[RFC] SHASH_DESC_ON_STACK macro

[RFC] SHASH_DESC_ON_STACK macro

[Gustavo A. R. Silva]

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

[RESEND] SHASH_DESC_ON_STACK macro

[Gustavo A. R. Silva]

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

Re: [RESEND] SHASH_DESC_ON_STACK macro

[Herbert Xu]

On Fri, Mar 23, 2018 at 02:09:46PM -0500, Gustavo A. R. Silva wrote:
> 
> 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?

descsize is capped at PAGE_SIZE / 8.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [RESEND] SHASH_DESC_ON_STACK macro

[Gustavo A. R. Silva]

Hi Herbert,

On 03/27/2018 05:07 AM, Herbert Xu wrote:
> On Fri, Mar 23, 2018 at 02:09:46PM -0500, Gustavo A. R. Silva wrote:
>>
>> 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?
> 
> descsize is capped at PAGE_SIZE / 8.
> 

Sorry for the late response. It seems I lost track of this email somehow.

OK. So based on your response I will propose a patch for this.

Thanks!
--
Gustavo