[PATCH v3 02/16] tracing/filters: Fix MATCH_END_ONLY regex matching for fixed-size strings

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

regex_match_end() calculates string length as "len - 1", assuming @len
is the string length including trailing '\0'. For dynamic strings
(filter_pred_strloc) and pointer strings (filter_pchar), this assumption
holds and allows O(1) constant-time suffix matching.

However, for fixed-size string fields (FILTER_STATIC_STRING) and "comm"
(FILTER_COMM), the callers pass the buffer capacity rather than the
actual string length (field->size for filter_pred_string, and
TASK_COMM_LEN for filter_pred_comm). Because these buffers hold
null-terminated strings (e.g. prev_comm in sched_switch holding "bash"
in a 16-byte buffer), regex_match_end() tests against the tail of the
buffer capacity (typically trailing '\0' padding at offset 13) rather
than the actual string end. Consequently, suffix matching like
comm ~ "*sh" or prev_comm ~ "*sh" fails for all strings shorter than
the buffer capacity.

To fix this without penalizing dynamic and pointer strings with an
unnecessary strnlen() scan on every event, introduce a dedicated match
function regex_match_end_str() for fixed-size buffers that uses
strnlen(str, len) to locate the string end within the buffer capacity.
In parse_pred(), assign regex_match_end_str to pred->regex->match for
FILTER_COMM and FILTER_STATIC_STRING fields when MATCH_END_ONLY is
requested.

Fixes: a3291c14ecf0 ("tracing/filters: Fix MATCH_END_ONLY filter matching")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
Changes in v3:
 - Split from zero-length string fix into a dedicated patch.
 - Introduce regex_match_end_str() for fixed-size buffers and assign
   it in parse_pred() for FILTER_COMM and FILTER_STATIC_STRING, avoiding
   strnlen() overhead for dynamic and pointer strings in regex_match_end().
---
 kernel/trace/trace_events_filter.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 59a0d3ba7274..f7c067a58ba4 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -1087,6 +1087,20 @@ static int regex_match_end(char *str, struct regex *r, int len)
 	return 0;
 }
 
+static int regex_match_end_str(char *str, struct regex *r, int len)
+{
+	int slen;
+
+	if (len < r->len)
+		return 0;
+
+	slen = strnlen(str, len);
+	if (slen >= r->len &&
+	    memcmp(str + slen - r->len, r->pattern, r->len) == 0)
+		return 1;
+	return 0;
+}
+
 static int regex_match_glob(char *str, struct regex *r, int len)
 {
 	return glob_match_len(r->pattern, str, len) ? 1 : 0;
@@ -2026,10 +2040,14 @@ static int parse_pred(const char *str, void *data,
 
 		if (field->filter_type == FILTER_COMM) {
 			pred->fn_num = FILTER_PRED_FN_COMM;
+			if (pred->regex->match == regex_match_end)
+				pred->regex->match = regex_match_end_str;
 
 		} else if (field->filter_type == FILTER_STATIC_STRING) {
 			pred->fn_num = FILTER_PRED_FN_STRING;
 			pred->regex->field_len = field->size;
+			if (pred->regex->match == regex_match_end)
+				pred->regex->match = regex_match_end_str;
 
 		} else if (field->filter_type == FILTER_DYN_STRING) {
 			pred->fn_num = FILTER_PRED_FN_STRLOC;