[PATCH] trace: don't use out-of-sync va_list in event printing

[Nikita Yushchenko]

If trace_seq becomes full, trace_seq_vprintf() no longer consumes
arguments from va_list, making va_list out of sync with format
processing by trace_check_vprintf().

This causes va_arg() in trace_check_vprintf() to return wrong
positional argument, which results into a WARN_ON_ONCE() hit.

ftrace_stress_test from LTP triggers this situation.

Fix it by explicitly avoiding further use if va_list at the point
when it's consistency can no longer be guaranteed.

Signed-off-by: Nikita Yushchenko <nikita.yushchenko@xxxxxxxxxxxxx>
---
 kernel/trace/trace.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a1adb29ef5c1..3f527c2e08f2 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3826,6 +3826,18 @@ void trace_check_vprintf(struct trace_iterator *iter, const char *fmt,
 		iter->fmt[i] = '\0';
 		trace_seq_vprintf(&iter->seq, iter->fmt, ap);
 
+		/*
+		 * If iter->seq is full, the above call no longer guarantees
+		 * that ap is in sync with fmt processing, and further calls
+		 * to va_arg() can return wrong positional arguments.
+		 *
+		 * Ensure that ap is no longer used in this case.
+		 */
+		if (iter->seq.full) {
+			p = "";
+			break;
+		}
+
 		if (star)
 			len = va_arg(ap, int);
 
-- 
2.30.2