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? 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 > > 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; >>>>>>>> >>>>>> >>>>>> >>>> >> > > . >
