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From: Rusty Russell <rusty@rustcorp.com.au>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
	virtualization <virtualization@lists.osdl.org>,
	jgarzik <jgarzik@pobox.com>
Subject: Re: [PATCH 5/8] lguest: trivial guest network driver
Date: Tue, 13 Feb 2007 13:15:18 +1100	[thread overview]
Message-ID: <1171332919.19842.74.camel@localhost.localdomain> (raw)
In-Reply-To: <20070212155553.GA15627@gondor.apana.org.au>

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);



  reply	other threads:[~2007-02-13  2:16 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-12  3:32 [PATCH 1/7] cleanup: paravirt unhandled fallthrough Rusty Russell
2007-02-12  3:33 ` [PATCH 2/7] cleanup: Initialize esp0 properly all the time Rusty Russell
2007-02-12  3:34   ` [PATCH 3/7] cleanup: Make hvc_console.c compile on non-PowerPC Rusty Russell
2007-02-12  3:35     ` [PATCH 4/7] cleanup: Move mce_disabled to asm/mce.h Rusty Russell
2007-02-12  3:36       ` [PATCH 5/7] cleanup: Rename cpu_gdt_descr and remove extern declaration from smpboot.c Rusty Russell
2007-02-12  3:37         ` [PATCH 6/7] cleanup: Remove extern declaration from mm/discontig.c, put in header Rusty Russell
2007-02-12  3:39           ` [PATCH 7/7] cleanup: make disable_acpi() valid w/o CONFIG_ACPI Rusty Russell
2007-02-12  3:41             ` [PATCH 1/2] lguest preparation: EXPORT_SYMBOL_GPL 5 functions Rusty Russell
2007-02-12  3:42               ` [PATCH 2/2] lguest preparation: expose futex infrastructure: get_futex_key, get_key_refs and drop_key_refs Rusty Russell
2007-02-12  3:44                 ` [PATCH 1/8] lguest: Kconfig and headers Rusty Russell
2007-02-12  3:46                   ` [PATCH 2/8] lguest: the host code (lg.ko) Rusty Russell
2007-02-12  3:48                     ` [PATCH 3/8] lguest: Guest code Rusty Russell
2007-02-12  3:50                       ` [PATCH 4/8] lguest: Makefile Rusty Russell
2007-02-12  3:52                         ` [PATCH 5/8] lguest: trivial guest network driver Rusty Russell
2007-02-12  3:53                           ` [PATCH 6/8] lguest: trivial guest console driver Rusty Russell
2007-02-12  3:54                             ` [PATCH 7/8] lguest: trivial guest block driver Rusty Russell
2007-02-12  3:55                               ` [PATCH 8/8] lguest: documentatation and example launcher Rusty Russell
2007-02-12  4:43                               ` [PATCH 7/8] lguest: trivial guest block driver Jens Axboe
2007-02-12  5:27                                 ` Rusty Russell
2007-02-12  5:32                                   ` Jens Axboe
2007-02-12  5:33                                     ` Jens Axboe
2007-02-12  7:09                                     ` Rusty Russell
2007-02-12 15:01                                       ` Jens Axboe
2007-02-13  0:25                                         ` Rusty Russell
2007-02-13  0:44                                           ` Jens Axboe
2007-02-12 15:55                           ` [PATCH 5/8] lguest: trivial guest network driver Herbert Xu
2007-02-13  2:15                             ` Rusty Russell [this message]
2007-02-13 14:06                               ` Herbert Xu
2007-02-14  4:47                                 ` Rusty Russell
2007-02-14 13:57                                   ` Herbert Xu
2007-02-14 23:00                                     ` Rusty Russell
2007-02-12 16:02                   ` [PATCH 1/8] lguest: Kconfig and headers James Morris
2007-02-13  5:09             ` [PATCH 7/7] cleanup: make disable_acpi() valid w/o CONFIG_ACPI Len Brown
2007-02-12  9:16         ` [PATCH 5/7] cleanup: Rename cpu_gdt_descr and remove extern declaration from smpboot.c Zachary Amsden

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