Re: max fsverity descriptor size?

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Dec 02, 2020 at 01:33:54PM -0500, Chris Mason wrote:
> 
> 
> On 2 Dec 2020, at 13:12, Eric Biggers wrote:
> 
> > +linux-fscrypt
> > 
> > On Wed, Dec 02, 2020 at 09:01:52AM -0500, Chris Mason wrote:
> > > Hi Eric,
> > > 
> > > I’m working on fsverity support in btrfs and wanted to check on the
> > > max size
> > > of the descriptor.  I can go up to any size, just wanted to make
> > > sure I had
> > > things correct in the disk format.
> > > 
> > > -chris
> > 
> > The implementations of fs-verity in ext4 and f2fs store the built-in
> > signature
> > (if there is one) appended to the 'struct fsverity_descriptor', and
> > limit the
> > total size of those two things combined to 16384 bytes.  See
> > FS_VERITY_MAX_DESCRIPTOR_SIZE in fs/verity/fsverity_private.h.
> > 
> > Note that there's nothing special about this particular number; it's
> > just an
> > implementation limit to prevent userspace doing weird things with
> > megabytes
> > "signatures".
> > 
> > If btrfs will be storing built-in signatures in the same way, it
> > probably should
> > use the same limit.  Preferably it would be done in a way such that it's
> > possible to increase the limit later if it's ever needed.
> > 
> 
> +Boris
> 
> Thanks Eric, the current btrfs code is just putting it in the btree, but
> I’ve got it setup so we won’t run into trouble if it spans multiple btree
> blocks.
> 
> Looks like the fs/verity/*.c are in charge of validating against the max
> size?  I’m not finding specific checks in ext4.

Yes, that's the case currently.

- Eric