Re: [PATCH bpf-next 01/16] bpf: Allow kfuncs return 'void *'

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Feb 6, 2024 at 2:04 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> From: Alexei Starovoitov <ast@kernel.org>
>
> Recognize return of 'void *' from kfunc as returning unknown scalar.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
>  kernel/bpf/verifier.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index ddaf09db1175..d9c2dbb3939f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -12353,6 +12353,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>                                         meta.func_name);
>                                 return -EFAULT;
>                         }
> +               } else if (btf_type_is_void(ptr_type)) {
> +                       /* kfunc returning 'void *' is equivalent to returning scalar */
> +                       mark_reg_unknown(env, regs, BPF_REG_0);

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

I think we should do a similar extension when passing `void *` into
global funcs. It's best to treat it as SCALAR instead of rejecting it
because we can't calculate the size. Currently users in practice just
have to define it as `uintptr_t` and then cast (or create static
wrappers doing the casting). Anyways, my point is that it makes sense
to treat `void *` as non-pointer.

>                 } else if (!__btf_type_is_struct(ptr_type)) {
>                         if (!meta.r0_size) {
>                                 __u32 sz;
> --
> 2.34.1
>