Re: replacement i_version counter for xfs

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2023-01-30 at 13:05 +1100, Dave Chinner wrote:
> On Wed, Jan 25, 2023 at 12:58:08PM -0500, Jeff Layton wrote:
> > On Wed, 2023-01-25 at 08:32 -0800, Darrick J. Wong wrote:
> > > On Wed, Jan 25, 2023 at 06:47:12AM -0500, Jeff Layton wrote:
> > > > Note that there are two other lingering issues with i_version. Neither
> > > > of these are xfs-specific, but they may inform the changes you want to
> > > > make there:
> > > > 
> > > > 1/ the ctime and i_version can roll backward on a crash.
> > > > 
> > > > 2/ the ctime and i_version are both currently updated before write data
> > > > is copied to the pagecache. It would be ideal if that were done
> > > > afterward instead. (FWIW, I have some draft patches for btrfs and ext4
> > > > for this, but they need a lot more testing.)
> > > 
> > > You might also want some means for xfs to tell the vfs that it already
> > > did the timestamp update (because, say, we had to allocate blocks).
> > > I wonder what people will say when we have to run a transaction before
> > > the write to peel off suid bits and another one after to update ctime.
> > > 
> > 
> > That's a great question! There is a related one too once I started
> > looking at this in more detail:
> > 
> > Most filesystems end up updating the timestamp via a the call to
> > file_update_time in __generic_file_write_iter. Today, that's called very
> > early in the function and if it fails, the write fails without changing
> > anything.
> > 
> > What do we do now if the write succeeds, but update_time fails? We don't
> 
> On XFS, the timestamp update will either succeed or cause the
> filesystem to shutdown as a failure with a dirty transaction is a
> fatal, unrecoverable error.
> 

Ok. So for xfs, we could move all of this to be afterward. Clearing
setuid bits is quite rare, so that would only rarely require a
transaction (in principle).

> > want to return an error on the write() since the data did get copied in.
> > Ignoring it seems wrong too though. There could even be some way to
> > exploit that by changing the contents while holding the timestamp and
> > version constant.
> 
> If the filesystem has shut down, it doesn't matter that the data got
> copied into the kernel - it's never going to make it to disk and
> attempts to read it back will also fail. There's nothing that can be
> exploited by such a failure on XFS - it's game over for everyone
> once the fs has shut down....
> 
> > At this point I'm leaning toward leaving the ctime and i_version to be
> > updated before the write, and just bumping the i_version a second time
> > after. In most cases the second bump will end up being a no-op, unless
> > an i_version query races in between.
> 
> Why not also bump ctime at write completion if a query races with
> the write()? Wouldn't that put ns-granularity ctime based change
> detection on a par with i_version?
> 
> Userspace isn't going to notice the difference - the ctime they
> observe indicates that it was changed during the syscall. So
> who/what is going to care if we bump ctime twice in the syscall
> instead of just once in this rare corner case?
> 

We could bump the ctime too in this situation, but it would be more
costly. In most cases the i_version bump will be a no-op. The only
exception would be when a query of i_version races in between the two
bumps. That wouldn't be the case with the ctime, which would almost
always require a second transaction.

-- 
Jeff Layton <jlayton@kernel.org>