Re: [PATCH bpf-next v3] virtio_net: add XDP meta data support

On 07/09/2019 05:04 AM, Jason Wang wrote:

On 2019/7/9 上午6:38, Daniel Borkmann wrote:

quoted

On 07/02/2019 04:11 PM, Yuya Kusakabe wrote:

quoted

On 7/2/19 5:33 PM, Jason Wang wrote:

quoted

On 2019/7/2 下午4:16, Yuya Kusakabe wrote:

quoted

This adds XDP meta data support to both receive_small() and
receive_mergeable().

Fixes: de8f3a83b0a0 ("bpf: add meta pointer for direct access")
Signed-off-by: Yuya Kusakabe <redacted>
---
v3:
   - fix preserve the vnet header in receive_small().
v2:
   - keep copy untouched in page_to_skb().
   - preserve the vnet header in receive_small().
   - fix indentation.
---
   drivers/net/virtio_net.c | 45 +++++++++++++++++++++++++++-------------
   1 file changed, 31 insertions(+), 14 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 4f3de0ac8b0b..03a1ae6fe267 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -371,7 +371,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,

                      struct receive_queue *rq,
                      struct page *page, unsigned int offset,
                      unsigned int len, unsigned int truesize,
-                   bool hdr_valid)
+                   bool hdr_valid, unsigned int metasize)
   {
       struct sk_buff *skb;
       struct virtio_net_hdr_mrg_rxbuf *hdr;

@@ -393,7 +393,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,

       else
           hdr_padded_len = sizeof(struct padded_vnet_hdr);
   -    if (hdr_valid)
+    if (hdr_valid && !metasize)
           memcpy(hdr, p, hdr_len);
         len -= hdr_len;

@@ -405,6 +405,11 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,

           copy = skb_tailroom(skb);
       skb_put_data(skb, p, copy);
   +    if (metasize) {
+        __skb_pull(skb, metasize);
+        skb_metadata_set(skb, metasize);
+    }
+
       len -= copy;
       offset += copy;
   @@ -644,6 +649,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
       unsigned int delta = 0;
       struct page *xdp_page;
       int err;
+    unsigned int metasize = 0;
         len -= vi->hdr_len;
       stats->bytes += len;

@@ -683,10 +689,13 @@ static struct sk_buff *receive_small(struct net_device *dev,

             xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
           xdp.data = xdp.data_hard_start + xdp_headroom;
-        xdp_set_data_meta_invalid(&xdp);
           xdp.data_end = xdp.data + len;
+        xdp.data_meta = xdp.data;
           xdp.rxq = &rq->xdp_rxq;
           orig_data = xdp.data;
+        /* Copy the vnet header to the front of data_hard_start to avoid
+         * overwriting by XDP meta data */
+        memcpy(xdp.data_hard_start - vi->hdr_len, xdp.data - vi->hdr_len, vi->hdr_len);

I'm not fully sure if I'm following this one correctly, probably just missing
something. Isn't the vnet header based on how we set up xdp.data_hard_start
earlier already in front of it? Wouldn't we copy invalid data from xdp.data -
vi->hdr_len into the vnet header at that point (given there can be up to 256
bytes of headroom between the two)? If it's relative to xdp.data and headroom
is >0, then BPF prog could otherwise mangle this; something doesn't add up to
me here. Could you clarify? Thx

Vnet headr sits just in front of xdp.data not xdp.data_hard_start. So it could be overwrote by metadata, that's why we need a copy here.

For the current code, you can adjust the xdp.data with a positive/negative offset
already via bpf_xdp_adjust_head() helper. If vnet headr sits just in front of
xdp.data, couldn't this be overridden today as well then? Anyway, just wondering
how this is handled differently?

Thanks,
Daniel