[PATCH 01/10] tracing/filters: Fix MATCH_END_ONLY regex matching for fixed-size strings

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

regex_match_end() calculated string length as "len - 1", assuming @len is
the string length including trailing '\0'. However, for fixed-size string
fields, filter_pred_string() passes field->size as @len, and for "comm",
filter_pred_comm() passes TASK_COMM_LEN as @len.

Because @len in these cases represents the buffer capacity rather than the
actual length of the string, regex_match_end() tests against the tail of
the buffer (typically trailing '\0' padding) rather than the end of the
string. Furthermore, if @len is 0 (denoting a dynamic string ending with
'\0'), "len - 1" underflows to -1 and never matches.

Compute the actual string length using strnlen(str, len) when @len is
non-zero, and strlen(str) when @len is zero.

Fixes: a3291c14ecf0 ("tracing/filters: Fix MATCH_END_ONLY filter matching")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 kernel/trace/trace_events_filter.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 5e74d2dae40a..18ce3ec00d34 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -1075,10 +1075,10 @@ static int regex_match_middle(char *str, struct regex *r, int len)
 
 static int regex_match_end(char *str, struct regex *r, int len)
 {
-	int strlen = len - 1;
+	int slen = len ? strnlen(str, len) : strlen(str);
 
-	if (strlen >= r->len &&
-	    memcmp(str + strlen - r->len, r->pattern, r->len) == 0)
+	if (slen >= r->len &&
+	    memcmp(str + slen - r->len, r->pattern, r->len) == 0)
 		return 1;
 	return 0;
 }