Re: [PATCH bpf-next v2 1/2] bpf: Allow access to const void pointer arguments in tracing programs

[Jiri Olsa]

On Thu, Apr 17, 2025 at 12:17:55AM +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
> 
> Changing the is_int_ptr to void and generic integer check and renaming
> it to is_void_or_int_ptr.
> 
> Cc: Leon Hwang <leon.hwang@xxxxxxxxx>
> Signed-off-by: KaFai Wan <kafai.wan@xxxxxxxxxxx>
> ---
>  kernel/bpf/btf.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 16ba36f34dfa..0b1724453b75 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6383,12 +6383,12 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
>  		return prog->aux->attach_btf;
>  }
>  
> -static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
> +static bool is_void_or_int_ptr(struct btf *btf, const struct btf_type *t)
>  {
>  	/* skip modifiers */
>  	t = btf_type_skip_modifiers(btf, t->type, NULL);
>  
> -	return btf_type_is_int(t);
> +	return btf_type_is_void(t) || btf_type_is_int(t);
>  }
>  
>  static u32 get_ctx_arg_idx(struct btf *btf, const struct btf_type *func_proto,
> @@ -6783,7 +6783,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>  		 */
>  		return true;

could we remove the above check then? 

        if (t->type == 0)
                /* This is a pointer to void.
                 * It is the same as scalar from the verifier safety pov.
                 * No further pointer walking is allowed.
                 */
                return true;

jirka

>  
> -	if (is_int_ptr(btf, t))
> +	if (is_void_or_int_ptr(btf, t))
>  		return true;
>  
>  	/* this is a pointer to another type */
> -- 
> 2.43.0
>