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

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Message-ID: <CAEf4BzZc3edO35FJwxgRscE4n5_qkpwQOJXjUAYjjfWwLkcANg@mail.gmail.com>
Date: Mon, 18 Dec 2023 10:03:06 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: andrii@...nel.org, eddyz87@...il.com, yonghong.song@...ux.dev, 
	alexei.starovoitov@...il.com, ast@...nel.org, daniel@...earbox.net, 
	john.fastabend@...il.com, martin.lau@...ux.dev, song@...nel.org, 
	kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org, 
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v4 3/3] selftests/bpf: add testcase to
 verifier_bounds.c for JMP_NE

On Sun, Dec 17, 2023 at 5:18 AM Menglong Dong <menglong8.dong@...il.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 = 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@...il.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 == 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_%=:   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?

> +{
> +       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
>