From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757144AbcFACvN (ORCPT ); Tue, 31 May 2016 22:51:13 -0400 Received: from bedivere.hansenpartnership.com ([66.63.167.143]:53218 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755471AbcFACvJ (ORCPT ); Tue, 31 May 2016 22:51:09 -0400 Message-ID: <1464749462.7732.15.camel@HansenPartnership.com> Subject: Re: [PATCH 1/1] shiftfs: uid/gid shifting bind mount From: James Bottomley To: Al Viro Cc: Djalal Harouni , Chris Mason , tytso@mit.edu, Serge Hallyn , Josh Triplett , "Eric W. Biederman" , Andy Lutomirski , Seth Forshee , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, Dongsu Park , David Herrmann , Miklos Szeredi , Alban Crequy Date: Tue, 31 May 2016 22:51:02 -0400 In-Reply-To: <20160601023959.GF14480@ZenIV.linux.org.uk> References: <1464740984.7732.5.camel@HansenPartnership.com> <1464741093.7732.7.camel@HansenPartnership.com> <20160601023959.GF14480@ZenIV.linux.org.uk> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2016-06-01 at 03:40 +0100, Al Viro wrote: > On Tue, May 31, 2016 at 08:31:33PM -0400, James Bottomley wrote: > > > > +static struct dentry *shiftfs_lookup(struct inode *dir, struct > > dentry *dentry, > > + unsigned int flags) > > +{ > > + struct dentry *real = dir->i_private, *new; > > + struct inode *reali = real->d_inode, *newi; > > + const struct cred *oldcred, *newcred; > > + > > + /* note: violation of usual fs rules here: dentries are > > never > > + * added with d_add. This is because we want no dentry > > cache > > + * for shiftfs. All lookups proceed through the dentry > > cache > > + * of the underlying filesystem, meaning we always see any > > + * changes in the underlying */ > > + > > + inode_lock(reali); > > + oldcred = shiftfs_new_creds(&newcred, dentry->d_sb); > > + new = lookup_one_len(dentry->d_name.name, real, dentry > > ->d_name.len); > > + shiftfs_old_creds(oldcred, &newcred); > > + inode_unlock(reali); > > + > > + if (IS_ERR(new)) > > + return new; > > + > > + dentry->d_fsdata = new; > > + > > + if (!new->d_inode) > > + return NULL; > > + > > + newi = shiftfs_new_inode(dentry->d_sb, new->d_inode > > ->i_mode, new); > > + if (!newi) { > > + dput(new); > > + return ERR_PTR(-ENOMEM); > > + } > > + > > + d_splice_alias(newi, dentry); > > + > > + return NULL; > > This is utter crap. First of all, d_splice_alias() *WILL* hash them, > so you get all the coherency problems, in spades. Moreover, if you > did manage to avoid hashing, you would get something absolutely > unusable. > * no mounting of anything on top of that thing > * performance shot to hell > and that's just for starters. I hadn't looked into the locking and > semantics issues - those would really depends upon how you would > achieve that "no dentry cache" thing; again, right now that's *not* > what your code is doing. Yes, sorry, after the new BUG_ON in the d_invalidate() code I didn't have much choice but to run two sets of dentry hashes; I'm afraid I just forgot to remove the comment (I did remove the blurb about the single dentry cache from the changelog). I'll remove it (the comment) in the next go around. > PS: and then there's the choice of name. I mean, just try to say it > over the phone several times in a row... But I did that especially for you, Al ... James