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SubjectRe: on stack dynamic allocations
Quoting David Daney <ddaney.cavm@gmail.com>:

> On 08/16/2012 02:20 PM, Kasatkin, Dmitry wrote:
>> Hello,
>>
>> Some places in the code uses variable-size allocation on stack..
>> For example from hmac_setkey():
>>
>> struct {
>> struct shash_desc shash;
>> char ctx[crypto_shash_descsize(hash)];
>> } desc;
>>
>>
>> sparse complains
>>
>> CHECK crypto/hmac.c
>> crypto/hmac.c:57:47: error: bad constant expression
>>
>> I like it instead of kmalloc..
>>
>> But what is position of kernel community about it?
>
> If you know that the range of crypto_shash_descsize(hash) is
> bounded, just use the upper bound.
>
> If the range of crypto_shash_descsize(hash) is unbounded, then the
> stack will overflow and ... BOOM!
>

Quick look shows that largest crypto_shash_descsize() would be with
hmac+s390/sha512, 16 + 332 = 348. Crypto-api also prevents registering
shash with descsize larger than (PAGE_SIZE / 8).

-Jussi



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