Re: [PATCH v2 bpf-next 1/3] bpf: Support for pointers beyond pkt_end.

Alexei Starovoitov
2020-11-12T23:59:39Z

On Thu, Nov 12, 2020 at 11:16:11AM -0800, John Fastabend wrote:
> Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@kernel.org>
> >
> > This patch adds the verifier support to recognize inlined branch conditions.
> > The LLVM knows that the branch evaluates to the same value, but the verifier
> > couldn't track it. Hence causing valid programs to be rejected.
> > The potential LLVM workaround: https://reviews.llvm.org/D87428
> > can have undesired side effects, since LLVM doesn't know that
> > skb->data/data_end are being compared. LLVM has to introduce extra boolean
> > variable and use inline_asm trick to force easier for the verifier assembly.
> >
> > Instead teach the verifier to recognize that
> > r1 = skb->data;
> > r1 += 10;
> > r2 = skb->data_end;
> > if (r1 > r2) {
> > here r1 points beyond packet_end and
> > subsequent
> > if (r1 > r2) // always evaluates to "true".
> > }
> >
> > Tested-by: Jiri Olsa <jolsa@redhat.com>
> > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > ---
> > include/linux/bpf_verifier.h | 2 +-
> > kernel/bpf/verifier.c | 129 +++++++++++++++++++++++++++++------
> > 2 files changed, 108 insertions(+), 23 deletions(-)
> >
>
> Thanks, we can remove another set of inline asm logic.

Awesome! Please contribute your C examples to selftests when possible.

> Acked-by: John Fastabend <john.fastabend@gmail.com>
>
> > if (pred >= 0) {
> > @@ -7517,7 +7601,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
> > */
> > if (!__is_pointer_value(false, dst_reg))
> > err = mark_chain_precision(env, insn->dst_reg);
> > - if (BPF_SRC(insn->code) == BPF_X && !err)
> > + if (BPF_SRC(insn->code) == BPF_X && !err &&
> > + !__is_pointer_value(false, src_reg))
>
> This could have been more specific with !type_is_pkt_pointer() correct? I
> think its fine as is though.

I actually meant to use __is_pointer_value() here for two reasons:
1. to match dst_reg check just few lines above.
2. mark_chain_precision() is for scalars only. If in the future
is_*_branch_taken() will support other kinds of pointers the more
precise !type_is_pkt_pointer() check would need to be modified.
That would be unnecessary code churn.

Thanks for the review!