[PATCH v4 3/4] samples/landlock: Extend sample tool to support LANDLOCK_ACCESS_FS_TRUNCATE

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

Update the sandboxer sample to restrict truncate actions. This is
automatically enabled by default if the running kernel supports
LANDLOCK_ACCESS_FS_TRUNCATE, expect for the paths listed in the
LL_FS_RW environment variable.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 samples/landlock/sandboxer.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
index 3e404e51ec64..771b6b10d519 100644
--- a/samples/landlock/sandboxer.c
+++ b/samples/landlock/sandboxer.c
@@ -76,7 +76,8 @@ static int parse_path(char *env_path, const char ***const path_list)
 #define ACCESS_FILE ( \
 	LANDLOCK_ACCESS_FS_EXECUTE | \
 	LANDLOCK_ACCESS_FS_WRITE_FILE | \
-	LANDLOCK_ACCESS_FS_READ_FILE)
+	LANDLOCK_ACCESS_FS_READ_FILE | \
+	LANDLOCK_ACCESS_FS_TRUNCATE)
 
 /* clang-format on */
 
@@ -160,10 +161,8 @@ static int populate_ruleset(const char *const env_var, const int ruleset_fd,
 	LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 	LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
 	LANDLOCK_ACCESS_FS_MAKE_SYM | \
-	LANDLOCK_ACCESS_FS_REFER)
-
-#define ACCESS_ABI_2 ( \
-	LANDLOCK_ACCESS_FS_REFER)
+	LANDLOCK_ACCESS_FS_REFER | \
+	LANDLOCK_ACCESS_FS_TRUNCATE)
 
 /* clang-format on */
 
@@ -226,11 +225,17 @@ 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;
+		__attribute__((fallthrough));
+	case 2:
+		/* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
+		ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
 	}
+	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);
-- 
2.37.2