[PATCH 0/2 v3] Open vSwitch zerocopy upcall

[PATCH 0/2 v3] Open vSwitch zerocopy upcall

[Thomas Graf]

Respin of the zerocopy patches for the openvswitch upcall.

V3: - Removed unneeded alignment of nlmsg_len after padding 
V2: - Added skb_zerocopy_headlen() to calculate headroom of destination
      buffer. This also takes care of the from->head_frag issue.
    - Attribute alignment for frag_list case
    - API documentation
    - performance data for CHECKSUM_PARTIAL tx case


Thomas Graf (2):
  net: Export skb_zerocopy() to zerocopy from one skb to another
  openvswitch: Use skb_zerocopy() for upcall

 include/linux/skbuff.h               |  3 ++
 net/core/skbuff.c                    | 85 ++++++++++++++++++++++++++++++++++++
 net/netfilter/nfnetlink_queue_core.c | 59 ++-----------------------
 net/openvswitch/datapath.c           | 52 +++++++++++++++++++---
 4 files changed, 137 insertions(+), 62 deletions(-)

-- 
1.8.3.1

[PATCH 1/2 net-next] net: Export skb_zerocopy() to zerocopy from one skb to another

[Thomas Graf]

Make the skb zerocopy logic written for nfnetlink queue available for
use by other modules.

Signed-off-by: Thomas Graf <tgraf-G/eBtMaohhA@public.gmane.org>
---
 include/linux/skbuff.h               |  3 ++
 net/core/skbuff.c                    | 85 ++++++++++++++++++++++++++++++++++++
 net/netfilter/nfnetlink_queue_core.c | 59 ++-----------------------
 3 files changed, 92 insertions(+), 55 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 2e153b6..bf26616 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2371,6 +2371,9 @@ int skb_splice_bits(struct sk_buff *skb, unsigned int offset,
 		    struct pipe_inode_info *pipe, unsigned int len,
 		    unsigned int flags);
 void skb_copy_and_csum_dev(const struct sk_buff *skb, u8 *to);
+extern unsigned int    skb_zerocopy_headlen(const struct sk_buff *from);
+extern void	       skb_zerocopy(struct sk_buff *to, const struct sk_buff *from,
+				    int len, int hlen);
 void skb_split(struct sk_buff *skb, struct sk_buff *skb1, const u32 len);
 int skb_shift(struct sk_buff *tgt, struct sk_buff *skb, int shiftlen);
 void skb_scrub_packet(struct sk_buff *skb, bool xnet);
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 3735fad..ab00e0b 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2102,6 +2102,91 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 }
 EXPORT_SYMBOL(skb_copy_and_csum_bits);
 
+ /**
+ *	skb_zerocopy_headlen - Calculate headroom needed for skb_zerocopy()
+ *	@from: source buffer
+ *
+ *	Calculates the amount of linear headroom needed in the 'to' skb passed
+ *	into skb_zerocopy().
+ */
+unsigned int
+skb_zerocopy_headlen(const struct sk_buff *from)
+{
+	unsigned int hlen = 0;
+
+	if (!from->head_frag ||
+	    skb_headlen(from) < L1_CACHE_BYTES ||
+	    skb_shinfo(from)->nr_frags >= MAX_SKB_FRAGS)
+		hlen = skb_headlen(from);
+
+	if (skb_has_frag_list(from))
+		hlen = from->len;
+
+	return hlen;
+}
+EXPORT_SYMBOL_GPL(skb_zerocopy_headlen);
+
+/**
+ *	skb_zerocopy - Zero copy skb to skb
+ *	@to: destination buffer
+ *	@source: source buffer
+ *	@len: number of bytes to copy from source buffer
+ *	@hlen: size of linear headroom in destination buffer
+ *
+ *	Copies up to `len` bytes from `from` to `to` by creating references
+ *	to the frags in the source buffer.
+ *
+ *	The `hlen` as calculated by skb_zerocopy_headlen() specifies the
+ *	headroom in the `to` buffer.
+ */
+void
+skb_zerocopy(struct sk_buff *to, const struct sk_buff *from, int len, int hlen)
+{
+	int i, j = 0;
+	int plen = 0; /* length of skb->head fragment */
+	struct page *page;
+	unsigned int offset;
+
+	BUG_ON(!from->head_frag && !hlen);
+
+	/* dont bother with small payloads */
+	if (len <= skb_tailroom(to)) {
+		skb_copy_bits(from, 0, skb_put(to, len), len);
+		return;
+	}
+
+	if (hlen) {
+		skb_copy_bits(from, 0, skb_put(to, hlen), hlen);
+		len -= hlen;
+	} else {
+		plen = min_t(int, skb_headlen(from), len);
+		if (plen) {
+			page = virt_to_head_page(from->head);
+			offset = from->data - (unsigned char *)page_address(page);
+			__skb_fill_page_desc(to, 0, page, offset, plen);
+			get_page(page);
+			j = 1;
+			len -= plen;
+		}
+	}
+
+	to->truesize += len + plen;
+	to->len += len + plen;
+	to->data_len += len + plen;
+
+	for (i = 0; i < skb_shinfo(from)->nr_frags; i++) {
+		if (!len)
+			break;
+		skb_shinfo(to)->frags[j] = skb_shinfo(from)->frags[i];
+		skb_shinfo(to)->frags[j].size = min_t(int, skb_shinfo(to)->frags[j].size, len);
+		len -= skb_shinfo(to)->frags[j].size;
+		skb_frag_ref(to, j);
+		j++;
+	}
+	skb_shinfo(to)->nr_frags = j;
+}
+EXPORT_SYMBOL_GPL(skb_zerocopy);
+
 void skb_copy_and_csum_dev(const struct sk_buff *skb, u8 *to)
 {
 	__wsum csum;
diff --git a/net/netfilter/nfnetlink_queue_core.c b/net/netfilter/nfnetlink_queue_core.c
index 21258cf..615ee12 100644
--- a/net/netfilter/nfnetlink_queue_core.c
+++ b/net/netfilter/nfnetlink_queue_core.c
@@ -235,51 +235,6 @@ nfqnl_flush(struct nfqnl_instance *queue, nfqnl_cmpfn cmpfn, unsigned long data)
 	spin_unlock_bh(&queue->lock);
 }
 
-static void
-nfqnl_zcopy(struct sk_buff *to, const struct sk_buff *from, int len, int hlen)
-{
-	int i, j = 0;
-	int plen = 0; /* length of skb->head fragment */
-	struct page *page;
-	unsigned int offset;
-
-	/* dont bother with small payloads */
-	if (len <= skb_tailroom(to)) {
-		skb_copy_bits(from, 0, skb_put(to, len), len);
-		return;
-	}
-
-	if (hlen) {
-		skb_copy_bits(from, 0, skb_put(to, hlen), hlen);
-		len -= hlen;
-	} else {
-		plen = min_t(int, skb_headlen(from), len);
-		if (plen) {
-			page = virt_to_head_page(from->head);
-			offset = from->data - (unsigned char *)page_address(page);
-			__skb_fill_page_desc(to, 0, page, offset, plen);
-			get_page(page);
-			j = 1;
-			len -= plen;
-		}
-	}
-
-	to->truesize += len + plen;
-	to->len += len + plen;
-	to->data_len += len + plen;
-
-	for (i = 0; i < skb_shinfo(from)->nr_frags; i++) {
-		if (!len)
-			break;
-		skb_shinfo(to)->frags[j] = skb_shinfo(from)->frags[i];
-		skb_shinfo(to)->frags[j].size = min_t(int, skb_shinfo(to)->frags[j].size, len);
-		len -= skb_shinfo(to)->frags[j].size;
-		skb_frag_ref(to, j);
-		j++;
-	}
-	skb_shinfo(to)->nr_frags = j;
-}
-
 static int
 nfqnl_put_packet_info(struct sk_buff *nlskb, struct sk_buff *packet,
 		      bool csum_verify)
@@ -304,7 +259,7 @@ nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 {
 	size_t size;
 	size_t data_len = 0, cap_len = 0;
-	int hlen = 0;
+	unsigned int hlen = 0;
 	struct sk_buff *skb;
 	struct nlattr *nla;
 	struct nfqnl_msg_packet_hdr *pmsg;
@@ -356,14 +311,8 @@ nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 		if (data_len > entskb->len)
 			data_len = entskb->len;
 
-		if (!entskb->head_frag ||
-		    skb_headlen(entskb) < L1_CACHE_BYTES ||
-		    skb_shinfo(entskb)->nr_frags >= MAX_SKB_FRAGS)
-			hlen = skb_headlen(entskb);
-
-		if (skb_has_frag_list(entskb))
-			hlen = entskb->len;
-		hlen = min_t(int, data_len, hlen);
+		hlen = skb_zerocopy_headlen(entskb);
+		hlen = min_t(unsigned int, hlen, data_len);
 		size += sizeof(struct nlattr) + hlen;
 		cap_len = entskb->len;
 		break;
@@ -504,7 +453,7 @@ nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 		nla->nla_type = NFQA_PAYLOAD;
 		nla->nla_len = nla_attr_size(data_len);
 
-		nfqnl_zcopy(skb, entskb, data_len, hlen);
+		skb_zerocopy(skb, entskb, data_len, hlen);
 	}
 
 	nlh->nlmsg_len = skb->len;
-- 
1.8.3.1

[PATCH 2/2 net-next] openvswitch: Use skb_zerocopy() for upcall

[Thomas Graf]

Use of skb_zerocopy() avoids the expensive call to memcpy() when
copying the packet data into the Netlink skb. Completes checksum
through skb_checksum_help() if needed.

Netlink messaged must be properly padded and aligned to meet
sanity checks of the user space counterpart.

Cost of memcpy is significantly reduced from:
+   7.48%       vhost-8471  [k] memcpy
+   5.57%     ovs-vswitchd  [k] memcpy
+   2.81%       vhost-8471  [k] csum_partial_copy_generic

to:
+   5.72%     ovs-vswitchd  [k] memcpy
+   3.32%       vhost-5153  [k] memcpy
+   0.68%       vhost-5153  [k] skb_zerocopy

(megaflows disabled)

Signed-off-by: Thomas Graf <tgraf@suug.ch>
---
 net/openvswitch/datapath.c | 52 +++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 45 insertions(+), 7 deletions(-)

diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index 1408adc..3f170e3 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -381,10 +381,11 @@ static size_t key_attr_size(void)
 }
 
 static size_t upcall_msg_size(const struct sk_buff *skb,
-			      const struct nlattr *userdata)
+			      const struct nlattr *userdata,
+			      unsigned int hdrlen)
 {
 	size_t size = NLMSG_ALIGN(sizeof(struct ovs_header))
-		+ nla_total_size(skb->len) /* OVS_PACKET_ATTR_PACKET */
+		+ nla_total_size(hdrlen) /* OVS_PACKET_ATTR_PACKET */
 		+ nla_total_size(key_attr_size()); /* OVS_PACKET_ATTR_KEY */
 
 	/* OVS_PACKET_ATTR_USERDATA */
@@ -402,6 +403,7 @@ static int queue_userspace_packet(struct net *net, int dp_ifindex,
 	struct sk_buff *nskb = NULL;
 	struct sk_buff *user_skb; /* to be queued to userspace */
 	struct nlattr *nla;
+	unsigned int plen, hlen;
 	int err;
 
 	if (vlan_tx_tag_present(skb)) {
@@ -422,7 +424,13 @@ static int queue_userspace_packet(struct net *net, int dp_ifindex,
 		goto out;
 	}
 
-	user_skb = genlmsg_new(upcall_msg_size(skb, upcall_info->userdata), GFP_ATOMIC);
+	/* Complete checksum if needed */
+	if (skb->ip_summed == CHECKSUM_PARTIAL &&
+	    (err = skb_checksum_help(skb)))
+		goto out;
+
+	hlen = skb_zerocopy_headlen(skb);
+	user_skb = genlmsg_new(upcall_msg_size(skb, upcall_info->userdata, hlen), GFP_ATOMIC);
 	if (!user_skb) {
 		err = -ENOMEM;
 		goto out;
@@ -441,13 +449,43 @@ static int queue_userspace_packet(struct net *net, int dp_ifindex,
 			  nla_len(upcall_info->userdata),
 			  nla_data(upcall_info->userdata));
 
-	nla = __nla_reserve(user_skb, OVS_PACKET_ATTR_PACKET, skb->len);
+	/* Only reserve room for attribute header, packet data is added
+	 * in skb_zerocopy() */
+	if (!(nla = nla_reserve(user_skb, OVS_PACKET_ATTR_PACKET, 0)))
+		goto out;
+	nla->nla_len = nla_attr_size(skb->len);
 
-	skb_copy_and_csum_dev(skb, nla_data(nla));
+	skb_zerocopy(user_skb, skb, skb->len, hlen);
 
-	genlmsg_end(user_skb, upcall);
-	err = genlmsg_unicast(net, user_skb, upcall_info->portid);
+	/* OVS user space expects the size of the message to be aligned to
+	 * NLA_ALIGNTO. Aligning nlmsg_len is not enough, the actual bytes
+	 * read must match nlmsg_len.
+	 */
+	plen = NLA_ALIGN(user_skb->len) - user_skb->len;
+	if (plen > 0) {
+		int nr_frags = skb_shinfo(user_skb)->nr_frags;
+
+		if (nr_frags) {
+			skb_frag_t *frag;
+
+			frag = &skb_shinfo(user_skb)->frags[nr_frags -1];
+			skb_frag_size_add(frag, plen);
+			BUG_ON(frag->size > PAGE_SIZE);
+
+			user_skb->truesize += plen;
+			user_skb->len += plen;
+			user_skb->data_len += plen;
+		} else {
+			/* The linear headroom is aligned to NLMSG_ALIGN by
+			 * genlmsg_new(), room must be available.
+			 */
+			memset(skb_put(user_skb, plen), 0, plen);
+		}
+	}
 
+	((struct nlmsghdr *) user_skb->data)->nlmsg_len = user_skb->len;
+
+	err = genlmsg_unicast(net, user_skb, upcall_info->portid);
 out:
 	kfree_skb(nskb);
 	return err;
-- 
1.8.3.1

Re: [PATCH 2/2 net-next] openvswitch: Use skb_zerocopy() for upcall

[Ben Hutchings]

On Fri, 2013-11-08 at 10:15 +0100, Thomas Graf wrote:
> Use of skb_zerocopy() avoids the expensive call to memcpy() when
> copying the packet data into the Netlink skb. Completes checksum
> through skb_checksum_help() if needed.
> 
> Netlink messaged must be properly padded and aligned to meet
> sanity checks of the user space counterpart.
> 
> Cost of memcpy is significantly reduced from:
> +   7.48%       vhost-8471  [k] memcpy
> +   5.57%     ovs-vswitchd  [k] memcpy
> +   2.81%       vhost-8471  [k] csum_partial_copy_generic
> 
> to:
> +   5.72%     ovs-vswitchd  [k] memcpy
> +   3.32%       vhost-5153  [k] memcpy
> +   0.68%       vhost-5153  [k] skb_zerocopy
> 
> (megaflows disabled)
> 
> Signed-off-by: Thomas Graf <tgraf@suug.ch>
> ---
>  net/openvswitch/datapath.c | 52 +++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 45 insertions(+), 7 deletions(-)
> 
> diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
> index 1408adc..3f170e3 100644
> --- a/net/openvswitch/datapath.c
> +++ b/net/openvswitch/datapath.c
[...]
> @@ -441,13 +449,43 @@ static int queue_userspace_packet(struct net *net, int dp_ifindex,
>  			  nla_len(upcall_info->userdata),
>  			  nla_data(upcall_info->userdata));
>  
> -	nla = __nla_reserve(user_skb, OVS_PACKET_ATTR_PACKET, skb->len);
> +	/* Only reserve room for attribute header, packet data is added
> +	 * in skb_zerocopy() */
> +	if (!(nla = nla_reserve(user_skb, OVS_PACKET_ATTR_PACKET, 0)))
> +		goto out;
> +	nla->nla_len = nla_attr_size(skb->len);
>  
> -	skb_copy_and_csum_dev(skb, nla_data(nla));
> +	skb_zerocopy(user_skb, skb, skb->len, hlen);
>  
> -	genlmsg_end(user_skb, upcall);
> -	err = genlmsg_unicast(net, user_skb, upcall_info->portid);
> +	/* OVS user space expects the size of the message to be aligned to
> +	 * NLA_ALIGNTO. Aligning nlmsg_len is not enough, the actual bytes
> +	 * read must match nlmsg_len.
> +	 */
> +	plen = NLA_ALIGN(user_skb->len) - user_skb->len;
> +	if (plen > 0) {
> +		int nr_frags = skb_shinfo(user_skb)->nr_frags;
> +
> +		if (nr_frags) {
> +			skb_frag_t *frag;
> +
> +			frag = &skb_shinfo(user_skb)->frags[nr_frags -1];
> +			skb_frag_size_add(frag, plen);

It looks like this is effectively padding with whatever happens to
follow the original packet content.  This could result in a small
information leak.  If the fragment has non-zero offset and already
extends to the end of a page, this could result in a segfault as the
next page may be unmapped.

Perhaps you could add the padding as an extra fragment pointing to a
preallocated zero page.  If the skb already has the maximum number of
fragments, you would have to copy the last fragment in order to add
padding.

> +			BUG_ON(frag->size > PAGE_SIZE);
[...]

I'm not sure that's a reasonable assumption either.  We certainly allow
fragments to be larger than PAGE_SIZE in the transmit path.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

Re: [PATCH 2/2 net-next] openvswitch: Use skb_zerocopy() for upcall

[Thomas Graf]

On 11/09/13 at 10:02pm, Ben Hutchings wrote:
> On Fri, 2013-11-08 at 10:15 +0100, Thomas Graf wrote:
> > Use of skb_zerocopy() avoids the expensive call to memcpy() when
> > copying the packet data into the Netlink skb. Completes checksum
> > through skb_checksum_help() if needed.
> > 
> > Netlink messaged must be properly padded and aligned to meet
> > sanity checks of the user space counterpart.
> > 
> > Cost of memcpy is significantly reduced from:
> > +   7.48%       vhost-8471  [k] memcpy
> > +   5.57%     ovs-vswitchd  [k] memcpy
> > +   2.81%       vhost-8471  [k] csum_partial_copy_generic
> > 
> > to:
> > +   5.72%     ovs-vswitchd  [k] memcpy
> > +   3.32%       vhost-5153  [k] memcpy
> > +   0.68%       vhost-5153  [k] skb_zerocopy
> > 
> > (megaflows disabled)
> > 
> > Signed-off-by: Thomas Graf <tgraf-G/eBtMaohhA@public.gmane.org>
> > ---
> >  net/openvswitch/datapath.c | 52 +++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 45 insertions(+), 7 deletions(-)
> > 
> > diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
> > index 1408adc..3f170e3 100644
> > --- a/net/openvswitch/datapath.c
> > +++ b/net/openvswitch/datapath.c
> [...]
> > @@ -441,13 +449,43 @@ static int queue_userspace_packet(struct net *net, int dp_ifindex,
> >  			  nla_len(upcall_info->userdata),
> >  			  nla_data(upcall_info->userdata));
> >  
> > -	nla = __nla_reserve(user_skb, OVS_PACKET_ATTR_PACKET, skb->len);
> > +	/* Only reserve room for attribute header, packet data is added
> > +	 * in skb_zerocopy() */
> > +	if (!(nla = nla_reserve(user_skb, OVS_PACKET_ATTR_PACKET, 0)))
> > +		goto out;
> > +	nla->nla_len = nla_attr_size(skb->len);
> >  
> > -	skb_copy_and_csum_dev(skb, nla_data(nla));
> > +	skb_zerocopy(user_skb, skb, skb->len, hlen);
> >  
> > -	genlmsg_end(user_skb, upcall);
> > -	err = genlmsg_unicast(net, user_skb, upcall_info->portid);
> > +	/* OVS user space expects the size of the message to be aligned to
> > +	 * NLA_ALIGNTO. Aligning nlmsg_len is not enough, the actual bytes
> > +	 * read must match nlmsg_len.
> > +	 */
> > +	plen = NLA_ALIGN(user_skb->len) - user_skb->len;
> > +	if (plen > 0) {
> > +		int nr_frags = skb_shinfo(user_skb)->nr_frags;
> > +
> > +		if (nr_frags) {
> > +			skb_frag_t *frag;
> > +
> > +			frag = &skb_shinfo(user_skb)->frags[nr_frags -1];
> > +			skb_frag_size_add(frag, plen);
> 
> It looks like this is effectively padding with whatever happens to
> follow the original packet content.  This could result in a small
> information leak.  If the fragment has non-zero offset and already
> extends to the end of a page, this could result in a segfault as the
> next page may be unmapped.
> 
> Perhaps you could add the padding as an extra fragment pointing to a
> preallocated zero page.  If the skb already has the maximum number of
> fragments, you would have to copy the last fragment in order to add
> padding.

You are right and thanks for the review Ben.

Realizing how complex this becomes I'm leaning towards avoiding
padding alltogether by fixing OVS user space to no longer enforce
it, signal this capability via a flag to the kernel and only
perform zerocopy for enabled OVS user space counterparts.

Re: [PATCH 2/2 net-next] openvswitch: Use skb_zerocopy() for upcall

[Jesse Gross]

On Mon, Nov 11, 2013 at 7:40 AM, Thomas Graf <tgraf@suug.ch> wrote:
> On 11/09/13 at 10:02pm, Ben Hutchings wrote:
>> On Fri, 2013-11-08 at 10:15 +0100, Thomas Graf wrote:
>> > Use of skb_zerocopy() avoids the expensive call to memcpy() when
>> > copying the packet data into the Netlink skb. Completes checksum
>> > through skb_checksum_help() if needed.
>> >
>> > Netlink messaged must be properly padded and aligned to meet
>> > sanity checks of the user space counterpart.
>> >
>> > Cost of memcpy is significantly reduced from:
>> > +   7.48%       vhost-8471  [k] memcpy
>> > +   5.57%     ovs-vswitchd  [k] memcpy
>> > +   2.81%       vhost-8471  [k] csum_partial_copy_generic
>> >
>> > to:
>> > +   5.72%     ovs-vswitchd  [k] memcpy
>> > +   3.32%       vhost-5153  [k] memcpy
>> > +   0.68%       vhost-5153  [k] skb_zerocopy
>> >
>> > (megaflows disabled)
>> >
>> > Signed-off-by: Thomas Graf <tgraf@suug.ch>
>> > ---
>> >  net/openvswitch/datapath.c | 52 +++++++++++++++++++++++++++++++++++++++-------
>> >  1 file changed, 45 insertions(+), 7 deletions(-)
>> >
>> > diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
>> > index 1408adc..3f170e3 100644
>> > --- a/net/openvswitch/datapath.c
>> > +++ b/net/openvswitch/datapath.c
>> [...]
>> > @@ -441,13 +449,43 @@ static int queue_userspace_packet(struct net *net, int dp_ifindex,
>> >                       nla_len(upcall_info->userdata),
>> >                       nla_data(upcall_info->userdata));
>> >
>> > -   nla = __nla_reserve(user_skb, OVS_PACKET_ATTR_PACKET, skb->len);
>> > +   /* Only reserve room for attribute header, packet data is added
>> > +    * in skb_zerocopy() */
>> > +   if (!(nla = nla_reserve(user_skb, OVS_PACKET_ATTR_PACKET, 0)))
>> > +           goto out;
>> > +   nla->nla_len = nla_attr_size(skb->len);
>> >
>> > -   skb_copy_and_csum_dev(skb, nla_data(nla));
>> > +   skb_zerocopy(user_skb, skb, skb->len, hlen);
>> >
>> > -   genlmsg_end(user_skb, upcall);
>> > -   err = genlmsg_unicast(net, user_skb, upcall_info->portid);
>> > +   /* OVS user space expects the size of the message to be aligned to
>> > +    * NLA_ALIGNTO. Aligning nlmsg_len is not enough, the actual bytes
>> > +    * read must match nlmsg_len.
>> > +    */
>> > +   plen = NLA_ALIGN(user_skb->len) - user_skb->len;
>> > +   if (plen > 0) {
>> > +           int nr_frags = skb_shinfo(user_skb)->nr_frags;
>> > +
>> > +           if (nr_frags) {
>> > +                   skb_frag_t *frag;
>> > +
>> > +                   frag = &skb_shinfo(user_skb)->frags[nr_frags -1];
>> > +                   skb_frag_size_add(frag, plen);
>>
>> It looks like this is effectively padding with whatever happens to
>> follow the original packet content.  This could result in a small
>> information leak.  If the fragment has non-zero offset and already
>> extends to the end of a page, this could result in a segfault as the
>> next page may be unmapped.
>>
>> Perhaps you could add the padding as an extra fragment pointing to a
>> preallocated zero page.  If the skb already has the maximum number of
>> fragments, you would have to copy the last fragment in order to add
>> padding.
>
> You are right and thanks for the review Ben.
>
> Realizing how complex this becomes I'm leaning towards avoiding
> padding alltogether by fixing OVS user space to no longer enforce
> it, signal this capability via a flag to the kernel and only
> perform zerocopy for enabled OVS user space counterparts.

It seems like at a minimum it would be a good idea to start by
patching userspace now. That would at least begin to limit the scope
of the problem.