Re: [PATCH bpf-next v2 0/3] Add XDP support for bpf_load_hdr_opt

[Martin KaFai Lau <kafai@fb.com>]

On Sat, Oct 09, 2021 at 12:20:27AM +0200, Toke Høiland-Jørgensen wrote:
> So if we can't fix the verifier, maybe we could come up with a more
> general helper for packet parsing? Something like:
> 
> bpf_for_each_pkt_chunk(ctx, offset, callback_fn, callback_arg)
> {
>   ptr = ctx->data + offset;
>   while (ptr < ctx->data_end) {
>     offset = callback_fn(ptr, ctx->data_end, callback_arg);
>     if (offset == 0)
>       return 0;
>     ptr += offset;
>   }
>   
>   // out of bounds before callback was done
>   return -EINVAL;
> }
>    
> This would work for parsing any kind of packet header or TLV-style data
> without having to teach the kernel about each header type. It'll have
> quite a bit of overhead if all the callbacks happen via indirect calls,
> but maybe the verifier can inline the calls (or at least turn them into
> direct CALL instructions)?
Direct call different callback_fn?  bpf_for_each_pkt_chunk() is a kernel
function.  It would be nice if the verifier could do that.

This for_each helper had been considered also.  Other than the need
to callback in a loop, the thought was to extend the existing
bpf_load_hdr_opt() because our initial feedback is the same
header handling logic cannot be used in xdp which is confusing.

I don't mind to go with the for_each helper.  However, with another
thought, if it needs to call a function in the loop anyway, I think
it could also be done in bpf by putting a global function in a loop.
Need to try and double check.