perf record: Ensure space for lost samples

Previous allocation didn't account for sample ID written after the
lost samples event. Switch from malloc/free to a stack allocation.
Reported-by: Milian Wolff <milian.wolff@kdab.com>
Closes: https://lore.kernel.org/linux-perf-users/23879991.0LEYPuXRzz@milian-workstation/Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240611050626.1223155-1-irogers@google.com

tools/lib/perf/include/perf/event.h

@@ -77,6 +77,12 @@ struct perf_record_lost_samples {
 	__u64			 lost;
 };
 
+#define MAX_ID_HDR_ENTRIES  6
+struct perf_record_lost_samples_and_ids {
+	struct perf_record_lost_samples lost;
+	__u64 sample_ids[MAX_ID_HDR_ENTRIES];
+};
+
 /*
  * PERF_FORMAT_ENABLED | PERF_FORMAT_RUNNING | PERF_FORMAT_ID | PERF_FORMAT_LOST
  */

tools/perf/builtin-record.c

@@ -1926,7 +1926,7 @@ static void __record__save_lost_samples(struct record *rec, struct evsel *evsel,
 static void record__read_lost_samples(struct record *rec)
 {
 	struct perf_session *session = rec->session;
-	struct perf_record_lost_samples *lost = NULL;
+	struct perf_record_lost_samples_and_ids lost;
 	struct evsel *evsel;
 
 	/* there was an error during record__open */
@@ -1951,20 +1951,13 @@ static void record__read_lost_samples(struct record *rec)
 
 				if (perf_evsel__read(&evsel->core, x, y, &count) < 0) {
 					pr_debug("read LOST count failed\n");
-					goto out;
+					return;
 				}
 
 				if (count.lost) {
-					if (!lost) {
-						lost = zalloc(sizeof(*lost) +
-							      session->machines.host.id_hdr_size);
-						if (!lost) {
-							pr_debug("Memory allocation failed\n");
-							return;
-						}
-						lost->header.type = PERF_RECORD_LOST_SAMPLES;
-					}
-					__record__save_lost_samples(rec, evsel, lost,
+					memset(&lost.lost, 0, sizeof(lost));
+					lost.lost.header.type = PERF_RECORD_LOST_SAMPLES;
+					__record__save_lost_samples(rec, evsel, &lost.lost,
 								    x, y, count.lost, 0);
 				}
 			}
@@ -1972,21 +1965,12 @@ static void record__read_lost_samples(struct record *rec)
 
 		lost_count = perf_bpf_filter__lost_count(evsel);
 		if (lost_count) {
-			if (!lost) {
-				lost = zalloc(sizeof(*lost) +
-					      session->machines.host.id_hdr_size);
-				if (!lost) {
-					pr_debug("Memory allocation failed\n");
-					return;
-				}
-				lost->header.type = PERF_RECORD_LOST_SAMPLES;
-			}
-			__record__save_lost_samples(rec, evsel, lost, 0, 0, lost_count,
+			memset(&lost.lost, 0, sizeof(lost));
+			lost.lost.header.type = PERF_RECORD_LOST_SAMPLES;
+			__record__save_lost_samples(rec, evsel, &lost.lost, 0, 0, lost_count,
 						    PERF_RECORD_MISC_LOST_SAMPLES_BPF);
 		}
 	}
-out:
-	free(lost);
 }
 
 static volatile sig_atomic_t workload_exec_errno;
@@ -3198,7 +3182,7 @@ static int switch_output_setup(struct record *rec)
 	unsigned long val;
 
 	/*
-	 * If we're using --switch-output-events, then we imply its 
+	 * If we're using --switch-output-events, then we imply its
 	 * --switch-output=signal, as we'll send a SIGUSR2 from the side band
 	 *  thread to its parent.
 	 */