struct n2_hmac_ctx *ctx = crypto_ahash_ctx(tfm);
        struct crypto_shash *child_shash = ctx->child_shash;
        struct crypto_ahash *fallback_tfm;
-       struct {
-               struct shash_desc shash;
-               char ctx[crypto_shash_descsize(child_shash)];
-       } desc;
+       SHASH_DESC_ON_STACK(shash, child_shash);
        int err, bs, ds;
 
        fallback_tfm = ctx->base.fallback_tfm;
        if (err)
                return err;
 
-       desc.shash.tfm = child_shash;
-       desc.shash.flags = crypto_ahash_get_flags(tfm) &
+       shash->tfm = child_shash;
+       shash->flags = crypto_ahash_get_flags(tfm) &
                CRYPTO_TFM_REQ_MAY_SLEEP;
 
        bs = crypto_shash_blocksize(child_shash);
        ds = crypto_shash_digestsize(child_shash);
        BUG_ON(ds > N2_HASH_KEY_MAX);
        if (keylen > bs) {
-               err = crypto_shash_digest(&desc.shash, key, keylen,
+               err = crypto_shash_digest(shash, key, keylen,
                                          ctx->hash_key);
                if (err)
                        return err;