On Tue, Jul 18, 2023 at 11:11 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote: > > On Tue, 18 Jul 2023 10:50:14 -0700 Alexei Starovoitov wrote: > > On Tue, Jul 18, 2023 at 10:18 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote: > > > > you're still missing the point. Pls read the whole patch series. > > > > > > Could you just tell me what the point is then? The "series" is one > > > patch plus some tiny selftests. I don't see any documentation for > > > how dynptrs are supposed to work either. > > > > > > As far as I can grasp this makes the "copy buffer" optional from > > > the kfunc-API perspective (of bpf_dynptr_slice()). > > > > > > > It is _not_ input validation. > > > > skb_copy_bits is a slow path. One extra check doesn't affect > > > > performance at all. So 'fast paths' isn't a valid argument here. > > > > The code is reusing > > > > if (likely(hlen - offset >= len)) > > > > return (void *)data + offset; > > > > which _is_ the fast path. > > > > > > > > What you're requesting is to copy paste > > > > the whole __skb_header_pointer into __skb_header_pointer2. > > > > Makes no sense. > > > > > > No, Alexei, the whole point of skb_header_pointer() is to pass > > > the secondary buffer, to make header parsing dependable. > > > > of course. No one argues about that. > > > > > Passing NULL buffer to skb_header_pointer() is absolutely nonsensical. > > > > Quick grep through the code proves you wrong: > > drivers/net/ethernet/broadcom/bnxt/bnxt.c > > __skb_header_pointer(NULL, start, sizeof(*hp), skb->data, > > skb_headlen(skb), NULL); > > > > was done before this patch. It's using __ variant on purpose > > and explicitly passing skb==NULL to exactly trigger that line > > to deliberately avoid the slow path. > > > > Another example: > > drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c > > skb_header_pointer(skb, 0, 0, NULL); > > > > This one I'm not sure about. Looks buggy. > > These are both Tx path for setting up offloads, Linux doesn't request > offloads for headers outside of the linear part. The ixgbevf code is > completely pointless, as you say. > > In general drivers are rarely a source of high quality code examples. > Having been directly involved in the bugs that lead to the bnxt code > being written - I was so happy that the driver started parsing Tx > packets *at all*, so I wasn't too fussed by the minor problems :( > > > > It should *not* be supported. We had enough prod problems with people > > > thinking that the entire header will be in the linear portion. > > > Then either the NIC can't parse the header, someone enables jumbo, > > > disables GRO, adds new HW, adds encap, etc etc and things implode. > > > > I don't see how this is related. > > NULL buffer allows to get a linear pointer and explicitly avoids > > slow path when it's not linear. > > Direct packet access via skb->data is there for those who want high > speed 🤷️ skb->data/data_end approach unfortunately doesn't work that well. Too much verifier fighting. That's why dynptr was introduced. > > > > If you want to support it in BPF that's up to you, but I think it's > > > entirely reasonable for me to request that you don't do such things > > > in general networking code. The function is 5 LoC, so a local BPF > > > copy seems fine. Although I'd suggest skb_header_pointer_misguided() > > > rather than __skb_header_pointer2() as the name :) > > > > If you insist we can, but bnxt is an example that buffer==NULL is > > a useful concept for networking and not bpf specific. > > It also doesn't make "people think the header is linear" any worse. > > My worry is that people will think that whether the buffer is needed or > not depends on _their program_, rather than on the underlying platform. > So if it works in testing without the buffer - the buffer must not be > required for their use case. Are you concerned about bpf progs breaking this way? I thought you're worried about the driver misusing skb_header_pointer() with buffer==NULL. We can remove !buffer check as in the attached patch, but I don't quite see how it would improve driver quality.
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0001-bpf-net-Introduce-skb_pointer_if_linear.patch
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