On Thu, 2020-09-17 at 05:54 -0700, Maciej Żenczykowski wrote: > On Thu, Sep 17, 2020 at 5:39 AM Jesper Dangaard Brouer > <brouer@xxxxxxxxxx> wrote: > > > > As you likely know[1] I'm looking into moving the MTU check (for > > TC-BPF) > > in __bpf_skb_max_len() when e.g. called by bpf_skb_adjust_room(), > > because when redirecting packets to another netdev it is not > > correct to > > limit the MTU based on the incoming netdev. > > > > I was looking at doing the MTU check in bpf_redirect() helper, > > because > > at this point we know the redirect to netdev, and returning an > > indication/error that MTU was exceed, would allow the BPF-prog > > logic to > > react, e.g. sending ICMP (instead of packet getting silently > > dropped). > > BUT this is not possible because bpf_redirect(index, flags) helper > > don't provide the packet context-object (so I cannot lookup the > > packet > > length). > > > > Seeking input: > > > > Should/can we change the bpf_redirect API or create a new helper > > with > > packet-context? > > > > Note: We have the same need for the packet context for XDP when > > redirecting the new multi-buffer packets, as not all destination > > netdev > > will support these new multi-buffer packets. > > > > I can of-cause do the MTU checks on kernel-side in skb_do_redirect, > > but > > then how do people debug this? as packet will basically be silently > > dropped. > > > > > > > > (Looking at how does BPF-prog logic handle MTU today) > > > > How do bpf_skb_adjust_room() report that the MTU was exceeded? > > Unfortunately it uses a common return code -ENOTSUPP which used for > > multiple cases (include MTU exceeded). Thus, the BPF-prog logic > > cannot > > use this reliably to know if this is a MTU exceeded event. (Looked > > BPF-prog code and they all simply exit with TC_ACT_SHOT for all > > error > > codes, cloudflare have the most advanced handling with > > metrics->errors_total_encap_adjust_failed++). > > > > > > [1] > > https://lore.kernel.org/bpf/159921182827.1260200.9699352760916903781.stgit@firesoul/ > > -- > > Best regards, > > Jesper Dangaard Brouer > > MSc.CS, Principal Kernel Engineer at Red Hat > > LinkedIn: http://www.linkedin.com/in/brouer > > > > (a) the current state of the world seems very hard to use correctly, > so adding new apis, > or even changing existing ones seems ok to me. > especially if this just means changing what error code they return > > (b) another complexity with bpf_redirect() is you can call it, it can > succeed, > but then you can not return TC_ACT_REDIRECT from the bpf program, > which effectively makes the earlier *successful* bpf_redirect() call > an utter no-op. > > (bpf_redirect() just determines what a future return TC_ACT_REDIRECT > will do) > > so if you bpf_redirect to interface with larger mtu, then increase > packet size, why would you redirect then touch the packet afterwards ? if you have a bad program, then it is a user issue. > then return TC_ACT_OK, then you potentially end up with excessively > large > packet egressing through original interface (with small mtu). > > My vote would be to return a new distinct error from bpf_redirect() > based on then current > packet size and interface being redirected to, save this interface > mtu > somewhere, > then in operations that increase packet size check against this saved > mtu, > for correctness you still have to check mtu after the bpf program is > done, > but this is then just to deal with braindead bpf code (that calls > bpf_redirect and returns TC_ACT_OK, or calls bpf_redirect() multiple > times, or something...). > Another solution is to have an exception function defined in the BPF_prog, this function by itself is another program that can be executed to notify the prog about any exception/err that happened after the main BPF_program exited and let the XDP program react by its own logic. example: BPF_prog: int XDP_main_prog(xdp_buff) { xdp_adjust_head/tail(xdp_buff); return xdp_redirect(ifindex, flags); } int XDP_exception(xdp_buff, excption_code) { if (excetption_code == XDP_REDIRECRT_MTU_EXCEEDED) { ICMP_response(xdp_buff); return XDP_TX; } return XDP_DROP; } netdev_driver_xdp_handle(): act = bpf_prog_run_xdp(prog, xdp); // Run XDP_main_prog if (act == XDP_REDIRECT) err = xdp_do_redirect(netdev, xdp, prog); if (err) { // Run XDP_exception() function in the user prog // finds the exception handler of active program act = bpf_prog_run_xdp_exciption(prog, xdp, err); // then handle exception action in the driver (XDP_TX/DROP/FORWARD).. } of-course a user program will be notified only on the first err .. if it fails on the 2nd time .. just drop.. -Saeed.
