Re: [PATCH v0 3/3] selftests/bpf: Extend tests with more coverage for sign extension

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2025-01-30 at 13:23 +0200, Dimitar Kanaliev wrote:
> This commit adds a few more tests related to tnum_scast that explicitly
> check cases with known / unknown sign bit, as well as values that cross
> zero (going from negative to positive).
> 
> Signed-off-by: Dimitar Kanaliev <dimitar.kanaliev@siteground.com>
> ---
>  .../selftests/bpf/progs/verifier_movsx.c      | 73 +++++++++++++++++++
>  1 file changed, 73 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/progs/verifier_movsx.c b/tools/testing/selftests/bpf/progs/verifier_movsx.c
> index 994bbc346d25..20abeec09dee 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_movsx.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_movsx.c
> @@ -327,6 +327,79 @@ label_%=: 	                                        \
>  	: __clobber_all);
>  }
>  
> +SEC("socket")
> +__description("MOV64SX, S8, unknown value")
> +__success __success_unpriv __retval(1)

Note: __retval() annotation is needed when one wants to execute the
      test using libbpf's bpf_prog_test_run_opts().
      The changes for register range tracking should not affect
      runtime behaviour (unless there is a bug in and some dead code
      elimination is done incorrectly).
      I suggest to add __log_level(2) annotation and check verifier
      log output using __msg() annotations to check what range is
      inferred for specific registers.
      As-is these new tests are passing on master as well,
      so the feature is effectively untested.

> +__naked void mov64sx_s8_unknown(void)
> +{
> +	asm volatile ("                                      \
> +	call %[bpf_get_prandom_u32];                         \
> +	r1 = r0;                                             \
> +	r1 &= 0xFF;      			             \
> +	r1 = (s8)r1;  					     \
> +	if r1 s>= -128 goto l0_%=;                           \
> +	r0 = 0;                                              \
> +	exit;                                                \
> +l0_%=:                                                       \
> +	if r1 s<= 127 goto l1_%=;                            \
> +	r0 = 0;                                              \
> +	exit;                                                \
> +l1_%=:                                                       \
> +	r0 = 1;                                              \
> +	exit;                                                \
> +"	:
> +	: __imm(bpf_get_prandom_u32)
> +	: __clobber_all);
> +}

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