[PATCH 7.1 313/438] tracing/filters: Fix false positive match in regex_match_full()

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

------------------

From: Masami Hiramatsu (Google) <[email protected]>

commit c22c7b735f9810ad276014f788f9aa5c879ec238 upstream.

regex_match_full() calls strncmp(str, r->pattern, len) where len is the
target field buffer size. When len is smaller than r->len (the filter
pattern length), strncmp() checks only len bytes of r->pattern against
str. If those len bytes match, strncmp() returns 0, resulting in a
false-positive match where a shorter string in a fixed-size field
matches a longer filter pattern.

For example, a 4-byte static string field containing "abcd" matched the
filter pattern "abcdefgh" because strncmp("abcd", "abcdefgh", 4)
returned 0. In this case, @len does NOT include '\0' because it is
fixed-size array.

Fix this by returning 0 (no match) early when len < r->len.

Fixes: 1889d20922d1 ("tracing/filters: Provide basic regex support")
Cc: [email protected]
Link: https://patch.msgid.link/178528488779.124250.5571741156199253769.stgit@devnote2
Assisted-by: Antigravity:gemini-3.5-flash
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 kernel/trace/trace_events_filter.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -1027,6 +1027,9 @@ static int regex_match_full(char *str, s
 	if (!len)
 		return strcmp(str, r->pattern) == 0;
 
+	if (len < r->len)
+		return 0;
+
 	return strncmp(str, r->pattern, len) == 0;
 }