[PATCH] libselinux: Workaround for heap overhead of pcre

[Inseob Kim <inseob@xxxxxxxxxx>]

pcre's behavior is changed so that pcre2_match always allocates heap for
match_data, rather than stack, regardless of size. The heap isn't freed
until explicitly calling pcre2_match_data_free. This new behavior may
result in heap overhead, which may increase the peak memory usage about
a few megabytes. It's because regex_match is first called for regex_data
objects, and then regex_data objects are freed at once.

To workaround it, free and reallocate match_data whenever we call
regex_match. It's fine because libselinux currently doesn't use
match_data, but use only the return value.

Signed-off-by: Inseob Kim <inseob@xxxxxxxxxx>
---
 libselinux/src/regex.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/libselinux/src/regex.c b/libselinux/src/regex.c
index 149a7973..2df282f1 100644
--- a/libselinux/src/regex.c
+++ b/libselinux/src/regex.c
@@ -213,10 +213,20 @@ void regex_data_free(struct regex_data *regex)
 int regex_match(struct regex_data *regex, char const *subject, int partial)
 {
 	int rc;
+	pcre2_match_data *new_match_data;
 	__pthread_mutex_lock(&regex->match_mutex);
+	new_match_data = pcre2_match_data_create_from_pattern(
+	    regex->regex, NULL);
 	rc = pcre2_match(
 	    regex->regex, (PCRE2_SPTR)subject, PCRE2_ZERO_TERMINATED, 0,
 	    partial ? PCRE2_PARTIAL_SOFT : 0, regex->match_data, NULL);
+	// pcre2_match allocates heap and it won't be freed until
+	// pcre2_match_data_free, resulting in heap overhead.
+	// Reallocate match_data to prevent such overhead, whenever possible.
+	if (new_match_data) {
+		pcre2_match_data_free(regex->match_data);
+		regex->match_data = new_match_data;
+	}
 	__pthread_mutex_unlock(&regex->match_mutex);
 	if (rc > 0)
 		return REGEX_MATCH;
-- 
2.39.0.314.g84b9a713c41-goog