ecryptfs: Annotate struct ecryptfs_message with __counted_by

Add the __counted_by() compiler attribute to the flexible array member
'data' to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Acked-by: Tyler Hicks <code@tyhicks.com>
Link: https://patch.msgid.link/20260112115314.739612-2-thorsten.blum@linux.dev
Signed-off-by: Kees Cook <kees@kernel.org>

Author: toblux

fs/ecryptfs/ecryptfs_kernel.h

@@ -359,7 +359,7 @@ struct ecryptfs_message {
 	/* Inherits from msg_ctx->index */
 	u32 index;
 	u32 data_len;
-	u8 data[];
+	u8 data[] __counted_by(data_len);
 };
 
 struct ecryptfs_msg_ctx {