[bpf-next,v5,4/4] selftests/bpf: add testcase to verifier_bounds.c for BPF_JNE

Add testcase for the logic that the verifier tracks the BPF_JNE for regs.
The assembly function "reg_not_equal_const()" and "reg_equal_const" 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@gmail.com>
---
v5:
- add some comments to the function that we add
- add reg_not_equal_const()
---
 .../selftests/bpf/progs/verifier_bounds.c     | 62 +++++++++++++++++++
 1 file changed, 62 insertions(+)

On Tue, Dec 19, 2023 at 5:51 AM 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_const()" and "reg_equal_const" 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@gmail.com>
> ---
> v5:
> - add some comments to the function that we add
> - add reg_not_equal_const()
> ---
>  .../selftests/bpf/progs/verifier_bounds.c     | 62 +++++++++++++++++++
>  1 file changed, 62 insertions(+)
>

LGTM

Acked-by: Andrii Nakryiko <andrii@kernel.org>

> diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
> index ec430b71730b..960998f16306 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
> @@ -1075,4 +1075,66 @@ l0_%=:   r0 = 0;                                         \
>         : __clobber_all);
>  }
>
> +SEC("tc")
> +__description("bounds check with JMP_NE for reg edge")
> +__success __retval(0)
> +__naked void reg_not_equal_const(void)
> +{
> +       asm volatile ("                                 \
> +       r6 = r1;                                        \
> +       r1 = 0;                                         \
> +       *(u64*)(r10 - 8) = r1;                          \
> +       call %[bpf_get_prandom_u32];                    \
> +       r4 = r0;                                        \
> +       r4 &= 7;                                        \
> +       if r4 != 0 goto l0_%=;                          \
> +       r0 = 0;                                         \
> +       exit;                                           \
> +l0_%=: r1 = r6;                                        \
> +       r2 = 0;                                         \
> +       r3 = r10;                                       \
> +       r3 += -8;                                       \
> +       r5 = 0;                                         \
> +       /* The 4th argument of bpf_skb_store_bytes is defined as \
> +        * ARG_CONST_SIZE, so 0 is not allowed. The 'r4 != 0' \
> +        * is providing us this exclusion of zero from initial \
> +        * [0, 7] range.                                \
> +        */                                             \
> +       call %[bpf_skb_store_bytes];                    \
> +       r0 = 0;                                         \
> +       exit;                                           \
> +"      :
> +       : __imm(bpf_get_prandom_u32),
> +         __imm(bpf_skb_store_bytes)
> +       : __clobber_all);
> +}
> +
> +SEC("tc")
> +__description("bounds check with JMP_EQ for reg edge")
> +__success __retval(0)
> +__naked void reg_equal_const(void)
> +{
> +       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;                                         \
> +       /* Just the same as what we do in reg_not_equal_const() */ \
> +       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
>