Re: [PATCH bpf-next 2/3] bpf: infer packet range for 'if pkt ==/!= pkt_end' comparisons

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-01-08 at 05:49 -0800, Maciej Żenczykowski wrote:
> (I've looked at all 3 patches, and they seem fine... but I don't claim
> to understand the intricacies of the verifier/registers/etc well
> enough to be certain)

Thank you.

> On Mon, Jan 8, 2024 at 5:28 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > 
> > Extend try_match_pkt_pointers() to handle == and != operations.
> > For instruction:
> > 
> >       .--------------- pointer to packet with some range R
> >       |     .--------- pointer to packet end
> >       v     v
> >   if rA == rB goto ...
> 
> it's possible this would be better without the 'goto' as just 'if (rA
> == rB) ...'

The idea was to show this as BPF asm instruction, which has syntax
'if rA == rB goto C'. I'll wait for more feedback and submit v2
with updated commit message to make it more clear.

> > It is valid to infer that R bytes are available in packet.
> > This change should allow verification of BPF generated for
> > C code like below:
> > 
> >   if (data + 42 != data_end) { ... }
> 
> this should probably be:
>   if (data + 42 != data_end) return;
>   ...

Makes sense, will update commit message.