From: Amir Goldstein <amir73il@gmail.com>
To: Al Viro <viro@zeniv.linux.org.uk>
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 09:01:36 +0200 [thread overview]
Message-ID: <CAOQ4uxiHH=e=Y5Xb3bkv+USxE0AftHiP935GGQEKkv54E17oDA@mail.gmail.com> (raw)
In-Reply-To: <20191103185133.GR26530@ZenIV.linux.org.uk>
On Sun, Nov 3, 2019 at 8:52 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> lower_dentry can't go from positive to negative (we have it pinned),
> but it *can* go from negative to positive. So fetching ->d_inode
> into a local variable, doing a blocking allocation, checking that
> now ->d_inode is non-NULL and feeding the value we'd fetched
> earlier to a function that won't accept NULL is not a good idea.
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
> diff --git a/fs/ecryptfs/inode.c b/fs/ecryptfs/inode.c
> index a905d5f4f3b0..3c2298721359 100644
> --- a/fs/ecryptfs/inode.c
> +++ b/fs/ecryptfs/inode.c
> @@ -319,7 +319,7 @@ static int ecryptfs_i_size_read(struct dentry *dentry, struct inode *inode)
> static struct dentry *ecryptfs_lookup_interpose(struct dentry *dentry,
> struct dentry *lower_dentry)
> {
> - struct inode *inode, *lower_inode = d_inode(lower_dentry);
> + struct inode *inode, *lower_inode;
> struct ecryptfs_dentry_info *dentry_info;
> struct vfsmount *lower_mnt;
> int rc = 0;
> @@ -339,7 +339,15 @@ static struct dentry *ecryptfs_lookup_interpose(struct dentry *dentry,
> dentry_info->lower_path.mnt = lower_mnt;
> dentry_info->lower_path.dentry = lower_dentry;
>
> - 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?
Thanks,
Amir.
[1] https://lore.kernel.org/linux-fsdevel/20190903135803.GA25692@hsiangkao-HP-ZHAN-66-Pro-G1/
next prev parent reply other threads:[~2019-11-13 7:01 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 ` Amir Goldstein [this message]
2019-11-13 12:52 ` [PATCH][RFC] ecryptfs_lookup_interpose(): lower_dentry->d_inode is not stable Al Viro
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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