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Subject: Re: [PATCH] ovl: fix null pointer when filesystem doesn't support
 direct IO
To:     Chengguang Xu <cgxu519@139.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
References: <20210918121346.12084-1-huangjianan@oppo.com>
 <3633c6e5-028c-fc77-3b8e-da9903f97ac5@139.com>
From:   Huang Jianan <huangjianan@oppo.com>
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Date:   Wed, 22 Sep 2021 11:39:18 +0800
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在 2021/9/22 9:56, Chengguang Xu 写道:
> 在 2021/9/18 20:13, 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:
>>
>> 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 fall
>> back to buffered io only when the file doesn't support direct_IO to
>> fix this problem.
>
>
> IMO, return error to user seems better option than fall back to
>
> buffered io directly.
>
Agreed, I will send v2 to fix it.

Thanks,
Jianan
>
> Thanks,
>
> Chengguang
>
>
>>
>> Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from 
>> overlayfs over xfs")
>> Signed-off-by: Huang Jianan <huangjianan@oppo.com>
>> ---
>>   fs/overlayfs/file.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
>> index d081faa55e83..998c60770b81 100644
>> --- a/fs/overlayfs/file.c
>> +++ b/fs/overlayfs/file.c
>> @@ -296,6 +296,10 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, 
>> struct iov_iter *iter)
>>       if (ret)
>>           return ret;
>>   +    if ((iocb->ki_flags & IOCB_DIRECT) && 
>> (!real.file->f_mapping->a_ops ||
>> +        !real.file->f_mapping->a_ops->direct_IO))
>> +        iocb->ki_flags &= ~IOCB_DIRECT;
>> +
>>       old_cred = ovl_override_creds(file_inode(file)->i_sb);
>>       if (is_sync_kiocb(iocb)) {
>>           ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,
>