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

On Wed, 28 Oct 2026 01:05:18 +0000, sashiko-bot@kernel.org [off-list ref] wrote:

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] tracing/filters: unhandled zero len in regex_match_end_str()
--

commit 92251abf42afdb93bef9298fec46088f078a4aa8
Author: Masami Hiramatsu (Google) [off-list ref]

tracing/filters: Fix MATCH_END_ONLY regex matching for fixed-size strings

This commit introduces regex_match_end_str() to handle suffix regex matching
on fixed-size string buffers using strnlen(), and assigns it in parse_pred()
for comm and static string fields.

quoted

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 59a0d3ba72740..f7c067a58ba41 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;
+}

[Severity: Medium]
How does regex_match_end_str() behave when len is 0?

For the ftrace/print event, the buf field is defined in
kernel/trace/trace_export.c with a size of 0:

#undef __dynamic_array
#define __dynamic_array(_type, _item) {					\
	.type = #_type "[]", .name = #_item,				\
	.size = 0, .align = __alignof__(_type),				\
	is_signed_type(_type), .filter_type = FILTER_OTHER },

Because its type is "char[]", filter_assign_type() classifies it as a static
string field while field->size remains 0. In parse_pred(),
pred->regex->field_len is set to field->size (0) and pred->regex->match is
set to 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;

When a trigger or eprobe with a suffix filter (such as buf ~ "*foo") is
evaluated via write_marker_to_buffer() -> event_triggers_call() ->
filter_match_preds() -> filter_pred_string(), regex_match_end_str() is called
with len == 0.

Good catch... this may be yet another issue.

I think we need another different set of matching functions for dynamic
string. Sharing it with static string (where len = 0 means zero length)
and dynamic string (where len = 0 means variable length string) is
a bad idea.

As noted in the comment above regex_match_full() (added in commit
10f20e9f9da9 ("tracing: Have zero size length in filter logic be full
string")), a len of zero indicates that str is a NUL-terminated string, which
regex_match_full(), regex_match_front(), and regex_match_middle() handle
explicitly:

static int regex_match_full(char *str, struct regex *r, int len)
{
	/* len of zero means str is dynamic and ends with '\0' */
	if (!len)
		return strcmp(str, r->pattern) == 0;

In regex_match_end_str(), however, when len is 0, the (len < r->len) check
returns 0 for any non-empty pattern, and strnlen(str, 0) also returns 0.

Should regex_match_end_str() check for !len and use strlen(str) in that case,
so that suffix filters on ftrace/print work as expected?

I rather like to have regex_match_end_dynstr() in this case.

Anyway, we should have a unit test for filters.

Thanks,

-- 
Masami Hiramatsu (Google) [off-list ref]