Subject: Re: [PATCH] ebpf: emit correct src_reg for conditional jumps

From: Daniel Borkmann <[email-masked]>
Date: Fri, 11 Sep 2015 10:45:29 +0200

On 09/11/2015 02:25 AM, Tycho Andersen wrote:
> Instead of always emitting BPF_REG_X, let's emit BPF_REG_X only when the
> source actually is BPF_X. This causes programs generated by the classic
> converter to not be importable via bpf(), as the eBPF verifier checks that
> the src_reg is correct or 0. While not a problem yet, this will be a
> problem when BPF_PROG_DUMP lands, and we can potentially dump and re-import
> programs generated by the converter.
>
> Signed-off-by: Tycho Andersen <[email-masked]>
> CC: Alexei Starovoitov <[email-masked]>
> CC: Daniel Borkmann <[email-masked]>

I think the description at the beginning could have been a bit clearer. I.e.
it's safe to zero insn->src_reg for BPF_SRC(fp->code) that is not X, because
we can either have X or K for classic jump compares, and in case of K we're
only interested in the immediate value, but not a possible src_reg, of course,
so eBPF converter should have zeroed it.

Yeah, it was never really the aim of the converter to cover something like
that requirement you seem to have:

   load classic BPF
     -> transform into eBPF
       -> dump that eBPF result to uspace
         -> load that eBPF via bpf(2)

Anyway, it doesn't cause an issue in the current code as stated, but it's good
if we fix it up nevertheless.

Looks good to me:

Acked-by: Daniel Borkmann <[email-masked]>