linux-kernel - Re: [PATCH v3] ovl: fix null pointer when filesystemdoesn'tsupportdirect IO

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Message-ID: <314324e7-02d7-dc43-b270-fb8117953549@139.com>
Date:   Wed, 22 Sep 2021 21:20:50 +0800
From:   Chengguang Xu <cgxu519@....com>
To:     Huang Jianan <huangjianan@...o.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 v3] ovl: fix null pointer when
 filesystemdoesn'tsupportdirect IO

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@...o.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.


Hi Miklos,

Is it right solution for this kind of issue? What do you think?


Thanks,

Chengguang



>
> Thanks,
> Jianan
>
>> Thanks,
>>
>> Chengguang
>>
>>
>>>
>>> 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 return
>>> -EINVAL 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 v2:
>>>   - Return error in ovl_open directly. (Chengguang Xu)
>>>
>>> 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..a0c99ea35daf 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))
>>> +        return -EINVAL;
>>> +
>>>       file->private_data = realfile;
>>>         return 0;
>>
>