Hey and I apologize for the late reply! On 9/15/2021 11:19 PM, Rob Sherwood wrote:> Definitely not an expert but no one has replied so I'll throw out my guess :-)
>> Check out https://lwn.net/Articles/794934/ for more info on 'bounded loops', but my guess is that the verifier doesn't have enough context to verify your loop is really bounded.
>> One trick might be to convert your while loop to a for(;;) loop, e.g., instead of :
>> https://github.com/gamemann/XDP-TCP-Header-Options/blob/master/src/xdp_prog.c#L81
> while ( optdata < 40) {... }
>
> you could try:
>
> for (optdata = 0; optdata < 40; optdata ++) { ... }
>
> I know from past attempts that just because it's obvious to humans
that there's not an infinite loop, it's not always obvious to the verifier.
> > Hope that helps (and is correct!), > > - Rob > .I did try a for loop before, but still ran into the same BPF verifier error :( I tried adding checks to prevent an infinite loop along with checks to make sure it doesn't go outside of ctx->data_end or below ctx->data, but no change. Thank you!
On 9/16/2021 5:32 AM, Toke Høiland-Jørgensen wrote: > Christian Deacon <gamemann@xxxxxxxxxxx> writes: > >> Hi everyone, >> >> >> I wasn't sure whether to submit this under XDP's mailing list or BPF's. >> However, since it's an XDP program, I figured I'd start here. The issue >> has to do with the BPF verifier, though. >> >> >> I am trying to parse TCP header options within XDP/BPF. In my case, I >> want to locate the 'timestamps' option and read/write to the sender and >> receive timestamps (the option's data, which is eight bytes in total I >> believe). > > We're doing just this in the 'pping' utility, see code here:> https://github.com/xdp-project/bpf-examples/blob/master/pping/pping_kern.c#L83
> > -Toke This code works great for me so far! Thank you!
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