Commit 70c8217a authored Jun 17, 2016 by Steven Rostedt (Red Hat) Committed by Steven Rostedt Jun 20, 2016

tracing: Handle NULL formats in hold_module_trace_bprintk_format()

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 3debb0a9 ("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@intel.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Fixes: 3debb0a9 ("tracing: Fix trace_printk() to print when not using bprintk()")
Cc: stable@vger.kernel.org # v3.5+
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

parent 5edb5649

kernel/trace/trace_printk.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -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;
		@@ -57,6 +61,7 @@ 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) {
		if (!IS_ERR(tb_fmt))
		*iter = tb_fmt->fmt;
		continue;
		}