Re: [PATCH v2 10/11] fs/buffer.c: support fsverity in block_read_full_folio()

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Jan 09, 2023 at 07:05:07PM -0800, Eric Biggers wrote:
> On Mon, Jan 09, 2023 at 06:37:59PM -0800, Andrew Morton wrote:
> > On Fri, 23 Dec 2022 12:36:37 -0800 Eric Biggers <ebiggers@kernel.org> wrote:
> > 
> > > After each filesystem block (as represented by a buffer_head) has been
> > > read from disk by block_read_full_folio(), verify it if needed.  The
> > > verification is done on the fsverity_read_workqueue.  Also allow reads
> > > of verity metadata past i_size, as required by ext4.
> > 
> > Sigh.  Do we reeeeealy need to mess with buffer.c in this fashion?  Did
> > any other subsystems feel a need to do this?
> 
> ext4 is currently the only filesystem that uses block_read_full_folio() and that
> supports fsverity.  However, since fsverity has a common infrastructure across
> filesystems, in fs/verity/, it makes sense to support it in the other filesystem
> infrastructure so that things aren't mutually exclusive for no reason.
> 
> Note that this applies to fscrypt too, which block_read_full_folio() (previously
> block_read_full_page()) already supports since v5.5.
> 
> If you'd prefer that block_read_full_folio() be copied into ext4, then modified
> to support fscrypt and fsverity, and then the fscrypt support removed from the
> original copy, we could do that.  That seems more like a workaround to avoid
> modifying certain files than an actually better solution, but it could be done.
> 
> > 
> > > This is needed to support fsverity on ext4 filesystems where the
> > > filesystem block size is less than the page size.
> > 
> > Does any real person actually do this?
> 
> Yes, on systems with the page size larger than 4K, the ext4 filesystem block
> size is often smaller than the page size.  ext4 encryption (fscrypt) originally
> had the same limitation, and Chandan Rajendra from IBM did significant work to
> solve it a few years ago, with the changes landing in v5.5.
> 
> - Eric

Any more thoughts on this from Andrew, the ext4 maintainers, or anyone else?

- Eric