Re: [PATCH bpf-next v4 3/3] selftests/bpf: add testcase to verifier_bounds.c for JMP_NE

[Menglong Dong]

On Tue, Dec 19, 2023 at 2:03 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Sun, Dec 17, 2023 at 5:18 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
> >
> > Add testcase for the logic that the verifier tracks the BPF_JNE for regs.
> > The assembly function "reg_not_equal()" that we add is exactly converted
> > from the following case:
> >
> >   u32 a = bpf_get_prandom_u32();
> >   u64 b = 0;
> >
> >   a %= 8;
> >   /* the "a > 0" here will be optimized to "a != 0" */
> >   if (a > 0) {
> >     /* now the range of a should be [1, 7] */
> >     bpf_skb_store_bytes(skb, 0, &b, a, 0);
> >   }
> >
> > Signed-off-by: Menglong Dong <menglong8.dong@xxxxxxxxx>
> > ---
> >  .../selftests/bpf/progs/verifier_bounds.c     | 27 +++++++++++++++++++
> >  1 file changed, 27 insertions(+)
> >
>
> LGTM, but please add a comment that we rely on bpf_skb_store_byte's
> 4th argument being defined as ARG_CONST_SIZE, so zero is not allowed.
> And that r4 == 0 check is providing us this exclusion of zero from
> initial [0, 7] range.
>

Okay, sounds great! BTW, should I add such a comment to the
commit log or to the assembly function?

>
> > diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
> > index ec430b71730b..3fe2ce2b3f21 100644
> > --- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
> > +++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
> > @@ -1075,4 +1075,31 @@ l0_%=:   r0 = 0;                                         \
> >         : __clobber_all);
> >  }
> >
> > +SEC("tc")
> > +__description("bounds check with JMP_NE for reg edge")
> > +__success __retval(0)
> > +__naked void reg_not_equal(void)
>
> technically, you are testing `r4 == 0` :) so maybe call the test
> reg_equal_const or something. And then add similar test where you
> actually have `r4 != 0`, called req_no_equal_const?
>

Yeah, that makes sense. I'll add such a test in the next version.

Thanks!
Menglong Dong

> > +{
> > +       asm volatile ("                                 \
> > +       r6 = r1;                                        \
> > +       r1 = 0;                                         \
> > +       *(u64*)(r10 - 8) = r1;                          \
> > +       call %[bpf_get_prandom_u32];                    \
> > +       r4 = r0;                                        \
> > +       r4 &= 7;                                        \
> > +       if r4 == 0 goto l0_%=;                          \
> > +       r1 = r6;                                        \
> > +       r2 = 0;                                         \
> > +       r3 = r10;                                       \
> > +       r3 += -8;                                       \
> > +       r5 = 0;                                         \
> > +       call %[bpf_skb_store_bytes];                    \
> > +l0_%=: r0 = 0;                                         \
> > +       exit;                                           \
> > +"      :
> > +       : __imm(bpf_get_prandom_u32),
> > +         __imm(bpf_skb_store_bytes)
> > +       : __clobber_all);
> > +}
> > +
> >  char _license[] SEC("license") = "GPL";
> > --
> > 2.39.2
> >