netdev - Re: [PATCH v2 net-next] tcp: md5: use kmalloc() backed scratch areas

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Date:	Tue, 28 Jun 2016 10:35:31 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] tcp: md5: use kmalloc() backed scratch areas

On Mon, Jun 27, 2016 at 8:41 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> On Mon, Jun 27, 2016 at 10:58:42AM -0700, Andy Lutomirski wrote:
>>
>> I wonder if it's worth switching from ahash to shash, though.  It
>> would probably be simpler and faster.
>
> No shash is not appropriate here because it needs to hash skb
> frags which are SG lists.
>

Do you mean this code:

    for (i = 0; i < shi->nr_frags; ++i) {
        const struct skb_frag_struct *f = &shi->frags[i];
        unsigned int offset = f->page_offset;
        struct page *page = skb_frag_page(f) + (offset >> PAGE_SHIFT);

        sg_set_page(&sg, page, skb_frag_size(f),
                offset_in_page(offset));
        ahash_request_set_crypt(req, &sg, NULL, skb_frag_size(f));
        if (crypto_ahash_update(req))
            return 1;
    }

I'm wondering why support for scatterlists is all-or-nothing.  Why
can't we initialize a hash object and then alternate between passing
it scatterlists and pointers?

I'm guessing that ahash enables async operation and shash is
synchronous only.  If I'm right, I understand why ahash requires a
scatterlist.  What I don't understand is why shash can't also accept a
scatterlist.  It appears that most of the ahash users in the tree
actually want synchronous crypto and are presumably using ahash for
some other reason such as ahash's ability to hash via scatterlist (in
this case, struct page *).

--Andy