[PATCH 3.14 32/53] tracing: Handle NULL formats in hold_module_trace_bprintk_format()

From: Greg Kroah-Hartman
Date: Mon Jul 25 2016 - 17:12:17 EST

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


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

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@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()")
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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;