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Message-ID: <9ef909de-1854-b4be-d272-2b4cda52329f@oppo.com>
Date: Wed, 22 Sep 2021 15:18:36 +0800
From: Huang Jianan <huangjianan@...o.com>
To: Chengguang Xu <cgxu519@....com>, linux-unionfs@...r.kernel.org,
miklos@...redi.hu, linux-erofs@...ts.ozlabs.org, xiang@...nel.org,
chao@...nel.org
Cc: guoweichao@...o.com, yh@...o.com, zhangshiming@...o.com,
guanyuwei@...o.com, jnhuang95@...il.com,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
cgxu519@...ernel.net
Subject: Re: [PATCH v2] ovl: fix null pointer when filesystem doesn't support
direct IO
在 2021/9/22 13:09, Chengguang Xu 写道:
> 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@...o.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.
>
I think that after reset a_ops, do_dentry_open will check
f_mapping->a_ops->direct_IO
and return error. But return error directly in ovl_open seems to be a
better solution, and
won't affect future extend of ovl_aops. Thanks for pointing this out.
Thanks,
Jianan
>
> Thanks,
>
> Chengguang
>
>
> [1]
> https://apc01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.spinics.net%2Flists%2Flinux-unionfs%2Fmsg08569.html&data=04%7C01%7Chuangjianan%40oppo.com%7Ce01c8bb9ad4e4ac2670008d97d87321c%7Cf1905eb1c35341c5951662b4a54b5ee6%7C0%7C0%7C637678842352759179%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=Wo2uMfbYhqDOzDPSwHci2AVtM9y9nNstmayb741gspQ%3D&reserved=0
>
>
>
>> +
>> file->private_data = realfile;
>> return 0;
>
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