From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f68.google.com ([209.85.210.68]:35309 "EHLO mail-ot1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726511AbeJHSCJ (ORCPT ); Mon, 8 Oct 2018 14:02:09 -0400 Received: by mail-ot1-f68.google.com with SMTP id j9-v6so19152427otl.2 for ; Mon, 08 Oct 2018 03:51:02 -0700 (PDT) MIME-Version: 1.0 References: <20180929103453.12025-1-cyphar@cyphar.com> <20180929131534.24472-1-cyphar@cyphar.com> <20181004162611.vdlujbdguvagalpt@ryuk> <20181005150728.mgqnpkbukpeu3bsm@ryuk> <20181006021002.6vzsdwd3klddbmji@ryuk> In-Reply-To: <20181006021002.6vzsdwd3klddbmji@ryuk> From: Jann Horn Date: Mon, 8 Oct 2018 12:50:34 +0200 Message-ID: Subject: Re: [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution Content-Type: text/plain; charset="UTF-8" Sender: linux-arch-owner@vger.kernel.org List-ID: To: cyphar@cyphar.com Cc: "Eric W. Biederman" , Al Viro , jlayton@kernel.org, Bruce Fields , Arnd Bergmann , shuah@kernel.org, David Howells , Andy Lutomirski , christian@brauner.io, Tycho Andersen , kernel list , linux-fsdevel@vger.kernel.org, linux-arch , linux-kselftest@vger.kernel.org, dev@opencontainers.org, containers@lists.linux-foundation.org, Linux API Message-ID: <20181008105034._jvsvyyoFZ96eq_dfbkQSls7Lv465x72lK35G3OyKP8@z> On Sat, Oct 6, 2018 at 4:10 AM Aleksa Sarai wrote: > On 2018-10-05, Jann Horn wrote: > > > What if we took rename_lock (call it nd->r_seq) at the start of the > > > resolution, and then only tried the __d_path-style check > > > > > > if (read_seqretry(&rename_lock, nd->r_seq) || > > > read_seqretry(&mount_lock, nd->m_seq)) > > > /* do the __d_path lookup. */ > > > > > > That way you would only hit the slow path if there were concurrent > > > renames or mounts *and* you are doing a path resolution with > > > AT_THIS_ROOT or AT_BENEATH. I've attached a modified patch that does > > > this (and after some testing it also appears to work). > > > > Yeah, I think that might do the job. > > *phew* I was all out of other ideas. :P > > > > --- > > > fs/namei.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++--- > > > 1 file changed, 46 insertions(+), 3 deletions(-) > > > > > > diff --git a/fs/namei.c b/fs/namei.c > > > index 6f995e6de6b1..12c9be175cb4 100644 > > > --- a/fs/namei.c > > > +++ b/fs/namei.c > > > @@ -493,7 +493,7 @@ struct nameidata { > > > struct path root; > > > struct inode *inode; /* path.dentry.d_inode */ > > > unsigned int flags; > > > - unsigned seq, m_seq; > > > + unsigned seq, m_seq, r_seq; > > > int last_type; > > > unsigned depth; > > > int total_link_count; > > > @@ -1375,6 +1375,27 @@ static int follow_dotdot_rcu(struct nameidata *nd) > > > return -EXDEV; > > > break; > > > } > > > + if (unlikely((nd->flags & (LOOKUP_BENEATH | LOOKUP_CHROOT)) && > > > + (read_seqretry(&rename_lock, nd->r_seq) || > > > + read_seqretry(&mount_lock, nd->m_seq)))) { > > > + char *pathbuf, *pathptr; > > > + > > > + nd->r_seq = read_seqbegin(&rename_lock); > > > + /* Cannot take m_seq here. */ > > > + > > > + pathbuf = kmalloc(PATH_MAX, GFP_ATOMIC); > > > + if (!pathbuf) > > > + return -ECHILD; > > > + pathptr = __d_path(&nd->path, &nd->root, pathbuf, PATH_MAX); > > > + kfree(pathbuf); > > > > You're doing this check before actually looking up the parent, right? > > So as long as I don't trigger the "path_equal(&nd->path, &nd->root)" > > check that you do for O_BENEATH, escaping up by one level is possible, > > right? You should probably move this check so that it happens after > > following "..". > > Yup, you're right. I'll do that. > > > (Also: I assume that you're going to get rid of that memory allocation > > in a future version.) > > Sure. Would you prefer adding some scratch space in nameidata, or that I > change __d_path so it accepts NULL as the buffer (and thus it doesn't > actually do any string operations)? Well, I think accepting a NULL buffer would be much cleaner; but keep in mind that I'm just someone making suggestions, Al Viro is the one who has to like your code. :P > > > if (nd->path.dentry != nd->path.mnt->mnt_root) { > > > int ret = path_parent_directory(&nd->path); > > > if (ret) > > > @@ -2269,6 +2311,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags) > > > nd->last_type = LAST_ROOT; /* if there are only slashes... */ > > > nd->flags = flags | LOOKUP_JUMPED | LOOKUP_PARENT; > > > nd->depth = 0; > > > + nd->m_seq = read_seqbegin(&mount_lock); > > > + nd->r_seq = read_seqbegin(&rename_lock); > > > > This means that now, attempting to perform a lookup while something is > > holding the rename_lock will spin on the lock. I don't know whether > > that's a problem in practice though. Does anyone on this thread know > > whether this is problematic? > > I could make it so that we only take &rename_lock > if (nd->flags & (FOLLOW_BENEATH | FOLLOW_CHROOT)), > since it's not used outside of that path. I think that might be a sensible change; but as I said, I don't actually know whether it's necessary, and it would be very helpful if someone who actually knows commented on this.