vfs: make vfs_symlink break delegations on parent dir

In order to add directory delegation support, we must break delegations
on the parent on any change to the directory.

Add a delegated_inode parameter to vfs_symlink() and have it break the
delegation. do_symlinkat() can then wait on the delegation break before
proceeding.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20251111-dir-deleg-ro-v6-12-52f3feebb2f2@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/ecryptfs/inode.c

@@ -479,7 +479,7 @@ static int ecryptfs_symlink(struct mnt_idmap *idmap,
 	if (rc)
 		goto out_lock;
 	rc = vfs_symlink(&nop_mnt_idmap, lower_dir, lower_dentry,
-			 encoded_symname);
+			 encoded_symname, NULL);
 	kfree(encoded_symname);
 	if (rc || d_really_is_negative(lower_dentry))
 		goto out_lock;

fs/init.c

@@ -209,7 +209,7 @@ int __init init_symlink(const char *oldname, const char *newname)
 	error = security_path_symlink(&path, dentry, oldname);
 	if (!error)
 		error = vfs_symlink(mnt_idmap(path.mnt), path.dentry->d_inode,
-				    dentry, oldname);
+				    dentry, oldname, NULL);
 	end_creating_path(&path, dentry);
 	return error;
 }

fs/namei.c

@@ -4845,6 +4845,7 @@ SYSCALL_DEFINE1(unlink, const char __user *, pathname)
  * @dir:	inode of the parent directory
  * @dentry:	dentry of the child symlink file
  * @oldname:	name of the file to link to
+ * @delegated_inode: returns victim inode, if the inode is delegated.
  *
  * Create a symlink.
  *
@@ -4855,7 +4856,8 @@ SYSCALL_DEFINE1(unlink, const char __user *, pathname)
  * raw inode simply pass @nop_mnt_idmap.
  */
 int vfs_symlink(struct mnt_idmap *idmap, struct inode *dir,
-		struct dentry *dentry, const char *oldname)
+		struct dentry *dentry, const char *oldname,
+		struct delegated_inode *delegated_inode)
 {
 	int error;
 
@@ -4870,6 +4872,10 @@ int vfs_symlink(struct mnt_idmap *idmap, struct inode *dir,
 	if (error)
 		return error;
 
+	error = try_break_deleg(dir, delegated_inode);
+	if (error)
+		return error;
+
 	error = dir->i_op->symlink(idmap, dir, dentry, oldname);
 	if (!error)
 		fsnotify_create(dir, dentry);
@@ -4883,6 +4889,7 @@ int do_symlinkat(struct filename *from, int newdfd, struct filename *to)
 	struct dentry *dentry;
 	struct path path;
 	unsigned int lookup_flags = 0;
+	struct delegated_inode delegated_inode = { };
 
 	if (IS_ERR(from)) {
 		error = PTR_ERR(from);
@@ -4897,8 +4904,13 @@ int do_symlinkat(struct filename *from, int newdfd, struct filename *to)
 	error = security_path_symlink(&path, dentry, from->name);
 	if (!error)
 		error = vfs_symlink(mnt_idmap(path.mnt), path.dentry->d_inode,
-				    dentry, from->name);
+				    dentry, from->name, &delegated_inode);
 	end_creating_path(&path, dentry);
+	if (is_delegated(&delegated_inode)) {
+		error = break_deleg_wait(&delegated_inode);
+		if (!error)
+			goto retry;
+	}
 	if (retry_estale(error, lookup_flags)) {
 		lookup_flags |= LOOKUP_REVAL;
 		goto retry;

fs/nfsd/vfs.c

@@ -1742,7 +1742,7 @@ nfsd_symlink(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	err = fh_fill_pre_attrs(fhp);
 	if (err != nfs_ok)
 		goto out_unlock;
-	host_err = vfs_symlink(&nop_mnt_idmap, d_inode(dentry), dnew, path);
+	host_err = vfs_symlink(&nop_mnt_idmap, d_inode(dentry), dnew, path, NULL);
 	err = nfserrno(host_err);
 	cerr = fh_compose(resfhp, fhp->fh_export, dnew, fhp);
 	if (!err)

fs/overlayfs/overlayfs.h

@@ -267,7 +267,7 @@ static inline int ovl_do_symlink(struct ovl_fs *ofs,
 				 struct inode *dir, struct dentry *dentry,
 				 const char *oldname)
 {
-	int err = vfs_symlink(ovl_upper_mnt_idmap(ofs), dir, dentry, oldname);
+	int err = vfs_symlink(ovl_upper_mnt_idmap(ofs), dir, dentry, oldname, NULL);
 
 	pr_debug("symlink(\"%s\", %pd2) = %i\n", oldname, dentry, err);
 	return err;

include/linux/fs.h

@@ -2118,7 +2118,7 @@ struct dentry *vfs_mkdir(struct mnt_idmap *, struct inode *,
 int vfs_mknod(struct mnt_idmap *, struct inode *, struct dentry *,
 	      umode_t, dev_t, struct delegated_inode *);
 int vfs_symlink(struct mnt_idmap *, struct inode *,
-		struct dentry *, const char *);
+		struct dentry *, const char *, struct delegated_inode *);
 int vfs_link(struct dentry *, struct mnt_idmap *, struct inode *,
 	     struct dentry *, struct delegated_inode *);
 int vfs_rmdir(struct mnt_idmap *, struct inode *, struct dentry *,