On Wed, Dec 18, 2019 at 11:02:39AM -0500, Willem de Bruijn wrote:

On Wed, Dec 18, 2019 at 8:35 AM Steffen Klassert
[off-list ref] wrote:

quoted

+struct sk_buff *skb_segment_list(struct sk_buff *skb,
+                                netdev_features_t features,
+                                unsigned int offset)
+{
+       struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
+       unsigned int tnl_hlen = skb_tnl_header_len(skb);
+       unsigned int delta_truesize = 0;
+       unsigned int delta_len = 0;
+       struct sk_buff *tail = NULL;
+       struct sk_buff *nskb;
+
+       skb_push(skb, -skb_network_offset(skb) + offset);
+
+       skb_shinfo(skb)->frag_list = NULL;
+
+       do {
+               nskb = list_skb;
+               list_skb = list_skb->next;
+
+               if (!tail)
+                       skb->next = nskb;
+               else
+                       tail->next = nskb;
+
+               tail = nskb;
+
+               delta_len += nskb->len;
+               delta_truesize += nskb->truesize;
+
+               skb_push(nskb, -skb_network_offset(nskb) + offset);
+
+               if (!secpath_exists(nskb))
+                       __skb_ext_copy(nskb, skb);

Of all the possible extensions, why is this only relevant to secpath?

I wrote this before we had these extensions and adjusted it
when the extensions where introduced to make it compile again.
I think we can just copy the extensions unconditionally.

More in general, this function open codes a variety of skb fields that
carry over from skb to nskb. How did you select this subset of fields?

I tried to find the subset of __copy_skb_header() that is needed to copy.
Some fields of nskb are still valid, and some (csum related) fields
should not be copied from skb to nskb.

quoted

+
+               memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
+
+               nskb->tstamp = skb->tstamp;
+               nskb->dev = skb->dev;
+               nskb->queue_mapping = skb->queue_mapping;
+
+               nskb->mac_len = skb->mac_len;
+               nskb->mac_header = skb->mac_header;
+               nskb->transport_header = skb->transport_header;
+               nskb->network_header = skb->network_header;
+               skb_dst_copy(nskb, skb);
+
+               skb_headers_offset_update(nskb, skb_headroom(nskb) - skb_headroom(skb));
+               skb_copy_from_linear_data_offset(skb, -tnl_hlen,
+                                                nskb->data - tnl_hlen,
+                                                offset + tnl_hlen);
+
+               if (skb_needs_linearize(nskb, features) &&
+                   __skb_linearize(nskb))
+                       goto err_linearize;
+
+       } while (list_skb);
+
+       skb->truesize = skb->truesize - delta_truesize;
+       skb->data_len = skb->data_len - delta_len;
+       skb->len = skb->len - delta_len;
+       skb->ip_summed = nskb->ip_summed;
+       skb->csum_level = nskb->csum_level;

This changed from the previous version, where nskb inherited ip_summed
and csum_level from skb. Why is that?

I had to set ip_summed to CHECKSUM_UNNECESSARY on GRO to
make sure the noone touches the checksum of the head
skb. Otherise netfilter etc. tries to touch the csum.

Before chaining I make sure that ip_summed and csum_level is
the same for all chained skbs and here I restore the original
value from nskb.