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From: Chengguang Xu <cgxu519@139.com>
To: Huang Jianan <huangjianan@oppo.com>,
	linux-unionfs@vger.kernel.org, miklos@szeredi.hu,
	linux-erofs@lists.ozlabs.org, xiang@kernel.org, chao@kernel.org
Cc: 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,
	cgxu519@mykernel.net
Subject: Re: [PATCH v2] ovl: fix null pointer when filesystem doesn't support direct IO
Date: Wed, 22 Sep 2021 13:09:43 +0800	[thread overview]
Message-ID: <4ccc5c89-eb13-5e91-9283-c94f755a9c17@139.com> (raw)
In-Reply-To: <20210922034700.15666-1-huangjianan@oppo.com>

在 2021/9/22 11:47, Huang Jianan 写道:
> 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:
>
> Kernel panic - not syncing: CFI failure (target: 0x0)
> CPU: 6 PID: 247 Comm: loop0
> Call Trace:
>   panic+0x188/0x45c
>   __cfi_slowpath+0x0/0x254
>   __cfi_slowpath+0x200/0x254
>   generic_file_read_iter+0x14c/0x150
>   vfs_iocb_iter_read+0xac/0x164
>   ovl_read_iter+0x13c/0x2fc
>   lo_rw_aio+0x2bc/0x458
>   loop_queue_work+0x4a4/0xbc0
>   kthread_worker_fn+0xf8/0x1d0
>   loop_kthread_worker_fn+0x24/0x38
>   kthread+0x29c/0x310
>   ret_from_fork+0x10/0x30
>
> The filesystem may only support direct_IO for some file types. For
> example, erofs supports direct_IO for uncompressed files. So reset
> f_mapping->a_ops to NULL when the file doesn't support direct_IO to
> fix this problem.
>
> Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from overlayfs over xfs")
> Signed-off-by: Huang Jianan <huangjianan@oppo.com>
> ---
> Change since v1:
>   - Return error to user rather than fall back to buffered io. (Chengguang Xu)
>
>   fs/overlayfs/file.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index d081faa55e83..38118d3b46f8 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -157,6 +157,10 @@ static int ovl_open(struct inode *inode, struct file *file)
>   	if (IS_ERR(realfile))
>   		return PTR_ERR(realfile);
>   
> +	if ((f->f_flags & O_DIRECT) && (!realfile->f_mapping->a_ops ||
> +		!realfile->f_mapping->a_ops->direct_IO))
> +		file->f_mapping->a_ops = NULL;


There are many other functions in a_ops and also address_space struct 
will be shared

between files which belong to same inode. Although overlayfs currently 
only defines

->direct_IO in a_ops, it will be extended in the future. (like 
containerized sycnfs [1])


It seems the simplest solution is directly return error to upper layer.


Thanks,

Chengguang


[1] https://www.spinics.net/lists/linux-unionfs/msg08569.html



> +
>   	file->private_data = realfile;
>   
>   	return 0;


  reply	other threads:[~2021-09-22  5:09 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-18 12:13 [PATCH] " 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 [this message]
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
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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