Re: [PATCH 18/26] xfs: use merkle tree offset as attr hash

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On Mon, May 20, 2024 at 05:39:59AM -0700, Christoph Hellwig wrote:
> On Fri, May 17, 2024 at 10:17:20AM -0700, Darrick J. Wong wrote:
> > >   Note that the verity metadata *must* be encrypted when the file is,
> > >   since it contains hashes of the plaintext data.
> > 
> > Refresh my memory of fscrypt -- does it encrypt directory names, xattr
> > names, and xattr values too?  Or does it only do that to file data?
> 
> It does encrypt the file names in the directories, but nothing in
> xattrs as far as I can tell.

Do we want that for user.* attrs?  That seems like quite an omission.

> > And if we copy the ext4 method of putting the merkle data after eof and
> > loading it into the pagecache, how much of the generic fs/verity cleanup
> > patches do we really need?
> 
> We shouldn't need anything.  A bunch of cleanup

Should we do the read/drop_merkle_tree_block cleanup anyway?

One of the advantages of xfs caching merkle tree blocks ourselves
is that we neither extend the usage of PageChecked when merkle blocksize
== pagesize nor become subject to the 1-million merkle block limit when
merkle blocksize < pagesize.  There's a tripping hazard if you mount a 4k
merkle block filesystem on a computer with 64k pages -- now you can't
open 6T verity files.

That said, it also sounds dumb to maintain a separate index for
pagecache pages to track a single bit.  Maybe we should port verity to
use xbitmap64 from xfs instead of single static buffer?

>                                                 and the support for not
> generating a hash for holes might still be nice to have, though.

Yeah, though I think that's only usable for xfs if merkle blocksize ==
fs blocksize, since xfs doesn't store sub-fsblock unwritten state.

--D




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