Re: XDP Software Issue - Payload Matching

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Christian Deacon <gamemann@gflclan.com> writes:

> Hey Toke,
>
>
> I apologize for the long delay on this. A lot has been going on recently!
>
>
> I attempted to match payload data using the packet's payload as the BPF 
> map key. Unfortunately, I didn't have any success with this. I stored my 
> findings here from last month:
>
>
> https://github.com/gamemann/XDP-Dynamic-Payload-Matching#section-methodfour-fail
>
>
> I'd assume I may be missing something here, though.
>
>
> I saw another XDP mailing list thread pop up recently regarding matching 
> TCP payload data. I believe this may be what they're trying to achieve 
> (being able to match dynamic payload data with XDP).
>
>
> I was wondering if you had any other ideas on how we can match packet 
> payload data against a BPF map.

That error ("invalid stack type R2 off=-16 access_size=150") comes from
this check in the verifier:

	if (off >= 0 || off < -MAX_BPF_STACK || off + access_size > 0 ||
	    access_size < 0 || (access_size == 0 && !zero_size_allowed)) {
		if (tnum_is_const(reg->var_off)) {
			verbose(env, "invalid stack type R%d off=%d access_size=%d\n",
				regno, off, access_size);
		} else {
                ..
                }
                return -EACCESS;
        }

which I think means that you're trying to use a 10-byte value as a
lookup key for a map that has a 150-byte key, which would make the
map key read through the end of the stack.

So basically, if you change

uint8_t hashkey[10];

to

uint8_t hashkey[150];

I think it ought to work?

-Toke