linux-ext4 - Re: [PATCH] ext4: Fix i_disksize exceeding i_size problem in paritally written case

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Date:   Sat, 18 Mar 2023 15:03:30 +0800
From:   Zhihao Cheng <chengzhihao1@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <tytso@....edu>, <adilger.kernel@...ger.ca>, <jack@...e.com>,
        <tudor.ambarus@...aro.org>, <linux-ext4@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>
Subject: Re: [PATCH] ext4: Fix i_disksize exceeding i_size problem in
 paritally written case

Hi, Jan
> On Fri 17-03-23 09:35:53, Zhihao Cheng wrote:
>> Following process makes i_disksize exceed i_size:
>>
>> generic_perform_write
>>   copied = iov_iter_copy_from_user_atomic(len) // copied < len
>>   ext4_da_write_end
>>   | ext4_update_i_disksize
>>   |  new_i_size = pos + copied;
>>   |  WRITE_ONCE(EXT4_I(inode)->i_disksize, newsize) // update i_disksize
>>   | generic_write_end
>>   |  copied = block_write_end(copied, len) // copied = 0
>>   |   if (unlikely(copied < len))
>>   |    if (!PageUptodate(page))
>>   |     copied = 0;
>>   |  if (pos + copied > inode->i_size) // return false
>>   if (unlikely(copied == 0))
>>    goto again;
>>   if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
>>    status = -EFAULT;
>>    break;
>>   }
>>
>> We get i_disksize greater than i_size here, which could trigger WARNING
>> check 'i_size_read(inode) < EXT4_I(inode)->i_disksize' while doing dio:
>>
>> ext4_dio_write_iter
>>   iomap_dio_rw
>>    __iomap_dio_rw // return err, length is not aligned to 512
>>   ext4_handle_inode_extension
>>    WARN_ON_ONCE(i_size_read(inode) < EXT4_I(inode)->i_disksize) // Oops
>>
>>   WARNING: CPU: 2 PID: 2609 at fs/ext4/file.c:319
>>   CPU: 2 PID: 2609 Comm: aa Not tainted 6.3.0-rc2
>>   RIP: 0010:ext4_file_write_iter+0xbc7
>>   Call Trace:
>>    vfs_write+0x3b1
>>    ksys_write+0x77
>>    do_syscall_64+0x39
>>
>> Fix it by putting block_write_end() before i_disksize updating just
>> like ext4_write_end() does.
>>
>> Fetch a reproducer in [Link].
>>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217209
>> Fixes: 64769240bd07f ("ext4: Add delayed allocation support in data=writeback mode")
>> Signed-off-by: Zhihao Cheng <chengzhihao1@...wei.com>
> 
> Good catch (although practically this will hardly have any negative
> effect). But rather than opencoding generic_write_end() I'd do:
> 
>          if (unlikely(copied < len) && !PageUptodate(page))
>                  copied = 0;
> 
> at the beginning of ext4_da_write_end() and that should solve these
> problems as well?
> 

Yes, your suggestion looks good, and I think we can put the checking 
just after ext4_write_inline_data_end(Line 3150)? On the one hand, we 
can pass original 'copied' value in trace_ext4_da_write_end(), one the 
other hand, ext4_write_inline_data_end() already has this checking.