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From: Menglong Dong <menglong8.dong@gmail.com>
Date: Tue, 19 Dec 2023 10:26:59 +0800
Message-ID: <CADxym3bSbxccDSnS1E2ywRMibCOtTb4Mmf0nMMB-YXtO5PonXQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next v4 3/3] selftests/bpf: add testcase to
 verifier_bounds.c for JMP_NE
To: Andrii Nakryiko <andrii.nakryiko@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 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@gma=
il.com> wrote:
> >
> > Add testcase for the logic that the verifier tracks the BPF_JNE for reg=
s.
> > The assembly function "reg_not_equal()" that we add is exactly converte=
d
> > 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?

>
> > diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tool=
s/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
> >