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References: <20231217131716.830290-1-menglong8.dong@gmail.com> <20231217131716.830290-4-menglong8.dong@gmail.com>
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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date: Mon, 18 Dec 2023 10:03:06 -0800
Message-ID: <CAEf4BzZc3edO35FJwxgRscE4n5_qkpwQOJXjUAYjjfWwLkcANg@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 Sun, Dec 17, 2023 at 5:18=E2=80=AFAM Menglong Dong <menglong8.dong@gmail=
.com> 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 =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.


> 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_%=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?

> +{
> +       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
>