[PATCH] selftests/landlock: fix the regex_escape() error check

[Sasha Levin <sashal@kernel.org>]

regex_escape() returns (char *)-ENOMEM when the buffer is too small, but
matches_log_fs_extra() checks the returned pointer with "< 0", which is
never true, and goes on with a bogus cursor. On 32-bit architectures gcc
also warns about the cast of the pointer in the error return:

  fs_test.c:7474:24: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

Compare against (char *)-ENOMEM instead.

The size left after the escaped path also subtracts the prefix a second
time, which can make a long path fail with -E2BIG. Compute it from the
cursor instead.

Fixes: 316d06b01130 ("selftests/landlock: Add audit tests for filesystem")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
Found with 32-bit builds of the selftests (gcc 16 and clang 21 with
-m32, with and without -D_TIME_BITS=64 -D_FILE_OFFSET_BITS=64);
KernelCI's 32-bit arm builds show the same warnings. The changed files
build without them now, and the x86_64 builds don't change.

 tools/testing/selftests/landlock/fs_test.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index f725882fa4a78..27a78434d1c21 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -7470,10 +7470,10 @@ static int matches_log_fs_extra(struct __test_metadata *const _metadata,
 	log_match_cursor = 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 (log_match_cursor == (char *)-ENOMEM)
+		return -ENOMEM;
 
-	log_match_remaining -= log_match_cursor - log_match;
+	log_match_remaining = sizeof(log_match) - (log_match_cursor - log_match);
 	chunk_len = snprintf(log_match_cursor, log_match_remaining,
 			     "\" dev=\"[^\"]\\+\" ino=[0-9]\\+%s$",
 			     extra ?: "");
-- 
2.53.0