pmu != NULL || info->name != NULL);
 	s = &state->aliases[state->index];
@@ -654,6 +665,8 @@ void perf_pmus__print_pmu_events(const struct print_callbacks *print_cb, void *p
 		perf_pmu__for_each_event(pmu, skip_duplicate_pmus, &state,
 					 perf_pmus__print_pmu_events__callback);
 	}
+	aliases = state.aliases;
+	len = state.index;
 	qsort(aliases, len, sizeof(struct sevent), cmp_sevent);
 	for (int j = 0; j < len; j++) {
 		/* Skip duplicates */
diff --git a/tools/perf/util/tp_pmu.c b/tools/perf/util/tp_pmu.c
index c2be8c9f9084..5ac732e06841 100644
--- a/tools/perf/util/tp_pmu.c
+++ b/tools/perf/util/tp_pmu.c
@@ -151,7 +151,9 @@ static int for_each_event_cb(void *state, const char *sys_name, const char *evt_
 
 static int for_each_event_sys_cb(void *state, const char *sys_name)
 {
-	return tp_pmu__for_each_tp_event(sys_name, state, for_each_event_cb);
+	int ret = tp_pmu__for_each_tp_event(sys_name, state, for_each_event_cb);
+
+	return ret == -ENOENT ? 0 : ret;
 }
 
 int tp_pmu__for_each_event(struct perf_pmu *pmu, void *state, pmu_event_callback cb)
@@ -176,7 +178,9 @@ static int num_events_cb(void *state, const char *sys_name __maybe_unused,
 
 static int num_events_sys_cb(void *state, const char *sys_name)
 {
-	return tp_pmu__for_each_tp_event(sys_name, state, num_events_cb);
+	int ret = tp_pmu__for_each_tp_event(sys_name, state, num_events_cb);
+
+	return ret == -ENOENT ? 0 : ret;
 }
 
 size_t tp_pmu__num_events(struct perf_pmu *pmu __maybe_unused)
-- 
2.56.0.rc1.315.gc6ed9934b7-goog