Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763625Ab3IEMDp (ORCPT ); Thu, 5 Sep 2013 08:03:45 -0400 Received: from mail-lb0-f177.google.com ([209.85.217.177]:41316 "EHLO mail-lb0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763454Ab3IEMDn (ORCPT ); Thu, 5 Sep 2013 08:03:43 -0400 MIME-Version: 1.0 X-Originating-IP: [86.59.245.170] In-Reply-To: <20130905120230.GA21170@tucsk.piliscsaba.szeredi.hu> References: <1378374284-1484-1-git-send-email-miklos@szeredi.hu> <1378374284-1484-5-git-send-email-miklos@szeredi.hu> <20130905111852.GP13318@ZenIV.linux.org.uk> <20130905120230.GA21170@tucsk.piliscsaba.szeredi.hu> Date: Thu, 5 Sep 2013 14:03:41 +0200 Message-ID: Subject: Re: [PATCH 04/11] vfs: check unlinked ancestors before mount From: Miklos Szeredi To: Al Viro Cc: Linux-Fsdevel , Kernel Mailing List , "mszeredi@suse.cz" , David Howells , Steven Whitehouse , Trond Myklebust , Greg Kroah-Hartman Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4940 Lines: 142 On Thu, Sep 5, 2013 at 2:02 PM, Miklos Szeredi wrote: > On Thu, Sep 05, 2013 at 01:32:10PM +0200, Miklos Szeredi wrote: >> On Thu, Sep 5, 2013 at 1:18 PM, Al Viro wrote: > >> > Something's really odd with locking here. You are take d_lock, do one >> > check, set flag, drop d_lock, grab rename_lock, do another check (taking >> > and dropping d_lock in process), and, in case that check fails, grab >> > d_lock again to clear the flag. >> > >> > At the very least it's a massive overkill. Just grab rename_lock, then >> > d_lock, then do the damn check and set the flag only on success. Moreover, >> > with rename_lock held, do you need d_lock on ancestors to mess with in >> > has_unlinked_ancestor()? >> >> Yes, we need hard exclusion for the __d_drop() part. rename_lock can >> provide one if we always take it for write in >> check_submounts_and_drop(). But if we only take it for read then >> that's not enough. >> >> And we do in fact also need DCACHE_MOUNTED set *before* checking >> ancestors. Otherwise check_submounts_and_drop() could succeed and >> has_unlinked_ancestor() return false, resulting in a dropped dentry >> and a mount below it. Though this is mostly theoretical at this >> point. > > Maybe something like this. Has less ugly locking. Untested. And is missing a "goto out;" > > Thanks, > Miklos > > > --- > fs/dcache.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++ > fs/internal.h | 1 + > fs/namespace.c | 11 +++++------ > 3 files changed, 55 insertions(+), 6 deletions(-) > > --- a/fs/dcache.c > +++ b/fs/dcache.c > @@ -1159,6 +1159,55 @@ int have_submounts(struct dentry *parent > } > EXPORT_SYMBOL(have_submounts); > > +static bool __has_unlinked_ancestor(struct dentry *dentry) > +{ > + struct dentry *this; > + > + for (this = dentry; !IS_ROOT(this); this = this->d_parent) { > + int is_unhashed; > + > + /* Need exclusion wrt. check_submounts_and_drop() */ > + spin_lock(&this->d_lock); > + is_unhashed = d_unhashed(this); > + spin_unlock(&this->d_lock); > + > + if (is_unhashed) > + return true; > + } > + return false; > +} > + > +/* > + * Called by mount code to check if the mountpoint is reachable (e.g. NFS can > + * unhash a directory dentry and then the complete subtree can become > + * unreachable). > + */ > +int d_set_mounted(struct dentry *dentry) > +{ > + int ret = 0; > + > + write_seqlock(&rename_lock); > + spin_lock(&dentry->d_lock); > + dentry->d_flags |= DCACHE_MOUNTED; > + if (!IS_ROOT(dentry)) { > + ret = -ENOENT; > + if (d_unhashed(dentry)) { > + dentry->d_flags &= ~DCACHE_MOUNTED; > + goto out; > + } > + spin_unlock(&dentry->d_lock); > + if (__has_unlinked_ancestor(dentry->d_parent)) { > + spin_lock(&dentry->d_lock); > + dentry->d_flags &= ~DCACHE_MOUNTED; > + spin_unlock(&dentry->d_lock); > + } > + ret = 0; > + } > +out: > + write_sequnlock(&rename_lock); > + return ret; > +} > + > /* > * Search the dentry child list of the specified parent, > * and move any unused dentries to the end of the unused > --- a/fs/internal.h > +++ b/fs/internal.h > @@ -126,6 +126,7 @@ extern int invalidate_inodes(struct supe > * dcache.c > */ > extern struct dentry *__d_alloc(struct super_block *, const struct qstr *); > +extern int d_set_mounted(struct dentry *dentry); > > /* > * read_write.c > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -611,6 +611,7 @@ static struct mountpoint *new_mountpoint > { > struct list_head *chain = mountpoint_hashtable + hash(NULL, dentry); > struct mountpoint *mp; > + int ret; > > list_for_each_entry(mp, chain, m_hash) { > if (mp->m_dentry == dentry) { > @@ -626,14 +627,12 @@ static struct mountpoint *new_mountpoint > if (!mp) > return ERR_PTR(-ENOMEM); > > - spin_lock(&dentry->d_lock); > - if (d_unlinked(dentry)) { > - spin_unlock(&dentry->d_lock); > + ret = d_set_mounted(dentry); > + if (ret) { > kfree(mp); > - return ERR_PTR(-ENOENT); > + return ERR_PTR(ret); > } > - dentry->d_flags |= DCACHE_MOUNTED; > - spin_unlock(&dentry->d_lock); > + > mp->m_dentry = dentry; > mp->m_count = 1; > list_add(&mp->m_hash, chain); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/