RE: [PATCH 1/2] tracing: Handle NULL formats in hold_module_trace_bprintk_format()

[Xing, Zhengjun]

I agree with you. You can also add me to the "Signed-off-by".

Best Regards,
Zhengjun

-----Original Message-----
From: Steven Rostedt [mailto:rostedt@xxxxxxxxxxx] 
Sent: Monday, June 20, 2016 9:53 PM
To: linux-kernel@xxxxxxxxxxxxxxx
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Xing, Zhengjun <zhengjun.xing@xxxxxxxxx>; Namhyung Kim <namhyung@xxxxxxxxxx>; stable@xxxxxxxxxxxxxxx
Subject: [PATCH 1/2] tracing: Handle NULL formats in hold_module_trace_bprintk_format()

From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

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@xxxxxxxxx

Reported-by: xingzhen <zhengjun.xing@xxxxxxxxx>
Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>
Fixes: 3debb0a9ddb ("tracing: Fix trace_printk() to print when not using bprintk()")
Cc: stable@xxxxxxxxxxxxxxx # v3.5+
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/trace/trace_printk.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c index f96f0383f6c6..ad1d6164e946 100644
--- a/kernel/trace/trace_printk.c
+++ b/kernel/trace/trace_printk.c
@@ -36,6 +36,10 @@ 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;
@@ -57,7 +61,8 @@ void hold_module_trace_bprintk_format(const char **start, const char **end)
 	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;
 		}
 
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2.8.0.rc3