[PATCH 18/27] selftests/landlock: Fix regex_escape() error handling in matches_log_fs_extra()

["Günther Noack" <gnoack3000@gmail.com>]

Use a more conventional error handling interface in regex_escape():
Return a ssize_t, which is either the number of bytes in the output,
or return -1 and set errno.

Before this change, the (log_match_cursor < 0) check in the call site
was never true because log_match_cursor is a pointer.

Assisted-by: LLM
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 tools/testing/selftests/landlock/common.h  | 20 ++++++++++++--------
 tools/testing/selftests/landlock/fs_test.c | 12 +++++++-----
 2 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/tools/testing/selftests/landlock/common.h b/tools/testing/selftests/landlock/common.h
index e1aa0ec02c96..91842f618a3a 100644
--- a/tools/testing/selftests/landlock/common.h
+++ b/tools/testing/selftests/landlock/common.h
@@ -264,11 +264,11 @@ static void __maybe_unused set_unix_address(struct service_fixture *const srv,
  * Escapes characters that have special meaning in POSIX Basic Regular
  * Expressions: $ * . [ \ ] ^
  *
- * Returns a pointer to the NUL terminator in @dst (cursor-style API for
- * chaining), or (char *)-ENOMEM if the buffer is too small.
+ * Returns the length of the escaped string (excluding the NUL terminator), or
+ * -1 with errno set to ENOMEM if the buffer is too small.
  */
-static __maybe_unused char *regex_escape(const char *const src, char *dst,
-					 size_t dst_size)
+static __maybe_unused ssize_t regex_escape(const char *const src, char *dst,
+					   size_t dst_size)
 {
 	char *d = dst;
 
@@ -282,21 +282,25 @@ static __maybe_unused char *regex_escape(const char *const src, char *dst,
 		case ']':
 		case '^':
 			if (d >= dst + dst_size - 2)
-				return (char *)-ENOMEM;
+				goto err_nomem;
 
 			*d++ = '\\';
 			*d++ = *s;
 			break;
 		default:
 			if (d >= dst + dst_size - 1)
-				return (char *)-ENOMEM;
+				goto err_nomem;
 
 			*d++ = *s;
 		}
 	}
 	if (d >= dst + dst_size - 1)
-		return (char *)-ENOMEM;
+		goto err_nomem;
 
 	*d = '\0';
-	return d;
+	return d - dst;
+
+err_nomem:
+	errno = ENOMEM;
+	return -1;
 }
diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index b1ce0f810e10..9abc2adef79d 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -7454,6 +7454,7 @@ static int matches_log_fs_extra(struct __test_metadata *const _metadata,
 	char log_match[log_match_remaining];
 	char *log_match_cursor = log_match;
 	size_t chunk_len;
+	ssize_t escaped_len;
 
 	chunk_len = snprintf(log_match_cursor, log_match_remaining,
 			     REGEX_LANDLOCK_PREFIX " blockers=%s path=\"",
@@ -7471,13 +7472,14 @@ static int matches_log_fs_extra(struct __test_metadata *const _metadata,
 
 	log_match_remaining -= chunk_len;
 	log_match_cursor += chunk_len;
-	log_match_cursor = regex_escape(absolute_path, log_match_cursor,
-					log_match_remaining);
+	escaped_len = regex_escape(absolute_path, log_match_cursor,
+				   log_match_remaining);
 	free(absolute_path);
-	if (log_match_cursor < 0)
-		return (long long)log_match_cursor;
+	if (escaped_len < 0)
+		return -errno;
 
-	log_match_remaining -= log_match_cursor - log_match;
+	log_match_remaining -= escaped_len;
+	log_match_cursor += escaped_len;
 	chunk_len = snprintf(log_match_cursor, log_match_remaining,
 			     "\" dev=\"[^\"]\\+\" ino=[0-9]\\+%s$",
 			     extra ?: "");
-- 
2.56.0