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From: Miklos Szeredi <miklos@szeredi.hu>
To: Chengguang Xu <cgxu519@139.com>
Cc: Huang Jianan <huangjianan@oppo.com>,
	overlayfs <linux-unionfs@vger.kernel.org>,
	linux-erofs@lists.ozlabs.org, xiang@kernel.org, chao@kernel.org,
	guoweichao@oppo.com, yh@oppo.com, zhangshiming@oppo.com,
	guanyuwei@oppo.com, jnhuang95@gmail.com,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Chengguang Xu <cgxu519@mykernel.net>
Subject: Re: [PATCH v3] ovl: fix null pointer when filesystemdoesn'tsupportdirect IO
Date: Wed, 22 Sep 2021 16:00:47 +0200	[thread overview]
Message-ID: <CAJfpegs_T5BQ+e79T=1fqTScjfaOyAftykmzK6=hdS=WhVvWsg@mail.gmail.com> (raw)
In-Reply-To: <314324e7-02d7-dc43-b270-fb8117953549@139.com>

On Wed, 22 Sept 2021 at 15:21, Chengguang Xu <cgxu519@139.com> wrote:
>
> 在 2021/9/22 16:24, Huang Jianan 写道:
> >
> >
> > 在 2021/9/22 16:06, Chengguang Xu 写道:
> >> 在 2021/9/22 15:23, Huang Jianan 写道:
> >>> From: Huang Jianan <huangjianan@oppo.com>
> >>>
> >>> At present, overlayfs provides overlayfs inode to users. Overlayfs
> >>> inode provides ovl_aops with noop_direct_IO to avoid open failure
> >>> with O_DIRECT. But some compressed filesystems, such as erofs and
> >>> squashfs, don't support direct_IO.
> >>>
> >>> Users who use f_mapping->a_ops->direct_IO to check O_DIRECT support,
> >>> will read file through this way. This will cause overlayfs to access
> >>> a non-existent direct_IO function and cause panic due to null pointer:
> >>
> >> I just looked around the code more closely, in open_with_fake_path(),
> >>
> >> do_dentry_open() has already checked O_DIRECT open flag and
> >> a_ops->direct_IO of underlying real address_space.
> >>
> >> Am I missing something?
> >>
> >>
> >
> > It seems that loop_update_dio will set lo->use_dio after open file
> > without set O_DIRECT.
> > loop_update_dio will check f_mapping->a_ops->direct_IO but it deal
> > with ovl_aops with
> > noop _direct_IO.
> >
> > So I think we still need a new aops?
>
>
> It means we should only set ->direct_IO for overlayfs inodes whose
> underlying fs has DIRECT IO ability.

First let's fix the oops: ovl_read_iter()/ovl_write_iter() must check
real file's ->direct_IO if IOCB_DIRECT is set in iocb->ki_flags and
return -EINVAL if not.

To fix the loop -> overlay -> squashfs case your suggestion of having
separate aops depending on the real inode's ->direct_IO sounds good.

Thanks,
Miklos

  reply	other threads:[~2021-09-22 14:01 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-18 12:13 [PATCH] ovl: fix null pointer when filesystem doesn't support direct IO Huang Jianan
2021-09-22  1:56 ` Chengguang Xu
2021-09-22  3:39   ` Huang Jianan
2021-09-22  3:47 ` [PATCH v2] " Huang Jianan
2021-09-22  5:09   ` Chengguang Xu
2021-09-22  7:18     ` Huang Jianan
2021-09-22  7:23       ` [PATCH v3] " Huang Jianan
2021-09-22  8:06         ` Chengguang Xu
2021-09-22  8:24           ` Huang Jianan
2021-09-22 13:20             ` [PATCH v3] ovl: fix null pointer when filesystemdoesn'tsupportdirect IO Chengguang Xu
2021-09-22 14:00               ` Miklos Szeredi [this message]
2021-09-27  9:38                 ` Miklos Szeredi
2021-09-28  7:01                   ` Huang Jianan
2021-09-28  7:17                     ` Miklos Szeredi

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