On 2021/5/7 16:25, Dongseok Yi wrote: > On Thu, May 06, 2021 at 09:53:45PM -0400, Willem de Bruijn wrote: >> 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. > > I think clearing the gso type will get an error at tcp4_gso_segment > because netif_needs_gso returns true in validate_xmit_skb. So the bpf_skb_proto_6_to_4() is called after validate_xmit_skb() and before tcp4_gso_segment()? If Yes, clearing the gso type here does not seem to help. > >> >>> 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. > > BPF program is usually out of control. Can we take a general approach? > The below 2 cases has no issue when mss upgrading. > 1) skb->data_len > mss + 20 > 2) skb->data_len < mss && skb->data_len > 20 > The corner case is when > 3) skb->data_len > mss && skb->data_len < mss + 20 As my understanding: Usually skb_headlen(skb) >= (mac header + ip/ipv6 header + udp/tcp header), other than that, there is no other guarantee as long as: skb->len = skb_headlen(skb) + skb->data_len So the cases should be: 1. skb->len - (mac header + ip/ipv6 header + udp/tcp header) > mss + len_diff 2. skb->len - (mac header + ip/ipv6 header + udp/tcp header) <= mss + len_diff The corner case is case 2. > > But to cover #3 case, we should check the condition Yunsheng Lin said. > What if we do mss upgrading for both #1 and #2 cases only? > > + unsigned short off_len = skb->data_len > shinfo->gso_size ? > + shinfo->gso_size : 0; > [...] > /* Due to IPv4 header, MSS can be upgraded. */ > - skb_increase_gso_size(shinfo, len_diff); > + if (skb->data_len - off_len > len_diff) > + skb_increase_gso_size(shinfo, 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; >>>>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>> >>>>> >>>> >>>> . >>>> >>> > > > . >
