Re: on stack dynamic allocations

Quoting David Daney [off-list ref]:

On 08/16/2012 02:20 PM, Kasatkin, Dmitry wrote:

quoted

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