Re: [PATCH bpf-next v3 2/2] bpf: Require ARG_PTR_TO_MEM with memory flag

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2026-01-20 at 16:28 +0800, Zesen Liu wrote:
> Add check to ensure that ARG_PTR_TO_MEM is used with either MEM_WRITE or
> MEM_RDONLY.
> 
> Using ARG_PTR_TO_MEM alone without flags does not make sense because:
> 
> - If the helper does not change the argument, missing MEM_RDONLY causes the
> verifier to incorrectly reject a read-only buffer.
> - If the helper does change the argument, missing MEM_WRITE causes the
> verifier to incorrectly assume the memory is unchanged, leading to errors
> in code optimization.
> 
> Co-developed-by: Shuran Liu <electronlsr@gmail.com>
> Signed-off-by: Shuran Liu <electronlsr@gmail.com>
> Co-developed-by: Peili Gao <gplhust955@gmail.com>
> Signed-off-by: Peili Gao <gplhust955@gmail.com>
> Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Zesen Liu <ftyghome@gmail.com>
> ---

At the moment, for helper arguments processing I see that MEM_RDONLY
influences verifier only when argument type is ARG_PTR_TO_DYNPTR or
ARG_PTR_TO_BTF_ID. Hence, I think this change is safe to apply,
effects are limited to the check_mem_arg_rw_flag_ok() below.

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>

>  kernel/bpf/verifier.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9de0ec0c3ed9..a89f5bc7eff7 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -10351,10 +10351,27 @@ static bool check_btf_id_ok(const struct bpf_func_proto *fn)
>  	return true;
>  }
>  
> +static bool check_mem_arg_rw_flag_ok(const struct bpf_func_proto *fn)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(fn->arg_type); i++) {
> +		enum bpf_arg_type arg_type = fn->arg_type[i];
> +
> +		if (base_type(arg_type) != ARG_PTR_TO_MEM)
> +			continue;
> +		if (!(arg_type & (MEM_WRITE | MEM_RDONLY)))
> +			return false;
> +	}
> +
> +	return true;
> +}
> +
>  static int check_func_proto(const struct bpf_func_proto *fn)
>  {
>  	return check_raw_mode_ok(fn) &&
>  	       check_arg_pair_ok(fn) &&
> +	       check_mem_arg_rw_flag_ok(fn) &&
>  	       check_btf_id_ok(fn) ? 0 : -EINVAL;
>  }
>