[PATCH 4.9 009/144] tracing: Quiet gcc warning about maybe unused link variable

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

commit 2519c1bbe38d7acacc9aacba303ca6f97482ed53 upstream.

Commit 57ea2a34adf4 ("tracing/kprobes: Fix trace_probe flags on
enable_trace_kprobe() failure") added an if statement that depends on another
if statement that gcc doesn't see will initialize the "link" variable and
gives the warning:

 "warning: 'link' may be used uninitialized in this function"

It is really a false positive, but to quiet the warning, and also to make
sure that it never actually is used uninitialized, initialize the "link"
variable to NULL and add an if (!WARN_ON_ONCE(!link)) where the compiler
thinks it could be used uninitialized.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 57ea2a34adf4 ("tracing/kprobes: Fix trace_probe flags on enable_trace_kprobe() failure")
Reported-by: kbuild test robot <lkp@xxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/trace/trace_kprobe.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -359,7 +359,7 @@ static struct trace_kprobe *find_trace_k
 static int
 enable_trace_kprobe(struct trace_kprobe *tk, struct trace_event_file *file)
 {
-	struct event_file_link *link;
+	struct event_file_link *link = NULL;
 	int ret = 0;
 
 	if (file) {
@@ -385,7 +385,9 @@ enable_trace_kprobe(struct trace_kprobe
 
 	if (ret) {
 		if (file) {
-			list_del_rcu(&link->list);
+			/* Notice the if is true on not WARN() */
+			if (!WARN_ON_ONCE(!link))
+				list_del_rcu(&link->list);
 			kfree(link);
 			tk->tp.flags &= ~TP_FLAG_TRACE;
 		} else {