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

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Date:   Tue, 8 Mar 2022 02:52:17 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Joanne Koong <joannelkoong@...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 Tue, Mar 08, 2022 at 12:58:13AM IST, Joanne Koong wrote:
> 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.
>

I don't have a problem with that. I was just avoiding the churn since
ARG_CONST_SIZE vs ARG_CONSTANT didn't seem confusing to me.

>
> >         /* 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

They already have their own arg_type_is_mem_size, I think the naming here has no
relation to the underlying compatible register types. ARG_CONSTANT is just a
stronger ARG_SCALAR, so it makes sense to put both under arg_type_is_scalar
umbrella.

> "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?

I don't think ARG_CONST_SIZE[_OR_ZERO] have any requirement that the scalar
value must be a known constant, so tnum_is_const check would be prohibitive.

I think the 'CONST' in their name is more misleading, so it might make sense to
drop that instead.

>
> >  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
> >

--
Kartikeya