linux-kernel - Re: Possible UDF locking error?

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Message-ID: <35961291-197a-0557-ff81-dc8686dd8661@digidescorp.com>
Date:   Mon, 25 Mar 2019 13:23:57 -0500
From:   Steve Magnani <steve.magnani@...idescorp.com>
To:     Jan Kara <jack@...e.cz>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: Possible UDF locking error?

On 3/25/19 11:42 AM, Jan Kara wrote:
> Hi!
>
> On Sat 23-03-19 15:14:05, Steve Magnani wrote:
>> ...
>>
>> In contrast, udf_setup_indirect_aext(), which constructs an AED,
>> has this sequence:
>>
>>    bh = udf_tgetblk(...);   // calls sb_getblk()
>>    lock_buffer(bh);
>>    memset(bh->b_data, 0, inode->i_sb->s_blocksize);
>>
>>    set_buffer_uptodate(bh);
>>    unlock_buffer(bh);
>>    mark_buffer_dirty_inode(bh);
>>
>>    // <snip>other code to populate AED data in the block</snip>
>>
>> In this case the population of the block occurs without
>> the protection of the lock.
>>
>> Because the block has been marked dirty, does this mean that
>> writeback could occur at any point during population?
> Yes. Thanks for noticing this!
>
>> There is one path through udf_setup_indirect_aext() where
>> mark_buffer_dirty_inode() gets called again after population is
>> complete, which I suppose could heal a partial writeout, but there is
>> also another path in which the buffer does not get marked dirty again.
> Generally, we add new extents to the created indirect extent which dirties
> the buffer and that should fix the problem. But you are definitely right
> that the code is suspicious and should be fixed. Will you send a patch?
>
> 								Honza


Sure. There's at least one other place where it looked like there might 
be a similar issue.

Steve