netdev - Re: [PATCH bpf-next v1 1/5] bpf: Add ARG_SCALAR and ARG

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Date:   Mon, 7 Mar 2022 11:28:13 -0800
From:   Joanne Koong <joannelkoong@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, Lorenz Bauer <linux@....io>,
        netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v1 1/5] bpf: Add ARG_SCALAR and ARG_CONSTANT

On Mon, Mar 7, 2022 at 1:24 AM Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
>
> In the next patch, we will introduce a new helper 'bpf_packet_pointer'
> that takes offset and len and returns a packet pointer. There we want to
> statically enforce offset is in range [0, 0xffff], and that len is a
> constant value, in range [1, 0xffff]. This also helps us avoid a
> pointless runtime check. To make these checks possible, we need to
> ensure we only get a scalar type. Although a lot of other argument types
> take scalars, their intent is different. Hence add general ARG_SCALAR
> and ARG_CONSTANT types, where the latter is also checked to be constant
> in addition to being scalar.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---
>  include/linux/bpf.h   |  2 ++
>  kernel/bpf/verifier.c | 13 +++++++++++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 88449fbbe063..7841d90b83df 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -391,6 +391,8 @@ enum bpf_arg_type {
>         ARG_PTR_TO_STACK,       /* pointer to stack */
>         ARG_PTR_TO_CONST_STR,   /* pointer to a null terminated read-only string */
>         ARG_PTR_TO_TIMER,       /* pointer to bpf_timer */
> +       ARG_SCALAR,             /* a scalar with any value(s) */
> +       ARG_CONSTANT,           /* a scalar with constant value */
>         __BPF_ARG_TYPE_MAX,
>

Should we rename ARG_CONST_SIZE and ARG_CONST_SIZE_OR_ZERO to
something like ARG_MEM_CONST_SIZE / ARG_MEM_CONST_SIZE_OR_ZERO to make
the interface more explicit? I think that would make it less confusing
to differentiate between ARG_CONST_SIZE and ARG_CONSTANT for arguments
that describe the length/size but are not associated with a memory
buffer.


>         /* Extended arg_types. */
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index ec3a7b6c9515..0373d5bd240f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5163,6 +5163,12 @@ static bool arg_type_is_int_ptr(enum bpf_arg_type type)
>                type == ARG_PTR_TO_LONG;
>  }
>
> +static bool arg_type_is_scalar(enum bpf_arg_type type)
> +{
> +       return type == ARG_SCALAR ||
> +              type == ARG_CONSTANT;
> +}
> +

I think this function name might be a bit misleading since
ARG_CONST_SIZE / ARG_CONST_SIZE_OR_ZERO / ARG_CONST_ALLOC_SIZE_OR_ZERO
are also scalar arg types. Maybe one alternative is to add a function
"arg_type_is_const" and then in check_func_arg, enforce that if
arg_type_is_const, then tnum_is_const(reg->var_off) must be true.
WDYT?

>  static int int_ptr_type_to_size(enum bpf_arg_type type)
>  {
>         if (type == ARG_PTR_TO_INT)
> @@ -5302,6 +5308,8 @@ static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
>         [ARG_PTR_TO_STACK]              = &stack_ptr_types,
>         [ARG_PTR_TO_CONST_STR]          = &const_str_ptr_types,
>         [ARG_PTR_TO_TIMER]              = &timer_types,
> +       [ARG_SCALAR]                    = &scalar_types,
> +       [ARG_CONSTANT]                  = &scalar_types,
>  };
>
>  static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
> @@ -5635,6 +5643,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>                         verbose(env, "string is not zero-terminated\n");
>                         return -EINVAL;
>                 }
> +       } else if (arg_type_is_scalar(arg_type)) {
> +               if (arg_type == ARG_CONSTANT && !tnum_is_const(reg->var_off)) {
> +                       verbose(env, "R%d is not a known constant\n", regno);
> +                       return -EACCES;
> +               }
>         }
>
>         return err;
> --
> 2.35.1
>