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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date: Mon, 18 Dec 2023 21:52:52 -0800
Message-ID: <CAEf4BzaKyARO0E+9N7myGoFbbQLMh14=6bnJ2x7ejSFpZQPMyA@mail.gmail.com>
Subject: Re: [PATCH bpf-next v4 3/3] selftests/bpf: add testcase to
 verifier_bounds.c for JMP_NE
To: Menglong Dong <menglong8.dong@gmail.com>
Cc: andrii@kernel.org, eddyz87@gmail.com, yonghong.song@linux.dev, 
	alexei.starovoitov@gmail.com, ast@kernel.org, daniel@iogearbox.net, 
	john.fastabend@gmail.com, martin.lau@linux.dev, song@kernel.org, 
	kpsingh@kernel.org, sdf@google.com, haoluo@google.com, jolsa@kernel.org, 
	bpf@vger.kernel.org, linux-kernel@vger.kernel.org
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On Mon, Dec 18, 2023 at 6:27=E2=80=AFPM Menglong Dong <menglong8.dong@gmail=
.com> wrote:
>
> On Tue, Dec 19, 2023 at 2:03=E2=80=AFAM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Sun, Dec 17, 2023 at 5:18=E2=80=AFAM Menglong Dong <menglong8.dong@g=
mail.com> wrote:
> > >
> > > Add testcase for the logic that the verifier tracks the BPF_JNE for r=
egs.
> > > The assembly function "reg_not_equal()" that we add is exactly conver=
ted
> > > from the following case:
> > >
> > >   u32 a =3D bpf_get_prandom_u32();
> > >   u64 b =3D 0;
> > >
> > >   a %=3D 8;
> > >   /* the "a > 0" here will be optimized to "a !=3D 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@gmail.com>
> > > ---
> > >  .../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 =3D=3D 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?
>

I'd leave it in the code, next to the function itself

> >
> > > diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/to=
ols/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_%=3D:   r0 =3D 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 =3D=3D 0` :) so maybe call the test
> > reg_equal_const or something. And then add similar test where you
> > actually have `r4 !=3D 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 =3D r1;                                        \
> > > +       r1 =3D 0;                                         \
> > > +       *(u64*)(r10 - 8) =3D r1;                          \
> > > +       call %[bpf_get_prandom_u32];                    \
> > > +       r4 =3D r0;                                        \
> > > +       r4 &=3D 7;                                        \
> > > +       if r4 =3D=3D 0 goto l0_%=3D;                          \
> > > +       r1 =3D r6;                                        \
> > > +       r2 =3D 0;                                         \
> > > +       r3 =3D r10;                                       \
> > > +       r3 +=3D -8;                                       \
> > > +       r5 =3D 0;                                         \
> > > +       call %[bpf_skb_store_bytes];                    \
> > > +l0_%=3D: r0 =3D 0;                                         \
> > > +       exit;                                           \
> > > +"      :
> > > +       : __imm(bpf_get_prandom_u32),
> > > +         __imm(bpf_skb_store_bytes)
> > > +       : __clobber_all);
> > > +}
> > > +
> > >  char _license[] SEC("license") =3D "GPL";
> > > --
> > > 2.39.2
> > >