on stack dynamic allocations

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?

- Dmitry