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SubjectRe: [PATCH 5/8] lguest: trivial guest network driver
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On Tue, 2007-02-13 at 02:55 +1100, Herbert Xu wrote:
> On Mon, Feb 12, 2007 at 02:52:01PM +1100, Rusty Russell wrote:
> >
> > +static void skb_to_dma(const struct sk_buff *skb, unsigned int len,
> > + struct lguest_dma *dma)
> > +{
> > + unsigned int i, seg;
> > +
> > + for (i = seg = 0; i < len; seg++, i += rest_of_page(skb->data + i)) {
>
> The length field from the skb covers the fragmented parts as well.
> So you don't want to use it as the boundary for the loop over the
> skb header data. As it is, if the skb has fragments, the first
> loop will try to read them off the header.

Good spotting! This function needs to be passed skb_headlen(skb),
rather than skb->len. Patch is below (I renamed the parameter as well,
for clarity).

It's fascinating why this "worked". Firstly, for inter-guest
communication, I am not calculating checksums, nor checking them.
Secondly, for guest->host, I turn off hw checksumming so the kernel
disables SG and this code is never executed. Thirdly, for virtbench's
inter-guest sendfile test does not check the data received is actually
correct. Finally, while we end up producing a packet which is larger
than skb->len of 1514, the DMA receive buffer for the other guest is
only 1514 bytes, so the result of the transfer is 1514 (==skb->len), so
no error reported by the driver.

==
lguest network driver fix: handle scatter-gather packets correctly

Thanks to Herbert Xu for noticing the bug: "len" here is skb_headlen(),
not skb->len. Renamed the function to clarify, too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r ed6484d145a4 drivers/net/lguest_net.c
--- a/drivers/net/lguest_net.c Tue Feb 13 11:30:39 2007 +1100
+++ b/drivers/net/lguest_net.c Tue Feb 13 12:07:15 2007 +1100
@@ -59,14 +59,14 @@ static unsigned long peer_addr(struct lg
return info->peer_phys + 4 * peernum;
}

-static void skb_to_dma(const struct sk_buff *skb, unsigned int len,
+static void skb_to_dma(const struct sk_buff *skb, unsigned int headlen,
struct lguest_dma *dma)
{
unsigned int i, seg;

- for (i = seg = 0; i < len; seg++, i += rest_of_page(skb->data + i)) {
+ for (i = seg = 0; i < headlen; seg++, i += rest_of_page(skb->data+i)) {
dma->addr[seg] = virt_to_phys(skb->data + i);
- dma->len[seg] = min((unsigned)(len - i),
+ dma->len[seg] = min((unsigned)(headlen - i),
rest_of_page(skb->data + i));
}
for (i = 0; i < skb_shinfo(skb)->nr_frags; i++, seg++) {
@@ -90,7 +90,7 @@ static void transfer_packet(struct net_d
struct lguestnet_info *info = dev->priv;
struct lguest_dma dma;

- skb_to_dma(skb, skb->len, &dma);
+ skb_to_dma(skb, skb_headlen(skb), &dma);
pr_debug("xfer length %04x (%u)\n", htons(skb->len), skb->len);

hcall(LHCALL_SEND_DMA, peer_addr(info,peernum), __pa(&dma),0);

-
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