linux-kernel - [PATCH 3.16 297/366] tracing: Quiet gcc warning about maybe unused link variable

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Date:   Sun, 11 Nov 2018 19:49:05 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        "kbuild test robot" <lkp@...el.com>
Subject: [PATCH 3.16 297/366] tracing: Quiet gcc warning about maybe
 unused link variable

3.16.61-rc1 review patch.  If anyone has any objections, please let me know.

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

From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>

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.

Fixes: 57ea2a34adf4 ("tracing/kprobes: Fix trace_probe flags on enable_trace_kprobe() failure")
Reported-by: kbuild test robot <lkp@...el.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 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
@@ -361,7 +361,7 @@ static struct trace_kprobe *find_trace_k
 static int
 enable_trace_kprobe(struct trace_kprobe *tk, struct ftrace_event_file *file)
 {
-	struct event_file_link *link;
+	struct event_file_link *link = NULL;
 	int ret = 0;
 
 	if (file) {
@@ -387,7 +387,9 @@ 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 {