[PATCH] samples/landlock: Document best-effort approach for LANDLOCK_ACCESS_FS_REFER

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

Add a comment to clarify how to handle best-effort backwards
compatibility for LANDLOCK_ACCESS_FS_REFER.

The "refer" access is special because these operations are always
forbidden in ABI 1, unlike most other operations, which are permitted
when using Landlock ABI levels where they are not supported yet.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 samples/landlock/sandboxer.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
index fd4237c64fb2..901acb383124 100644
--- a/samples/landlock/sandboxer.c
+++ b/samples/landlock/sandboxer.c
@@ -234,7 +234,21 @@ int main(const int argc, char *const argv[], char *const *const envp)
 	/* Best-effort security. */
 	switch (abi) {
 	case 1:
-		/* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */
+		/*
+		 * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
+		 *
+		 * Note: The "refer" operations (file renaming and linking
+		 * across different directories) are always forbidden when using
+		 * Landlock with ABI 1.
+		 *
+		 * If only ABI 1 is available, the sample tool knowingly forbids
+		 * refer operations.
+		 *
+		 * If a program *needs* to do refer operations after enabling
+		 * Landlock, it can not use Landlock at ABI level 1.  To be
+		 * compatible across different kernels, such programs should
+		 * fall back to not using Landlock instead.
+		 */
 		ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
 		__attribute__((fallthrough));
 	case 2:

base-commit: 4bc90a766ea5af69c12ca1ea00b7fc5fe1d68831
-- 
2.38.1