linux-kernel - [PATCH 3.16 204/305] tracing: Handle NULL formats in hold_module_trace_bprintk

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Date:	Sat, 13 Aug 2016 18:42:51 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Namhyung Kim" <namhyung@...nel.org>,
	"Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
	"xingzhen" <zhengjun.xing@...el.com>
Subject: [PATCH 3.16 204/305] tracing: Handle NULL formats in
 hold_module_trace_bprintk_format()

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

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

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

commit 70c8217acd4383e069fe1898bbad36ea4fcdbdcc upstream.

If a task uses a non constant string for the format parameter in
trace_printk(), then the trace_printk_fmt variable is set to NULL. This
variable is then saved in the __trace_printk_fmt section.

The function hold_module_trace_bprintk_format() checks to see if duplicate
formats are used by modules, and reuses them if so (saves them to the list
if it is new). But this function calls lookup_format() that does a strcmp()
to the value (which is now NULL) and can cause a kernel oops.

This wasn't an issue till 3debb0a9ddb ("tracing: Fix trace_printk() to print
when not using bprintk()") which added "__used" to the trace_printk_fmt
variable, and before that, the kernel simply optimized it out (no NULL value
was saved).

The fix is simply to handle the NULL pointer in lookup_format() and have the
caller ignore the value if it was NULL.

Link: http://lkml.kernel.org/r/1464769870-18344-1-git-send-email-zhengjun.xing@intel.com

Reported-by: xingzhen <zhengjun.xing@...el.com>
Acked-by: Namhyung Kim <namhyung@...nel.org>
Fixes: 3debb0a9ddb ("tracing: Fix trace_printk() to print when not using bprintk()")
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/trace/trace_printk.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/kernel/trace/trace_printk.c
+++ b/kernel/trace/trace_printk.c
@@ -38,6 +38,10 @@ struct trace_bprintk_fmt {
 static inline struct trace_bprintk_fmt *lookup_format(const char *fmt)
 {
 	struct trace_bprintk_fmt *pos;
+
+	if (!fmt)
+		return ERR_PTR(-EINVAL);
+
 	list_for_each_entry(pos, &trace_bprintk_fmt_list, list) {
 		if (!strcmp(pos->fmt, fmt))
 			return pos;
@@ -59,7 +63,8 @@ void hold_module_trace_bprintk_format(co
 	for (iter = start; iter < end; iter++) {
 		struct trace_bprintk_fmt *tb_fmt = lookup_format(*iter);
 		if (tb_fmt) {
-			*iter = tb_fmt->fmt;
+			if (!IS_ERR(tb_fmt))
+				*iter = tb_fmt->fmt;
 			continue;
 		}