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

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

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

quoted

What happens if we have a large metadata that occupies all headroom here?

Thanks

Do you mean a large "XDP" metadata? If a large metadata is a large "XDP" metadata, I think we can not use a metadata that occupies all headroom. The size of metadata limited by bpf_xdp_adjust_meta() as below.
bpf_xdp_adjust_meta() in net/core/filter.c:
	if (unlikely((metalen & (sizeof(__u32) - 1)) ||
		     (metalen > 32)))
		return -EACCES;

Thanks.

quoted

quoted

          act = bpf_prog_run_xdp(xdp_prog, &xdp);
          stats->xdp_packets++;
  @@ -695,9 +704,11 @@ static struct sk_buff *receive_small(struct net_device *dev,
              /* Recalculate length in case bpf program changed it */
              delta = orig_data - xdp.data;
              len = xdp.data_end - xdp.data;
+            metasize = xdp.data - xdp.data_meta;
              break;
          case XDP_TX:
              stats->xdp_tx++;
+            xdp.data_meta = xdp.data;
              xdpf = convert_to_xdp_frame(&xdp);
              if (unlikely(!xdpf))
                  goto err_xdp;

@@ -736,10 +747,12 @@ static struct sk_buff *receive_small(struct net_device *dev,

      skb_reserve(skb, headroom - delta);
      skb_put(skb, len);
      if (!delta) {
-        buf += header_offset;
-        memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);
+        memcpy(skb_vnet_hdr(skb), buf + VIRTNET_RX_PAD, vi->hdr_len);
      } /* keep zeroed vnet hdr since packet was changed by bpf */
  +    if (metasize)
+        skb_metadata_set(skb, metasize);
+
  err:
      return skb;
  @@ -760,8 +773,8 @@ static struct sk_buff *receive_big(struct net_device *dev,
                     struct virtnet_rq_stats *stats)
  {
      struct page *page = buf;
-    struct sk_buff *skb = page_to_skb(vi, rq, page, 0, len,
-                      PAGE_SIZE, true);
+    struct sk_buff *skb =
+        page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, true, 0);
        stats->bytes += len - vi->hdr_len;
      if (unlikely(!skb))

@@ -793,6 +806,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

      unsigned int truesize;
      unsigned int headroom = mergeable_ctx_to_headroom(ctx);
      int err;
+    unsigned int metasize = 0;
        head_skb = NULL;
      stats->bytes += len - vi->hdr_len;

@@ -839,8 +853,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

          data = page_address(xdp_page) + offset;
          xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;
          xdp.data = data + vi->hdr_len;
-        xdp_set_data_meta_invalid(&xdp);
          xdp.data_end = xdp.data + (len - vi->hdr_len);
+        xdp.data_meta = xdp.data;
          xdp.rxq = &rq->xdp_rxq;
            act = bpf_prog_run_xdp(xdp_prog, &xdp);

@@ -852,8 +866,9 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

               * adjustments. Note other cases do not build an
               * skb and avoid using offset
               */
-            offset = xdp.data -
-                    page_address(xdp_page) - vi->hdr_len;
+            metasize = xdp.data - xdp.data_meta;
+            offset = xdp.data - page_address(xdp_page) -
+                 vi->hdr_len - metasize;
                /* recalculate len if xdp.data or xdp.data_end were
               * adjusted

@@ -863,14 +878,15 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

              if (unlikely(xdp_page != page)) {
                  rcu_read_unlock();
                  put_page(page);
-                head_skb = page_to_skb(vi, rq, xdp_page,
-                               offset, len,
-                               PAGE_SIZE, false);
+                head_skb = page_to_skb(vi, rq, xdp_page, offset,
+                               len, PAGE_SIZE, false,
+                               metasize);
                  return head_skb;
              }
              break;
          case XDP_TX:
              stats->xdp_tx++;
+            xdp.data_meta = xdp.data;
              xdpf = convert_to_xdp_frame(&xdp);
              if (unlikely(!xdpf))
                  goto err_xdp;

@@ -921,7 +937,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

          goto err_skb;
      }
  -    head_skb = page_to_skb(vi, rq, page, offset, len, truesize, !xdp_prog);
+    head_skb = page_to_skb(vi, rq, page, offset, len, truesize, !xdp_prog,
+                   metasize);
      curr_skb = head_skb;
        if (unlikely(!curr_skb))