vfs: default to generic_readlink()

If i_op->readlink is NULL, but i_op->get_link is set then vfs_readlink()
defaults to calling generic_readlink().

The IOP_DEFAULT_READLINK flag indicates that the above conditions are met
and the default action can be taken.

Signed-off-by: Miklos Szeredi <[e-mail]>

Documentation/filesystems/porting

···
 [mandatory]
 	->rename() has an added flags argument.  Any flags not handled by the
         filesystem should result in EINVAL being returned.
+--
+[recommended]
+	->readlink is optional for symlinks.  Don't set, unless filesystem needs
+	to fake something for readlink(2).

Documentation/filesystems/vfs.txt

···
 	exist; this is checked by the VFS.  Unlike plain rename,
 	source and target may be of different type.
 
-  readlink: called by the readlink(2) system call. Only required if
-	you want to support reading symbolic links
-
   get_link: called by the VFS to follow a symbolic link to the
 	inode it points to.  Only required if you want to support
 	symbolic links.  This method returns the symlink body
···
 	May be called in RCU mode; that is indicated by NULL dentry
 	argument.  If request can't be handled without leaving RCU mode,
 	have it return ERR_PTR(-ECHILD).
+
+  readlink: this is now just an override for use by readlink(2) for the
+	cases when ->get_link uses nd_jump_link() or object is not in
+	fact a symlink.  Normally filesystems should only implement
+	->get_link for symlinks and readlink(2) will automatically use
+	that.
 
   permission: called by the VFS to check for access rights on a POSIX-like
   	filesystem.

fs/namei.c

···
 {
 	struct inode *inode = d_inode(dentry);
 
-	if (!inode->i_op->readlink)
-		return -EINVAL;
+	if (unlikely(!(inode->i_opflags & IOP_DEFAULT_READLINK))) {
+		if (unlikely(inode->i_op->readlink))
+			return inode->i_op->readlink(dentry, buffer, buflen);
 
-	return inode->i_op->readlink(dentry, buffer, buflen);
+		if (!d_is_symlink(dentry))
+			return -EINVAL;
+
+		spin_lock(&inode->i_lock);
+		inode->i_opflags |= IOP_DEFAULT_READLINK;
+		spin_unlock(&inode->i_lock);
+	}
+
+	return generic_readlink(dentry, buffer, buflen);
 }
 EXPORT_SYMBOL(vfs_readlink);
 

include/linux/fs.h

···
 #define IOP_LOOKUP	0x0002
 #define IOP_NOFOLLOW	0x0004
 #define IOP_XATTR	0x0008
+#define IOP_DEFAULT_READLINK	0x0010
 
 /*
  * Keep mostly read-only and often accessed (especially for