Date:   Fri, 25 Sep 2020 09:22:22 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 bpf-next 01/13] bpf: Move the PTR_TO_BTF_ID check to check_reg_type()

On Fri, 25 Sep 2020 at 01:03, Martin KaFai Lau <kafai@...com> wrote:
>
> check_reg_type() checks whether a reg can be used as an arg of a
> func_proto.  For PTR_TO_BTF_ID, the check is actually not
> completely done until the reg->btf_id is pointing to a
> kernel struct that is acceptable by the func_proto.
>
> Thus, this patch moves the btf_id check into check_reg_type().
> "arg_type" and "arg_btf_id" are passed to check_reg_type() instead of
> "compatible".  The compatible_reg_types[] usage is localized in
> check_reg_type() now.
>
> The "if (!btf_id) verbose(...); " is also removed since it won't happen.
>
> Signed-off-by: Martin KaFai Lau <kafai@...com>

Acked-by: Lorenz Bauer <lmb@...udflare.com>

> ---
>  kernel/bpf/verifier.c | 60 ++++++++++++++++++++++---------------------
>  1 file changed, 31 insertions(+), 29 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 42dee5dcbc74..945fa2b4d096 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4028,19 +4028,27 @@ static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
>  };
>
>  static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
> -                         const struct bpf_reg_types *compatible)
> +                         enum bpf_arg_type arg_type,
> +                         const u32 *arg_btf_id)
>  {
>         struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
>         enum bpf_reg_type expected, type = reg->type;
> +       const struct bpf_reg_types *compatible;
>         int i, j;
>
> +       compatible = compatible_reg_types[arg_type];
> +       if (!compatible) {
> +               verbose(env, "verifier internal error: unsupported arg type %d\n", arg_type);
> +               return -EFAULT;
> +       }
> +
>         for (i = 0; i < ARRAY_SIZE(compatible->types); i++) {
>                 expected = compatible->types[i];
>                 if (expected == NOT_INIT)
>                         break;
>
>                 if (type == expected)
> -                       return 0;
> +                       goto found;
>         }
>
>         verbose(env, "R%d type=%s expected=", regno, reg_type_str[type]);
> @@ -4048,6 +4056,25 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
>                 verbose(env, "%s, ", reg_type_str[compatible->types[j]]);
>         verbose(env, "%s\n", reg_type_str[compatible->types[j]]);
>         return -EACCES;
> +
> +found:
> +       if (type == PTR_TO_BTF_ID) {
> +               if (!btf_struct_ids_match(&env->log, reg->off, reg->btf_id,
> +                                         *arg_btf_id)) {
> +                       verbose(env, "R%d is of type %s but %s is expected\n",
> +                               regno, kernel_type_name(reg->btf_id),
> +                               kernel_type_name(*arg_btf_id));
> +                       return -EACCES;
> +               }
> +
> +               if (!tnum_is_const(reg->var_off) || reg->var_off.value) {
> +                       verbose(env, "R%d is a pointer to in-kernel struct with non-zero offset\n",
> +                               regno);
> +                       return -EACCES;
> +               }
> +       }
> +
> +       return 0;
>  }
>
>  static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> @@ -4057,7 +4084,6 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>         u32 regno = BPF_REG_1 + arg;
>         struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
>         enum bpf_arg_type arg_type = fn->arg_type[arg];
> -       const struct bpf_reg_types *compatible;
>         enum bpf_reg_type type = reg->type;
>         int err = 0;
>
> @@ -4097,35 +4123,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>                  */
>                 goto skip_type_check;
>
> -       compatible = compatible_reg_types[arg_type];
> -       if (!compatible) {
> -               verbose(env, "verifier internal error: unsupported arg type %d\n", arg_type);
> -               return -EFAULT;
> -       }
> -
> -       err = check_reg_type(env, regno, compatible);
> +       err = check_reg_type(env, regno, arg_type, fn->arg_btf_id[arg]);
>         if (err)
>                 return err;
>
> -       if (type == PTR_TO_BTF_ID) {
> -               const u32 *btf_id = fn->arg_btf_id[arg];
> -
> -               if (!btf_id) {
> -                       verbose(env, "verifier internal error: missing BTF ID\n");
> -                       return -EFAULT;
> -               }
> -
> -               if (!btf_struct_ids_match(&env->log, reg->off, reg->btf_id, *btf_id)) {
> -                       verbose(env, "R%d is of type %s but %s is expected\n",
> -                               regno, kernel_type_name(reg->btf_id), kernel_type_name(*btf_id));
> -                       return -EACCES;
> -               }
> -               if (!tnum_is_const(reg->var_off) || reg->var_off.value) {
> -                       verbose(env, "R%d is a pointer to in-kernel struct with non-zero offset\n",
> -                               regno);
> -                       return -EACCES;
> -               }
> -       } else if (type == PTR_TO_CTX) {
> +       if (type == PTR_TO_CTX) {
>                 err = check_ctx_reg(env, reg, regno);
>                 if (err < 0)
>                         return err;
> --
> 2.24.1
>


-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

www.cloudflare.com

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