Re: [PATCH v2 1/3] fscrypt: add support for IV_INO_LBLK_64 policies

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Nov 05, 2019 at 10:35:44PM -0500, Theodore Y. Ts'o wrote:
> On Thu, Oct 24, 2019 at 02:54:36PM -0700, Eric Biggers wrote:
> > @@ -83,6 +118,10 @@ bool fscrypt_supported_policy(const union fscrypt_policy *policy_u,
> >  			return false;
> >  		}
> >  
> > +		if ((policy->flags & FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64) &&
> > +		    !supported_iv_ino_lblk_64_policy(policy, inode))
> > +			return false;
> > +
> >  		if (memchr_inv(policy->__reserved, 0,
> >  			       sizeof(policy->__reserved))) {
> >  			fscrypt_warn(inode,
> 
> fscrypt_supported_policy is getting more and more complicated, and
> supported_iv_ino_lblk_64_policy calls a fs-supplied callback function,
> etc.  And we need to use this every single time we need to set up an
> inode.  Granted that compared to the crypto, even if it is ICE, it's
> probably small beer --- but perhaps we should think about caching some
> of what fscrypt_supported_policy does on a per-file system basis at
> some point?

I don't think this will make any difference given everything else that needs to
be done to set up a file's key.  Also, anything extra we spend here will be far
less than the amount of time we save with IV_INO_LBLK_64 policies by not having
to do the key derivation and tfm allocation for every file.

Christoph suggested replacing ->has_stable_inodes() and
->get_ino_and_lblk_bits() with a new SB_* flag like SB_IV_INO_LBLK_64_SUPPORT.
But I don't like that that would result in worse error messages and would "leak"
a specific fscrypt policy flag into filesystems rather than having the
filesystems declare their properties.

If we really wanted to optimize fscrypt_get_encryption_info(), I think we
probably shouldn't try to microoptimize fscrypt_supported_policy(), but rather
take advantage of the fact that fscrypt_has_permitted_context() already ran.
E.g., we could cache the xattr, or skip both the keyring lookup and
fscrypt_supported_policy() by grabbing them from the parent directory.

- Eric