Re: [PATCH] Btrfs: allow empty subvol= again

[Omar Sandoval <osandov@osandov.com>]

On Mon, May 21, 2018 at 05:07:19PM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> I got a report that after upgrading to 4.16, someone's filesystems
> weren't mounting:
> 
> [   23.845852] BTRFS info (device loop0): unrecognized mount option 'subvol='
> 
> Before 4.16, this mounted the default subvolume. It turns out that this
> empty "subvol=" is actually an application bug, but it was causing the
> application to fail, so it's an ABI break if you squint.
> 
> The generic parsing code we use for mount options (match_token())
> doesn't match an empty string as "%s". Previously, setup_root_args()
> removed the "subvol=" string, but the mount path was cleaned up to not
> need that. Add a dummy Opt_subvol_empty to fix this.
> 
> Fixes: 312c89fbca06 ("btrfs: cleanup btrfs_mount() using btrfs_mount_root()")
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
> Hi, Dave,
> 
> This is silly but IMO it's a regression. If you agree, this should
> probably go to 4.17 + stable. I'll submit an xfstests test shortly.
> 
> Thanks!

Update, we fixed the userspace bug now so I'd be fine if we dropped this
patch. I guess there's still a possibility that there are other users
that hit this, so I'll leave it up to you, Dave, whether it's important
enough to care.