On Sun, Dec 17, 2023 at 5:18 AM Menglong Dong <menglong8.dong@xxxxxxxxx> 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@xxxxxxxxx>
> ---
> .../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
>
