[RFC] SHASH_DESC_ON_STACK macro

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

Hi Herbert,

There is an ongoing effort to remove all VLAs from the code base [1] and 
while working on that I came across the following macro at 
include/crypto/hash.h:154:

#define SHASH_DESC_ON_STACK(shash, ctx)                           \
         char __##shash##_desc[sizeof(struct shash_desc) +         \
                 crypto_shash_descsize(ctx)] CRYPTO_MINALIGN_ATTR; \
         struct shash_desc *shash = (struct shash_desc *)__##shash##_desc


Currently, this macro is being used in 46 different places.

I wonder how big can tfm->descsize can get?

Do you think it is feasible to replace the call to crypto_shash_descsize 
with a constant value and get rid of 46 VLA warnings?

I have sent some patches to replace the use of this macro with dynamic 
memory allocation instead, but it seems that this is not a suitable 
solution for all cases due to performance issues.

[1] https://lkml.org/lkml/2018/3/7/621

I'd really appreciate any feedback.
Thanks
--
Gustavo