Re: [PATCH] fs: Fix freeze_bdev()/thaw_bdev() accounting of bd_fsfreeze_sb

[Bob Peterson <rpeterso@redhat.com>]

----- Original Message -----
> Can someone pick this up?  Maybe through Jens' block tree as that is
> where my commit this is fixing up came from.
> 
> For reference:
> 
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> 
> On Thu, Dec 24, 2020 at 04:49:54AM +0000, Satya Tangirala wrote:
> > freeze/thaw_bdev() currently use bdev->bd_fsfreeze_count to infer
> > whether or not bdev->bd_fsfreeze_sb is valid (it's valid iff
> > bd_fsfreeze_count is non-zero). thaw_bdev() doesn't nullify
> > bd_fsfreeze_sb.
> > 
> > But this means a freeze_bdev() call followed by a thaw_bdev() call can
> > leave bd_fsfreeze_sb with a non-null value, while bd_fsfreeze_count is
> > zero. If freeze_bdev() is called again, and this time
> > get_active_super() returns NULL (e.g. because the FS is unmounted),
> > we'll end up with bd_fsfreeze_count > 0, but bd_fsfreeze_sb is
> > *untouched* - it stays the same (now garbage) value. A subsequent
> > thaw_bdev() will decide that the bd_fsfreeze_sb value is legitimate
> > (since bd_fsfreeze_count > 0), and attempt to use it.
> > 
> > Fix this by always setting bd_fsfreeze_sb to NULL when
> > bd_fsfreeze_count is successfully decremented to 0 in thaw_sb().
> > Alternatively, we could set bd_fsfreeze_sb to whatever
> > get_active_super() returns in freeze_bdev() whenever bd_fsfreeze_count
> > is successfully incremented to 1 from 0 (which can be achieved cleanly
> > by moving the line currently setting bd_fsfreeze_sb to immediately
> > after the "sync:" label, but it might be a little too subtle/easily
> > overlooked in future).
> > 
> > This fixes the currently panicking xfstests generic/085.
> > 
> > Fixes: 040f04bd2e82 ("fs: simplify freeze_bdev/thaw_bdev")
> > Signed-off-by: Satya Tangirala <satyat@google.com>
> > ---
> >  fs/block_dev.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > index 9e56ee1f2652..12a811a9ae4b 100644
> > --- a/fs/block_dev.c
> > +++ b/fs/block_dev.c
> > @@ -606,6 +606,8 @@ int thaw_bdev(struct block_device *bdev)
> >  		error = thaw_super(sb);
> >  	if (error)
> >  		bdev->bd_fsfreeze_count++;
> > +	else
> > +		bdev->bd_fsfreeze_sb = NULL;
> >  out:
> >  	mutex_unlock(&bdev->bd_fsfreeze_mutex);
> >  	return error;
> > --
> > 2.29.2.729.g45daf8777d-goog
> ---end quoted text---
> 
> 
Funny you should ask. I came across this bug in my testing of gfs2
and my patch is slightly different. I was wondering who to send it to.
Perhaps Viro?

Regards,

Bob Peterson