On Wed, May 5, 2021 at 10:27 PM Dongseok Yi <dseok.yi@xxxxxxxxxxx> wrote: > > On Wed, May 05, 2021 at 09:45:37PM -0400, Willem de Bruijn wrote: > > On Wed, May 5, 2021 at 8:45 PM Dongseok Yi <dseok.yi@xxxxxxxxxxx> wrote: > > > > > > On Wed, May 05, 2021 at 10:55:10PM +0200, Daniel Borkmann wrote: > > > > On 4/29/21 12:08 PM, Dongseok Yi wrote: > > > > > tcp_gso_segment check for the size of GROed payload if it is bigger > > > > > than the mss. bpf_skb_proto_6_to_4 increases mss, but the mss can be > > > > > bigger than the size of GROed payload unexpectedly if data_len is not > > > > > big enough. > > > > > > > > > > Assume that skb gso_size = 1372 and data_len = 8. bpf_skb_proto_6_to_4 > > > > Is this a typo and is this intended to read skb->data_len = 1380? > > This is not a typo. I intended skb->data_len = 8. > > > > > The issue is that payload length (1380) is greater than mss with ipv6 > > (1372), but less than mss with ipv4 (1392). > > > > I don't understand data_len = 8 or why the patch compares > > skb->data_len to len_diff (20). > > skb_gro_receive(): > unsigned int len = skb_gro_len(skb); > [...] > done: > NAPI_GRO_CB(p)->count++; > p->data_len += len; > > head_skb's data_len is the sum of skb_gro_len for each skb of the frags. > data_len could be 8 if server sent a small size packet and it is GROed > to head_skb. > > Please let me know if I am missing something. This is my understanding of the data path. This is a forwarding path for TCP traffic. GRO is enabled and will coalesce multiple segments into a single large packet. In bad cases, the coalesced packet payload is > MSS, but < MSS + 20. Somewhere between GRO and GSO you have a BPF program that converts the IPv6 address to IPv4. There is no concept of head_skb at the time of this BPF program. It is a single SKB, with an skb linear part and multiple data items in the frags (no frag_list). When entering the GSO stack, this single skb now has a payload length < MSS. So it would just make a valid TCP packet on its own? skb_gro_len is only relevant inside the GRO stack. It internally casts the skb->cb[] to NAPI_GRO_CB. This field is a scratch area that may be reused for other purposes later by other layers of the datapath. It is not safe to read this inside bpf_skb_proto_6_to_4. > > > > One simple solution if this packet no longer needs to be segmented > > might be to reset the gso_type completely. > > I am not sure gso_type can be cleared even when GSO is needed. > > > > > In general, I would advocate using BPF_F_ADJ_ROOM_FIXED_GSO. When > > converting from IPv6 to IPv4, fixed gso will end up building packets > > that are slightly below the MTU. That opportunity cost is negligible > > (especially with TSO). Unfortunately, I see that that flag is > > available for bpf_skb_adjust_room but not for bpf_skb_proto_6_to_4. > > > > > > > > > would increse the gso_size to 1392. tcp_gso_segment will get an error > > > > > with 1380 <= 1392. > > > > > > > > > > Check for the size of GROed payload if it is really bigger than target > > > > > mss when increase mss. > > > > > > > > > > Fixes: 6578171a7ff0 (bpf: add bpf_skb_change_proto helper) > > > > > Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx> > > > > > --- > > > > > net/core/filter.c | 4 +++- > > > > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > > > > > > > diff --git a/net/core/filter.c b/net/core/filter.c > > > > > index 9323d34..3f79e3c 100644 > > > > > --- a/net/core/filter.c > > > > > +++ b/net/core/filter.c > > > > > @@ -3308,7 +3308,9 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb) > > > > > } > > > > > > > > > > /* Due to IPv4 header, MSS can be upgraded. */ > > > > > - skb_increase_gso_size(shinfo, len_diff); > > > > > + if (skb->data_len > len_diff) > > > > > > > > Could you elaborate some more on what this has to do with data_len specifically > > > > here? I'm not sure I follow exactly your above commit description. Are you saying > > > > that you're hitting in tcp_gso_segment(): > > > > > > > > [...] > > > > mss = skb_shinfo(skb)->gso_size; > > > > if (unlikely(skb->len <= mss)) > > > > goto out; > > > > [...] > > > > > > Yes, right > > > > > > > > > > > Please provide more context on the bug, thanks! > > > > > > tcp_gso_segment(): > > > [...] > > > __skb_pull(skb, thlen); > > > > > > mss = skb_shinfo(skb)->gso_size; > > > if (unlikely(skb->len <= mss)) > > > [...] > > > > > > skb->len will have total GROed TCP payload size after __skb_pull. > > > skb->len <= mss will not be happened in a normal GROed situation. But > > > bpf_skb_proto_6_to_4 would upgrade MSS by increasing gso_size, it can > > > hit an error condition. > > > > > > We should ensure the following condition. > > > total GROed TCP payload > the original mss + (IPv6 size - IPv4 size) > > > > > > Due to > > > total GROed TCP payload = the original mss + skb->data_len > > > IPv6 size - IPv4 size = len_diff > > > > > > Finally, we can get the condition. > > > skb->data_len > len_diff > > > > > > > > > > > > + skb_increase_gso_size(shinfo, len_diff); > > > > > + > > > > > /* Header must be checked, and gso_segs recomputed. */ > > > > > shinfo->gso_type |= SKB_GSO_DODGY; > > > > > shinfo->gso_segs = 0; > > > > > > > > > > > >
