Because we drop capabilities (most importantly, CAP_DAC_OVERRIDE), if a
user runs the selftests under a Linux source checked out by a non-root
user, the test will fail even when ran under sudo, and will print a
"Permission denied" error.  This creates a confusing situation if they
does not realize that the test drops capabilities, and can mislead users
to think there's something wrong with the test or landlock.

This patch produces output that looks like:

   # #  RUN           layout0.ruleset_with_unknown_access ...
   # # fs_test.c:240:ruleset_with_unknown_access:Expected 0 (0) == mkdir(path, 0700) (-1)
   # # fs_test.c:244:ruleset_with_unknown_access:Failed to create directory "tmp": Permission denied
   # # fs_test.c:230:ruleset_with_unknown_access:Hint: fs_tests requires permissions for uid 0 on test directory /home/mao/landlock-selftests/tools/testing/selftests/landlock and files under it (even when running as root).
   # # fs_test.c:232:ruleset_with_unknown_access:      Try chmod a+rwX -R /home/mao/landlock-selftests/tools/testing/selftests/landlock
   # # ruleset_with_unknown_access: Test terminated by assertion
   # #          FAIL  layout0.ruleset_with_unknown_access

Signed-off-by: Tingmao Wang <redacted>
---
  tools/testing/selftests/landlock/fs_test.c | 35 +++++++++++++++++++---
  1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index e65e6cc80e22..21ed8afcc060 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c

@@ -216,14 +216,37 @@ static void mkdir_parents(struct __test_metadata *const _metadata,
  	free(walker);
  }
  
+static void
+maybe_warn_about_permission_on_cwd(struct __test_metadata *const _metadata,
+				   int err)
+{
+	char abspath_buf[255];
+
+	if (err == EACCES) {
+		const char *realp = realpath(".", abspath_buf);
+		if (realp == NULL) {
+			realp = ".";
+		}
+		TH_LOG("Hint: fs_tests requires permissions for uid %u on test directory %s and files under it (even when running as root).",
+		       getuid(), realp);
+		TH_LOG("      Try chmod a+rwX -R %s", realp);