Re: [PATCH v4 02/29] rxrpc: Avoid using stack memory in SG lists in rxkad

From: David Howells
Date: Tue Jun 28 2016 - 03:52:31 EST


Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> - skcipher_request_set_crypt(req, &sg[1], &sg[0], sizeof(tmpbuf), iv.x);
> + skcipher_request_set_crypt(req, &sg, &sg, sizeof(tmpbuf), iv.x);

Don't the sg's have to be different? Aren't they both altered by the process
of reading/writing from them?

> struct rxrpc_skb_priv *sp;
> ...
> + swap(tmpbuf.xl, *(__be64 *)sp);
> +
> + sg_init_one(&sg, sp, sizeof(tmpbuf));

???? I assume you're assuming that the rxrpc_skb_priv struct contents can
arbitrarily replaced temporarily...

And using an XCHG-equivalent instruction? This won't work on a 32-bit arch
(apart from one that sports CMPXCHG8 or similar).

> /*
> - * load a scatterlist with a potentially split-page buffer
> + * load a scatterlist
> */
> -static void rxkad_sg_set_buf2(struct scatterlist sg[2],
> +static void rxkad_sg_set_buf2(struct scatterlist sg[1],
> void *buf, size_t buflen)
> {
> - int nsg = 1;
> -
> - sg_init_table(sg, 2);
> -
> + sg_init_table(sg, 1);
> sg_set_buf(&sg[0], buf, buflen);
> - if (sg[0].offset + buflen > PAGE_SIZE) {
> - /* the buffer was split over two pages */
> - sg[0].length = PAGE_SIZE - sg[0].offset;
> - sg_set_buf(&sg[1], buf + sg[0].length, buflen - sg[0].length);
> - nsg++;
> - }
> -
> - sg_mark_end(&sg[nsg - 1]);
> -
> - ASSERTCMP(sg[0].length + sg[1].length, ==, buflen);
> }

This should be a separate patch.

David