Re: [PATCH v4 05/16] fs-verity: add Kconfig and the helper functions for hashing

[Eric Biggers <ebiggers@kernel.org>]

On Sat, Jun 15, 2019 at 08:57:31AM -0400, Theodore Ts'o wrote:
> On Thu, Jun 06, 2019 at 08:51:54AM -0700, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@google.com>
> > 
> > Add the beginnings of the fs/verity/ support layer, including the
> > Kconfig option and various helper functions for hashing.  To start, only
> > SHA-256 is supported, but other hash algorithms can easily be added.
> > 
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> 
> Looks good; you can add:
> 
> Reviewed-off-by: Theodore Ts'o <tytso@mit.edu>
> 
> One thought for consideration below...
> 
> 
> > +
> > +/*
> > + * Maximum depth of the Merkle tree.  Up to 64 levels are theoretically possible
> > + * with a very small block size, but we'd like to limit stack usage during
> > + * verification, and in practice this is plenty.  E.g., with SHA-256 and 4K
> > + * blocks, a file with size UINT64_MAX bytes needs just 8 levels.
> > + */
> > +#define FS_VERITY_MAX_LEVELS		16
> 
> Maybe we should make FS_VERITY_MAX_LEVELS 8 for now?  This is an
> implementation-level restriction, and currently we don't support any
> architectures that have a page size < 4k.  We can always bump this
> number up in the future if it ever becomes necessary, and limiting max
> levels to 8 saves almost 100 bytes of stack space in verify_page().
> 
> 						- Ted

Yes, I agree.  I'll reduce MAX_LEVELS to 8 for now and tweak the comment.

- Eric