rtla: Abort on nr_cpus mismatch with tracer

In rare situations, sysfs might report a different number of CPUs than
the kernel sees, for example, when /sys is not real sysfs.

In tracefs mode, RTLA will discard any samples that are reported on CPUs
higher than the maximum in sysfs. However, in BPF mode, the incorrectly
sized BPF_MAP_LOOKUP_ELEM syscall on percpu maps will cause a buffer
overflow.

To cover for this corner case, write "all" to osnoise/cpus, and read
back the real kernel-side list of all possible cpus. If it doesn't match
RTLA's nr_cpus (read from sysfs), abort with an error.

Assisted-by: Claude:claude-opus-4-6
Link: https://lore.kernel.org/r/20260814135511.2207190-4-tglozar@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>

tools/tracing/rtla/src/common.c

@@ -62,11 +62,22 @@ static void unset_signals(struct common_params *params)
 int
 common_apply_config(struct osnoise_tool *tool, struct common_params *params)
 {
-	int retval, i;
+	int retval, i, tracer_cpus;
 
 	if (!params->sleep_time)
 		params->sleep_time = 1;
 
+	tracer_cpus = osnoise_get_cpu_count();
+	if (tracer_cpus < 0) {
+		err_msg("Failed to get tracer cpu count from osnoise/cpus\n");
+		goto out_err;
+	}
+	if (tracer_cpus != nr_cpus) {
+		err_msg("Kernel reports %d cpus but rtla sees %d\n",
+			tracer_cpus, nr_cpus);
+		goto out_err;
+	}
+
 	retval = osnoise_set_cpus(tool->context, params->cpus ? params->cpus : "all");
 	if (retval) {
 		err_msg("Failed to apply CPUs config\n");

tools/tracing/rtla/src/common.h

@@ -168,6 +168,7 @@ should_continue_tracing(const struct common_params *params)
 int
 common_threshold_handler(const struct osnoise_tool *tool);
 
+int osnoise_get_cpu_count(void);
 int osnoise_set_cpus(struct osnoise_context *context, char *cpus);
 void osnoise_restore_cpus(struct osnoise_context *context);
 

tools/tracing/rtla/src/osnoise.c

@@ -124,6 +124,50 @@ void osnoise_put_cpus(struct osnoise_context *context)
 	context->orig_cpus = NULL;
 }
 
+/*
+ * osnoise_get_cpu_count - get the number of CPUs as seen by the osnoise tracer
+ *
+ * Write "all" to osnoise/cpus and read back the real kernel-side list
+ * of all possible cpus. Restore the original value afterwards.
+ *
+ * Returns the number of CPUs, or -1 on error.
+ */
+int osnoise_get_cpu_count(void)
+{
+	char *orig, *readback;
+	int max_cpu;
+
+	orig = tracefs_instance_file_read(NULL, "osnoise/cpus", NULL);
+	if (!orig)
+		return -1;
+
+	if (tracefs_instance_file_write(NULL, "osnoise/cpus", "all\n") < 0) {
+		free(orig);
+		return -1;
+	}
+
+	readback = tracefs_instance_file_read(NULL, "osnoise/cpus", NULL);
+	if (!readback) {
+		tracefs_instance_file_write(NULL, "osnoise/cpus", orig);
+		free(orig);
+		return -1;
+	}
+
+	max_cpu = get_max_cpu_from_list(readback);
+	free(readback);
+
+	if (tracefs_instance_file_write(NULL, "osnoise/cpus", orig) < 0) {
+		free(orig);
+		return -1;
+	}
+	free(orig);
+
+	if (max_cpu < 0)
+		return -1;
+
+	return max_cpu + 1;
+}
+
 /*
  * osnoise_read_ll_config - read a long long value from a config
  *

tools/tracing/rtla/src/utils.c

@@ -161,6 +161,22 @@ static int max_cpu_callback(int i, void *data)
 	return 0;
 }
 
+/*
+ * get_max_cpu_from_list - get the maximum CPU in a CPU list
+ *
+ * Returns maximum CPU number, or -1 on error
+ */
+int get_max_cpu_from_list(const char *cpu_list)
+{
+	int max_cpu = -1;
+
+	if (cpu_list_iterate(cpu_list, max_cpu_callback, &max_cpu) < 0)
+		return -1;
+
+	return max_cpu;
+}
+
+
 static int tmp_cpu_set_callback(int i, void *data)
 {
 	bool *cpu_set = data;

tools/tracing/rtla/src/utils.h

@@ -50,6 +50,7 @@ long parse_seconds_duration(char *val);
 void get_duration(time_t start_time, char *output, int output_size);
 
 int cpu_list_iterate(const char *cpu_list, int (*callback)(int, void *), void *data);
+int get_max_cpu_from_list(const char *cpu_list);
 int get_possible_cpus(void);
 
 long long get_llong_from_str(char *start);