On Thu, May 6, 2021 at 9:45 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote: > > On 2021/5/7 9:25, Willem de Bruijn wrote: > >>>> 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. > >> > >> Your understanding is right. The data path is GRO -> BPF 6 to 4 -> > >> GSO. > >> > >>> > >>> 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). > >> > >> Sorry for the confusion. head_skb what I mentioned was a skb linear > >> part. I'm considering a single SKB with frags too. > >> > >>> > >>> 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. > >> > >> The condition what I made uses skb->data_len not skb_gro_len. Does > >> skb->data_len have a different meaning on each layer? As I know, > >> data_len indicates the amount of frags or frag_list. skb->data_len > >> should be > 20 in the sample case because the payload size of the skb > >> linear part is the same with mss. > > > > Ah, got it. > > > > data_len is the length of the skb minus the length in the skb linear > > section (as seen in skb_headlen). > > > > So this gso skb consists of two segments, the first one entirely > > linear, the payload of the second is in skb_shinfo(skb)->frags[0]. > > > > It is not guaranteed that gso skbs built from two individual skbs end > > up looking like that. Only protocol headers in the linear segment and > > the payload of both in frags is common. > > > >> We can modify netif_needs_gso as another option to hit > >> skb_needs_linearize in validate_xmit_skb. But I think we should compare > >> skb->gso_size and skb->data_len too to check if mss exceed a payload > >> size. > > > > The rest of the stack does not build such gso packets with payload len > > < mss, so we should not have to add workarounds in the gso hot path > > for this. > > > > Also no need to linearize this skb. I think that if the bpf program > > would just clear the gso type, the packet would be sent correctly. > > Unless I'm missing something. > > Does the checksum/len field in ip and tcp/udp header need adjusting > before clearing gso type as the packet has became bigger? gro takes care of this. see for instance inet_gro_complete for updates to the ip header. > Also, instead of testing skb->data_len, may test the skb->len? > > skb->len - (mac header + ip/ipv6 header + udp/tcp header) > mss + len_diff Yes. Essentially doing the same calculation as the gso code that is causing the packet to be dropped. > > > > But I don't mean to argue that it should do that in production. > > Instead, not playing mss games would solve this and stay close to the > > original datapath if no bpf program had been present. Including > > maintaining the GSO invariant of sending out the same chain of packets > > as received (bar the IPv6 to IPv4 change). > > > > This could be achieved by adding support for the flag > > BPF_F_ADJ_ROOM_FIXED_GSO in the flags field of bpf_skb_change_proto. > > And similar to bpf_skb_net_shrink: > > > > /* Due to header shrink, MSS can be upgraded. */ > > if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO)) > > skb_increase_gso_size(shinfo, len_diff); > > > > The other case, from IPv4 to IPv6 is more difficult to address, as not > > reducing the MSS will result in packets exceeding MTU. That calls for > > workarounds like MSS clamping. Anyway, that is out of scope here. > > > > > > > >>> > >>> > >>>>> > >>>>> 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; > >>>>>>>> > >>>>>> > >>>>>> > >>>> > >> > > > > . > > >
