Re: [PATCH v2 01/12] fs-verity: add a documentation file

[Matthew Wilcox]

On Fri, Dec 21, 2018 at 11:28:13AM -0500, Theodore Y. Ts'o wrote:
> On Fri, Dec 21, 2018 at 07:53:54AM -0800, Matthew Wilcox wrote:
> > In contrast to "we'll just fix it up later" (which usually applies
> > to in-kernel interfaces), we have a policy of not breaking userspace,
> > so accepting this interface means setting it in stone.  We should get
> > it right.
> 
> I'm not convinced it's a "fix", but my point is that if later on you
> want to add extra complexity transforming
> 
>      ioctl(fd, FS_IOC_ENABLE_VERITY);
> 
> so it does the equivalent of
> 
>      ioctl(fd, FS_IOC_ENABLE_VERITY_NOW_WITH_EXTRA_USELESS_COMPLEXITY,
> 	   fd, sizeof_data, sizeof_verity_data);

I disagree with your EXTRA_USELESS_COMPLEXITY appendage.  The interface
you designed reflects the implementation you did in ext4, so I understand
why it seems simple from your point of view.  From the user point of view,
it looks completely weird.  You write a file, being a series of bytes,
then all of a sudden have to know that it's composed of blocks, seek
to the next block, write a header, then this Merkle data structure,
then write a footer which isn't allowed to cross a block boundary
for some unknowable reason.  It seems much more logical to have the
header+Merkle+footer as a separate data stream which the filesystem can
then layout according to its own rules.