[PATCH v3 2/2] tracing: protect trace_probe_log with mutex

[Paul Cacheux via B4 Relay <devnull+paulcacheux.gmail.com@kernel.org>]

From: Paul Cacheux <paulcacheux@gmail.com>

The shared trace_probe_log variable can be accessed and modified
by multiple processes using tracefs at the same time, this new
mutex will guarantee it's always in a coherent state.

There is no guarantee that multiple errors happening at the same
time will each have the correct error message, but at least this
won't crash.

Fixes: ab105a4fb894 ("tracing: Use tracing error_log with probe events")
Signed-off-by: Paul Cacheux <paulcacheux@gmail.com>
---
 kernel/trace/trace_probe.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
index 2eeecb6c95eea55502b83af6775b7b6f0cc5ab94..d538964c87a52b06646ec1b8c3469be7399f04fa 100644
--- a/kernel/trace/trace_probe.c
+++ b/kernel/trace/trace_probe.c
@@ -153,10 +153,19 @@ static const struct fetch_type *find_fetch_type(const char *type, unsigned long
 	return NULL;
 }
 
+/*
+ * The trace_probe_log_lock only protects against the individual
+ * modification of the trace_probe_log. It does not protect against
+ * the log from producing garbage if two probes use it at the same
+ * time. That would only happen if two admins were trying to add
+ * probes simultaneously which they shouldn't be doing.
+ */
+static DEFINE_MUTEX(trace_probe_log_lock);
 static struct trace_probe_log trace_probe_log;
 
 void trace_probe_log_init(const char *subsystem, int argc, const char **argv)
 {
+	guard(mutex)(&trace_probe_log_lock);
 	trace_probe_log.subsystem = subsystem;
 	trace_probe_log.argc = argc;
 	trace_probe_log.argv = argv;
@@ -165,11 +174,13 @@ void trace_probe_log_init(const char *subsystem, int argc, const char **argv)
 
 void trace_probe_log_clear(void)
 {
+	guard(mutex)(&trace_probe_log_lock);
 	memset(&trace_probe_log, 0, sizeof(trace_probe_log));
 }
 
 void trace_probe_log_set_index(int index)
 {
+	guard(mutex)(&trace_probe_log_lock);
 	trace_probe_log.index = index;
 }
 
@@ -178,6 +189,8 @@ void __trace_probe_log_err(int offset, int err_type)
 	char *command, *p;
 	int i, len = 0, pos = 0;
 
+	guard(mutex)(&trace_probe_log_lock);
+
 	if (!trace_probe_log.argv)
 		return;
 

-- 
2.49.0