allow the temp files created by open() to be linked to

O_TMPFILE | O_CREAT => linkat() with AT_SYMLINK_FOLLOW and /proc/self/fd/<n>
as oldpath (i.e. flink()) will create a link
O_TMPFILE | O_CREAT | O_EXCL => ENOENT on attempt to link those guys
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/inode.c

@@ -333,8 +333,10 @@ EXPORT_SYMBOL(set_nlink);
  */
 void inc_nlink(struct inode *inode)
 {
-	if (WARN_ON(inode->i_nlink == 0))
+	if (unlikely(inode->i_nlink == 0)) {
+		WARN_ON(!(inode->i_state & I_LINKABLE));
 		atomic_long_dec(&inode->i_sb->s_remove_count);
+	}
 
 	inode->__i_nlink++;
 }

fs/namei.c

@@ -2948,8 +2948,14 @@ static int do_tmpfile(int dfd, struct filename *pathname,
 	if (error)
 		goto out2;
 	error = open_check_o_direct(file);
-	if (error)
+	if (error) {
 		fput(file);
+	} else if (!(op->open_flag & O_EXCL)) {
+		struct inode *inode = file_inode(file);
+		spin_lock(&inode->i_lock);
+		inode->i_state |= I_LINKABLE;
+		spin_unlock(&inode->i_lock);
+	}
 out2:
 	mnt_drop_write(nd->path.mnt);
 out:
@@ -3628,12 +3634,18 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de
 
 	mutex_lock(&inode->i_mutex);
 	/* Make sure we don't allow creating hardlink to an unlinked file */
-	if (inode->i_nlink == 0)
+	if (inode->i_nlink == 0 && !(inode->i_state & I_LINKABLE))
 		error =  -ENOENT;
 	else if (max_links && inode->i_nlink >= max_links)
 		error = -EMLINK;
 	else
 		error = dir->i_op->link(old_dentry, dir, new_dentry);
+
+	if (!error && (inode->i_state & I_LINKABLE)) {
+		spin_lock(&inode->i_lock);
+		inode->i_state &= ~I_LINKABLE;
+		spin_unlock(&inode->i_lock);
+	}
 	mutex_unlock(&inode->i_mutex);
 	if (!error)
 		fsnotify_link(dir, inode, new_dentry);

include/linux/fs.h

@@ -1744,6 +1744,7 @@ struct super_operations {
 #define I_REFERENCED		(1 << 8)
 #define __I_DIO_WAKEUP		9
 #define I_DIO_WAKEUP		(1 << I_DIO_WAKEUP)
+#define I_LINKABLE		(1 << 10)
 
 #define I_DIRTY (I_DIRTY_SYNC | I_DIRTY_DATASYNC | I_DIRTY_PAGES)