[PATCH] tracing: fix va_list breakage in trace_check_vprintf()

[Nikita Yushchenko <nikita.yushchenko@virtuozzo.com>]

When trace_check_vprintf() extracts parts of the format string and
passes those to trace_seq_printf() together with va_list, it expects
that trace_seq_printf() consumes arguments from va_list, as defined
by the passed format substring.

However, trace_seq_printf() has a special path for overflow handling,
that does not consume any arguments from va_list. This causes va_list
to get out of sync with format string processing, the next va_arg()
inside trace_check_vprintf() gets wrong argument, and WARN_ON_ONCE()
hits.

This situation easily triggers by ftrace_stress_test from LTP.

Fix that by adding a dummy vsnprintf() call to the overflow path inside
trace_seq_printf() to ensure args from va_list are still consumed.

Signed-off-by: Nikita Yushchenko <nikita.yushchenko@virtuozzo.com>
---
 kernel/trace/trace_seq.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_seq.c b/kernel/trace/trace_seq.c
index 9c90b3a7dce2..3551b5e18aa2 100644
--- a/kernel/trace/trace_seq.c
+++ b/kernel/trace/trace_seq.c
@@ -141,9 +141,15 @@ EXPORT_SYMBOL_GPL(trace_seq_bitmask);
 void trace_seq_vprintf(struct trace_seq *s, const char *fmt, va_list args)
 {
 	unsigned int save_len = s->seq.len;
+	char buf[1];
 
-	if (s->full)
+	if (s->full) {
+		/* Consume args from va_list before returning, some callers
+		 * expect that.
+		 */
+		vsnprintf(buf, sizeof(buf), fmt, args);
 		return;
+	}
 
 	__trace_seq_init(s);
 
-- 
2.30.2