vfs: add RENAME_WHITEOUT

This adds a new RENAME_WHITEOUT flag.  This flag makes rename() create a
whiteout of source.  The whiteout creation is atomic relative to the
rename.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/namei.c

@@ -4209,12 +4209,16 @@ SYSCALL_DEFINE5(renameat2, int, olddfd, const char __user *, oldname,
 	bool should_retry = false;
 	int error;
 
-	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE))
+	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE | RENAME_WHITEOUT))
 		return -EINVAL;
 
-	if ((flags & RENAME_NOREPLACE) && (flags & RENAME_EXCHANGE))
+	if ((flags & (RENAME_NOREPLACE | RENAME_WHITEOUT)) &&
+	    (flags & RENAME_EXCHANGE))
 		return -EINVAL;
 
+	if ((flags & RENAME_WHITEOUT) && !capable(CAP_MKNOD))
+		return -EPERM;
+
 retry:
 	from = user_path_parent(olddfd, oldname, &oldnd, lookup_flags);
 	if (IS_ERR(from)) {

include/uapi/linux/fs.h

@@ -37,6 +37,7 @@
 
 #define RENAME_NOREPLACE	(1 << 0)	/* Don't overwrite target */
 #define RENAME_EXCHANGE		(1 << 1)	/* Exchange source and dest */
+#define RENAME_WHITEOUT		(1 << 2)	/* Whiteout source */
 
 struct fstrim_range {
 	__u64 start;