Re: [PATCH bpf-next 1/2] bpf: Allow access to const void pointer arguments in tracing programs

[Jiri Olsa <olsajiri@gmail.com>]

On Sun, Apr 13, 2025 at 01:06:25AM +0800, KaFai Wan wrote:
> Adding support to access arguments with const void pointer arguments
> in tracing programs.
> 
> Currently we allow tracing programs to access void pointers. If we try to
> access argument which is pointer to const void like 2nd argument in kfree,
> verifier will fail to load the program with;
> 
> 0: R1=ctx() R10=fp0
> ; asm volatile ("r2 = *(u64 *)(r1 + 8); ");
> 0: (79) r2 = *(u64 *)(r1 +8)
> func 'kfree' arg1 type UNKNOWN is not a struct
> 
> Adding is_void_ptr to generic void  pointer check.
> 
> Cc: Leon Hwang <leon.hwang@linux.dev>
> Signed-off-by: KaFai Wan <kafai.wan@hotmail.com>
> ---
>  kernel/bpf/btf.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 16ba36f34dfa..e11d3afd0562 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6383,6 +6383,14 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
>  		return prog->aux->attach_btf;
>  }
>  
> +static bool is_void_ptr(struct btf *btf, const struct btf_type *t)
> +{
> +	/* skip modifiers */
> +	t = btf_type_skip_modifiers(btf, t->type, NULL);
> +
> +	return t->type == 0;

I think you can use btf_type_is_void in here

> +}
> +
>  static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
>  {
>  	/* skip modifiers */
> @@ -6776,7 +6784,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>  		}
>  	}
>  
> -	if (t->type == 0)
> +	if (is_void_ptr(btf, t))

lgtm,

nit, the is_void_ptr name suggest there's also ptr check in the helper function,
which is not the case. I understand it follows is_int_ptr name, but perhaps we
could rename both helpers to is_void and is_int ... feel free to ignore ;-)

jirka


>  		/* This is a pointer to void.
>  		 * It is the same as scalar from the verifier safety pov.
>  		 * No further pointer walking is allowed.
> -- 
> 2.43.0
>