Re: Possible UDF locking error?

[Steve Magnani]

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!"

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