linux-kernel - Re: [PATCH bpf-next v2 2/6] bpf: Add a ARG_PTR_TO_CONST

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Message-ID: <CAEf4BzZEgHyodDqj7exrEo+eBOzHEnsvkc003vxq3dcRVZXE2A@mail.gmail.com>
Date:   Fri, 26 Mar 2021 15:23:39 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Florent Revest <revest@...omium.org>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yonghong Song <yhs@...com>, KP Singh <kpsingh@...nel.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 2/6] bpf: Add a ARG_PTR_TO_CONST_STR argument type

On Tue, Mar 23, 2021 at 7:23 PM Florent Revest <revest@...omium.org> wrote:
>
> This type provides the guarantee that an argument is going to be a const
> pointer to somewhere in a read-only map value. It also checks that this
> pointer is followed by a zero character before the end of the map value.
>
> Signed-off-by: Florent Revest <revest@...omium.org>
> ---
>  include/linux/bpf.h   |  1 +
>  kernel/bpf/verifier.c | 38 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 39 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index a25730eaa148..7b5319d75b3e 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -308,6 +308,7 @@ enum bpf_arg_type {
>         ARG_PTR_TO_PERCPU_BTF_ID,       /* pointer to in-kernel percpu type */
>         ARG_PTR_TO_FUNC,        /* pointer to a bpf program function */
>         ARG_PTR_TO_STACK_OR_NULL,       /* pointer to stack or NULL */
> +       ARG_PTR_TO_CONST_STR,   /* pointer to a null terminated read-only string */
>         __BPF_ARG_TYPE_MAX,
>  };
>

[...]

> +
> +               map_off = reg->off + reg->var_off.value;
> +               err = map->ops->map_direct_value_addr(map, &map_addr, map_off);
> +               if (err)
> +                       return err;
> +
> +               str_ptr = (char *)(long)(map_addr);
> +               if (!strnchr(str_ptr + map_off,
> +                            map->value_size - reg->off - map_off, 0))

you are double subtracting reg->off here. isn't map->value_size -
map_off what you want?

> +                       verbose(env, "string is not zero-terminated\n");

I'd prefer `return -EINVAL;`, but at least set err, otherwise what's the point?

>         }
>
>         return err;
> --
> 2.31.0.291.g576ba9dcdaf-goog
>