RE: [PATCH bpf-next] selftests/bpf: Make sure libbpf doesn't enforce the signature of a func pointer.

[John Fastabend <john.fastabend@gmail.com>]

Kui-Feng Lee wrote:
> The verifier in the kernel checks the signatures of struct_ops
> operators. Libbpf should not verify it in order to allow flexibility in
> loading different implementations of an operator with different signatures
> to try to comply with the kernel, even if the signature defined in the BPF
> programs does not match with the implementations and the kernel.
> 
> This feature enables user space applications to manage the variations
> between different versions of the kernel by attempting various
> implementations of an operator.

What is the utility of this? I'm missing what difference it would be
if libbpf rejected vs kernel rejecting it? For backwards compat the
kernel will fail or libbpf might throw an error and user will have to
fixup signature regardless right? Why not get the error as early as
possible.

> 
> This is a follow-up of the commit c911fc61a7ce ("libbpf: Skip zeroed or
> null fields if not found in the kernel type.")
> 
> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
> ---
>  .../bpf/prog_tests/test_struct_ops_module.c   | 24 +++++++++++++++++++
>  .../selftests/bpf/progs/struct_ops_module.c   | 13 ++++++++++
>  2 files changed, 37 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
> index 098776d00ab4..7cf2b9ddd3e1 100644
> --- a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
> +++ b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
> @@ -138,11 +138,35 @@ static void test_struct_ops_not_zeroed(void)
>  	struct_ops_module__destroy(skel);
>  }
>  
> +/* The signature of an implementation might not match the signature of the
> + * function pointer prototype defined in the BPF program. This mismatch
> + * should be allowed as long as the behavior of the operator program
> + * adheres to the signature in the kernel. Libbpf should not enforce the
> + * signature; rather, let the kernel verifier handle the enforcement.
> + */
> +static void test_struct_ops_incompatible(void)
> +{
> +	struct struct_ops_module *skel;
> +	struct bpf_link *link;
> +
> +	skel = struct_ops_module__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "open_and_load"))
> +		return;
> +
> +	link = bpf_map__attach_struct_ops(skel->maps.testmod_incompatible);
> +	if (ASSERT_OK_PTR(link, "attach_struct_ops"))
> +		bpf_link__destroy(link);
> +
> +	struct_ops_module__destroy(skel);
> +}
> +
>  void serial_test_struct_ops_module(void)
>  {
>  	if (test__start_subtest("test_struct_ops_load"))
>  		test_struct_ops_load();
>  	if (test__start_subtest("test_struct_ops_not_zeroed"))
>  		test_struct_ops_not_zeroed();
> +	if (test__start_subtest("test_struct_ops_incompatible"))
> +		test_struct_ops_incompatible();
>  }
>  
> diff --git a/tools/testing/selftests/bpf/progs/struct_ops_module.c b/tools/testing/selftests/bpf/progs/struct_ops_module.c
> index 86e1e50c5531..63b065dae002 100644
> --- a/tools/testing/selftests/bpf/progs/struct_ops_module.c
> +++ b/tools/testing/selftests/bpf/progs/struct_ops_module.c
> @@ -68,3 +68,16 @@ struct bpf_testmod_ops___zeroed testmod_zeroed = {
>  	.test_1 = (void *)test_1,
>  	.test_2 = (void *)test_2_v2,
>  };
> +
> +struct bpf_testmod_ops___incompatible {
> +	int (*test_1)(void);
> +	void (*test_2)(int *a);
> +	int data;
> +};
> +
> +SEC(".struct_ops.link")
> +struct bpf_testmod_ops___incompatible testmod_incompatible = {
> +	.test_1 = (void *)test_1,
> +	.test_2 = (void *)test_2,
> +	.data = 3,
> +};
> -- 
> 2.34.1
> 
>