[PATCH 10/13] iomap: use a function pointer for dio submits

[gaoxiang25@huawei.com (Gao Xiang)]

On Thu, Aug 08, 2019@07:21:39PM +0800, Gao Xiang wrote:
> On Thu, Aug 08, 2019@05:29:47PM +0800, Gao Xiang wrote:
> > On Thu, Aug 08, 2019@06:16:47PM +1000, Dave Chinner wrote:
> > > On Wed, Aug 07, 2019@10:49:36PM -0700, Eric Biggers wrote:
> > > > FWIW, the only order that actually makes sense is decrypt->decompress->verity.
> > > 
> > > *nod*
> > > 
> > > Especially once we get the inline encryption support for fscrypt so
> > > the storage layer can offload the encrypt/decrypt to hardware via
> > > the bio containing plaintext. That pretty much forces fscrypt to be
> > > the lowest layer of the filesystem transformation stack.  This
> > > hardware offload capability also places lots of limits on what you
> > > can do with block-based verity layers below the filesystem. e.g.
> > > using dm-verity when you don't know if there's hardware encryption
> > > below or software encryption on top becomes problematic...
> 
> ...and I'm not talking of fs-verity, I personally think fs-verity
> is great. I am only talking about a generic stuff.
> 
> In order to know which level becomes problematic, there even could
> be another choice "decrypt->verity1->decompress->verity2" for such
> requirement (assuming verity1/2 themselves are absolutely bug-free),
> verity1 can be a strong merkle tree and verity2 is a weak form (just
> like a simple Adler-32/crc32 in compressed block), thus we can locate
> whether it's a decrypt or decompress bug.
> 
> Many compression algorithm containers already have such a weak
> form such as gzip algorithm, so there is no need to add such
> an extra step to postprocess.
> 
> and I have no idea which (decrypt->verity1->decompress->verity2 or
> decrypt->decompress->verity) is faster since verity2 is rather simple.
> However, if we use the only strong form in the end, there could be
> a lot of extra IO and expensive multiple-level computations if files
> are highly compressible.
> 
> On the other hand, such verity2 can be computed offline / avoided
> by fuzzer tools for read-only scenerios (for example, after building
> these images and do a full image verification with the given kernel)
> in order to make sure its stability (In any case, I'm talking about
> how to make those algorithms bug-free).
> 
> All I want to say is I think "decrypt->verity->decompress" is
> reasonable as well.

... And another fundamental concern is that if we don't verify earlier
(I mean on-disk data), then untrusted data will be transformed
(decompressed and even decrypted if no inline encryption) with risk,
and it seems _vulnerable_ if such decrypt / decompress algorithms have
_security issues_ (such as Buffer Overflow). It seems that it's less
security than do verity earlier.

Thanks,
Gao Xiang

> 
> Thanks,
> Gao Xiang
> 
> > 
> > Add a word, I was just talking benefits between "decrypt->decompress->
> > verity" and "decrypt->verity->decompress", I think both forms are
> > compatible with inline en/decryption. I don't care which level
> > "decrypt" is at... But maybe some user cares. Am I missing something?
> > 
> > Thanks,
> > Gao Xiang
> >