samples/landlock: Print hints about ABI versions

Extend the help with the latest Landlock ABI version supported by the
sandboxer.

Inform users about the sandboxer or the kernel not being up-to-date.

Make the version check code easier to update and harder to misuse.

Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20220923154207.3311629-2-mic@digikod.net

Author: l0kod

samples/landlock/sandboxer.c

@@ -162,11 +162,10 @@ static int populate_ruleset(const char *const env_var, const int ruleset_fd,
 	LANDLOCK_ACCESS_FS_MAKE_SYM | \
 	LANDLOCK_ACCESS_FS_REFER)
 
-#define ACCESS_ABI_2 ( \
-	LANDLOCK_ACCESS_FS_REFER)
-
 /* clang-format on */
 
+#define LANDLOCK_ABI_LAST 2
+
 int main(const int argc, char *const argv[], char *const *const envp)
 {
 	const char *cmd_path;
@@ -196,8 +195,12 @@ int main(const int argc, char *const argv[], char *const *const envp)
 			"\nexample:\n"
 			"%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
 			"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
-			"%s bash -i\n",
+			"%s bash -i\n\n",
 			ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
+		fprintf(stderr,
+			"This sandboxer can use Landlock features "
+			"up to ABI version %d.\n",
+			LANDLOCK_ABI_LAST);
 		return 1;
 	}
 
@@ -225,12 +228,30 @@ int main(const int argc, char *const argv[], char *const *const envp)
 		}
 		return 1;
 	}
+
 	/* Best-effort security. */
-	if (abi < 2) {
-		ruleset_attr.handled_access_fs &= ~ACCESS_ABI_2;
-		access_fs_ro &= ~ACCESS_ABI_2;
-		access_fs_rw &= ~ACCESS_ABI_2;
+	switch (abi) {
+	case 1:
+		/* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */
+		ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
+
+		fprintf(stderr,
+			"Hint: You should update the running kernel "
+			"to leverage Landlock features "
+			"provided by ABI version %d (instead of %d).\n",
+			LANDLOCK_ABI_LAST, abi);
+		__attribute__((fallthrough));
+	case LANDLOCK_ABI_LAST:
+		break;
+	default:
+		fprintf(stderr,
+			"Hint: You should update this sandboxer "
+			"to leverage Landlock features "
+			"provided by ABI version %d (instead of %d).\n",
+			abi, LANDLOCK_ABI_LAST);
 	}
+	access_fs_ro &= ruleset_attr.handled_access_fs;
+	access_fs_rw &= ruleset_attr.handled_access_fs;
 
 	ruleset_fd =
 		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);