From: Al Viro <viro@zeniv.linux.org.uk>
To: Amir Goldstein <amir73il@gmail.com>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Ritesh Harjani <riteshh@linux.ibm.com>,
linux-kernel <linux-kernel@vger.kernel.org>,
wugyuan@cn.ibm.com, Jeff Layton <jlayton@kernel.org>,
Gao Xiang <hsiangkao@aol.com>, Jan Kara <jack@suse.cz>,
Linus Torvalds <torvalds@linux-foundation.org>,
ecryptfs@vger.kernel.org
Subject: Re: [PATCH][RFC] ecryptfs_lookup_interpose(): lower_dentry->d_inode is not stable
Date: Wed, 13 Nov 2019 12:52:16 +0000 [thread overview]
Message-ID: <20191113125216.GF26530@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAOQ4uxiHH=e=Y5Xb3bkv+USxE0AftHiP935GGQEKkv54E17oDA@mail.gmail.com>
On Wed, Nov 13, 2019 at 09:01:36AM +0200, Amir Goldstein wrote:
> > - if (d_really_is_negative(lower_dentry)) {
> > + /*
> > + * negative dentry can go positive under us here - its parent is not
> > + * locked. That's OK and that could happen just as we return from
> > + * ecryptfs_lookup() anyway. Just need to be careful and fetch
> > + * ->d_inode only once - it's not stable here.
> > + */
> > + lower_inode = READ_ONCE(lower_dentry->d_inode);
> > +
> > + if (!lower_inode) {
> > /* We want to add because we couldn't find in lower */
> > d_add(dentry, NULL);
> > return NULL;
>
> Sigh!
>
> Open coding a human readable macro to solve a subtle lookup race.
> That doesn't sound like a scalable solution.
> I have a feeling this is not the last patch we will be seeing along
> those lines.
>
> Seeing that developers already confused about when they should use
> d_really_is_negative() over d_is_negative() [1] and we probably
> don't want to add d_really_really_is_negative(), how about
> applying that READ_ONCE into d_really_is_negative() and
> re-purpose it as a macro to be used when races with lookup are
> a concern?
Would you care to explain what that "fix" would've achieved here,
considering the fact that barriers are no-ops on UP and this is
*NOT* an SMP race?
And it's very much present on UP - we have
fetch ->d_inode into local variable
do blocking allocation
check if ->d_inode is NULL now
if it is not, use the value in local variable and expect it to be non-NULL
That's not a case of missing barriers. At all. And no redefinition of
d_really_is_negative() is going to help - it can't retroactively affect
the value explicitly fetched into a local variable some time prior to
that.
There are other patches dealing with ->d_inode accesses, but they are
generally not along the same lines. The problem is rarely the same...
next prev parent reply other threads:[~2019-11-13 12:52 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-27 4:42 [PATCH RESEND 1/1] vfs: Really check for inode ptr in lookup_fast Ritesh Harjani
2019-10-15 4:07 ` Ritesh Harjani
2019-10-22 13:38 ` Ritesh Harjani
2019-10-22 14:37 ` Al Viro
2019-10-22 14:50 ` Al Viro
2019-10-22 20:11 ` Al Viro
2019-10-23 11:05 ` Ritesh Harjani
2019-11-01 23:46 ` Al Viro
2019-11-02 6:17 ` Al Viro
2019-11-02 17:24 ` Paul E. McKenney
2019-11-02 17:22 ` Paul E. McKenney
2019-11-02 18:08 ` Al Viro
2019-11-03 14:41 ` Paul E. McKenney
2019-11-03 16:35 ` [RFC] lookup_one_len_unlocked() lousy calling conventions Al Viro
2019-11-03 18:20 ` Al Viro
2019-11-03 18:51 ` [PATCH][RFC] ecryptfs_lookup_interpose(): lower_dentry->d_inode is not stable Al Viro
2019-11-03 19:03 ` [PATCH][RFC] ecryptfs_lookup_interpose(): lower_dentry->d_parent is not stable either Al Viro
2019-11-13 7:01 ` [PATCH][RFC] ecryptfs_lookup_interpose(): lower_dentry->d_inode is not stable Amir Goldstein
2019-11-13 12:52 ` Al Viro [this message]
2019-11-13 16:22 ` Amir Goldstein
2019-11-13 20:18 ` Jean-Louis Biasini
2019-11-03 17:05 ` [PATCH][RFC] ecryptfs unlink/rmdir breakage (similar to caught in ecryptfs rename last year) Al Viro
2019-11-09 3:13 ` [PATCH][RFC] race in exportfs_decode_fh() Al Viro
2019-11-09 16:55 ` Linus Torvalds
2019-11-09 18:26 ` Al Viro
2019-11-11 9:16 ` Christoph Hellwig
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