linux-kernel - Re: Possible UDF locking error?

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Date:   Sat, 30 Mar 2019 14:49:46 -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?

Jan -

On 3/25/19 11:42 AM, Jan Kara wrote:
> Hi!
>
> On Sat 23-03-19 15:14:05, Steve Magnani wrote:
>> I have been hunting a UDF bug that occasionally results in generation
>> of an Allocation Extent Descriptor with an incorrect tagLocation. So
>> far I haven't been able to see a path through the code that could
>> cause that. But, I noticed some inconsistency in locking during
>> AED generation and wonder if it could result in random corruption.
>>
>> The function udf_update_inode() has this general pattern:
>>
>>    bh = udf_tgetblk(...);   // calls sb_getblk()
>>    lock_buffer(bh);
>>    memset(bh->b_data, 0, inode->i_sb->s_blocksize);
>>    // <snip>other code to populate FE/EFE data in the block</snip>
>>    set_buffer_uptodate(bh);
>>    unlock_buffer(bh);
>>    mark_buffer_dirty(bh);
>>
>> This I can understand - the lock is held for as long as the buffer
>> contents are being assembled.
>>
>> 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?

I did a little archaeology to see how the code evolved to this point. 
It's been like this a long time.

I also did some research to understand why filesystems use lock_buffer() 
sometimes but not others. For example, the FAT driver never calls it. I 
ran across this thread from 2011:

        https://lkml.org/lkml/2011/5/16/402

...from which I conclude that while it is correct in a strict sense to 
hold a lock on a buffer any time its contents are being modified, 
performance considerations make it preferable (or at least reasonable) 
to make some modifications without a lock provided it's known that a 
subsequent write-out will "fix" any potential partial write out before 
anyone else tries to read the block. I doubt that UDF sees common use 
with DIF/DIX block devices, which might make a decision in favor of 
performance a little easier. Since the FAT driver doesn't contain 
Darrick's proposed changes I assume a decision was made that performance 
was more important there.

Certainly the call to udf_setup_indirect_aext() from udf_add_aext() 
meets those criteria. But udf_table_free_blocks() may not dirty the AED 
block.

So if this looks reasonable I will resend as a formal patch:

--- a/fs/udf/inode.c	2019-03-30 11:28:38.637759458 -0500
+++ b/fs/udf/inode.c	2019-03-30 11:33:00.357761250 -0500
@@ -1873,9 +1873,6 @@ int udf_setup_indirect_aext(struct inode
  		return -EIO;
  	lock_buffer(bh);
  	memset(bh->b_data, 0x00, sb->s_blocksize);
-	set_buffer_uptodate(bh);
-	unlock_buffer(bh);
-	mark_buffer_dirty_inode(bh, inode);
  
  	aed = (struct allocExtDesc *)(bh->b_data);
  	if (!UDF_QUERY_FLAG(sb, UDF_FLAG_STRICT)) {
@@ -1890,6 +1887,9 @@ int udf_setup_indirect_aext(struct inode
  	udf_new_tag(bh->b_data, TAG_IDENT_AED, ver, 1, block,
  		    sizeof(struct tag));
  
+	set_buffer_uptodate(bh);
+	unlock_buffer(bh);
+
  	nepos.block = neloc;
  	nepos.offset = sizeof(struct allocExtDesc);
  	nepos.bh = bh;
@@ -1913,6 +1913,8 @@ int udf_setup_indirect_aext(struct inode
  	} else {
  		__udf_add_aext(inode, epos, &nepos.block,
  			       sb->s_blocksize | EXT_NEXT_EXTENT_ALLOCDECS, 0);
+		/* Make sure completed AED gets written out */
+		mark_buffer_dirty_inode(nepos.bh, inode);
  	}
  
  	brelse(epos->bh);

------------------------------------------------------------------------
  Steven J. Magnani               "I claim this network for MARS!
  www.digidescorp.com               Earthling, return my space modulator!"

  #include <standard.disclaimer>