Re: [PATCH 1/9] crypto: add zbufsize() interface

[Kees Cook <keescook@chromium.org>]

On Wed, Aug 08, 2018 at 10:53:19AM +0800, Herbert Xu wrote:
> On Tue, Aug 07, 2018 at 11:10:10AM -0700, Kees Cook wrote:
> >
> > > Please don't add new features to the old compress interface.  Any
> > > new improvements should be added to scomp/acomp only.  Users who
> > > need new features should be converted.
> > 
> > So, keep crypto_scomp_zbufsize() and drop crypto_comp_zbufsize() and
> > crypto_zbufsize()? Should I add crypto_acomp_zbufsize()?
> 
> Yes and yes.  acomp is the primary interface and should support
> all the features in scomp.

*thread necromancy*

Okay, I'm looking at this again because of the need in the module loader
to know "worst case decompression size"[1]. I am at a loss for how (or
why) the acomp interface is the "primary interface".

For modules, all that would be wanted is this, where the buffer size can
be allocated on demand:

u8 *decompressed = NULL;
size_t decompressed_size = 0;

decompressed = decompress(decompressed, compressed, compressed_size, &decompressed_size);

For pstore, the compressed_size is fixed and the decompression buffer
must be preallocated (for catching panic dumps), so the worst-case size
needs to be known in advance:

u8 *decompressed = NULL;
size_t decompressed_worst_size = 0;
size_t decompressed_size = 0;

worst_case(&decompressed_worst_size, compressed_size);

decompressed = kmalloc(decompressed_worst_size, GFP_KERNEL);
...
decompressed_size = decompressed_worst_size;
decompress(decompressed, compressed, compressed_size, &decompressed_size);


I don't see anything like this in the kernel for handling a simple
buffer-to-buffer decompression besides crypto_comp_decompress(). The
acomp interface is wildly over-complex for this. What the right
way to do this? (I can't find any documentation that discusses
compress/decompress[2].)

-Kees

[1] https://lore.kernel.org/linux-modules/YaMYJv539OEBz5B%2F@google.com/
[2] https://www.kernel.org/doc/html/latest/crypto/api-samples.html

-- 
Kees Cook