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

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Tue, Mar 08, 2022 at 11:12:13AM IST, Andrii Nakryiko wrote:
> On Sun, Mar 6, 2022 at 3:43 PM Kumar Kartikeya Dwivedi <memxor@gmail.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@gmail.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) */
>
> What's the difference between ARG_ANYTHING and ARG_SCALAR?
>

ARG_SCALAR only accepts reg->type == SCALAR, ARG_ANYTHING accepts anything as
long as reg->type != NOT_INIT (due to SRC_OP for check_reg_arg and early return
without further checks).

> > +       ARG_CONSTANT,           /* a scalar with constant value */
>
> This ARG_CONSTANT serves a very similar purpose as
> ARG_CONST_ALLOC_SIZE_OR_ZERO, tbh. The only difference is that one is
> used to set meta->mem_size and this one is used (through extra func_id
> special handling) to set meta->ret_pkt_len. But meta->mem_size and
> meta->ret_pkt_len mean the same thing: how many bytes are directly
> accessible through a pointer returned from the helper. So I feel like
> there is some opportunity to unify and generalize, instead of adding
> more custom variants of constants. WDYT?
>

I see, indeed it would make sense to make both equivalent, since
CONST_ALLOC_SIZE must also be a constant. Joanne also mentioned consolidating,
but I didn't understand how that would work for ARG_CONSTANT and ARG_CONST_SIZE
ones.

I'm wondering whether we can take a step back and should go with the following
convention:

ARG_MEM_SIZE, and two type flags, ARG_ZERO | ARG_CONSTANT

Old				New (in bpf_func_proto)
-------------------------------------------------------------------
ARG_CONST_SIZE			ARG_MEM_SIZE
ARG_CONST_SIZE_OR_ZERO		ARG_MEM_SIZE | ARG_ZERO
ARG_CONST_ALLOC_SIZE		ARG_MEM_SIZE | ARG_CONST
ARG_CONST_ALLOC_SIZE_OR_ZERO	ARG_MEM_SIZE | ARG_CONST | ARG_ZERO
ARG_CONSTANT (mine)		ARG_MEM_SIZE | ARG_CONST

When we detect ARG_CONST, we always set meta->mem_size, which can be used to
refine returned pointer range, otherwise meta->mem_size = -1 by default (so it
will be -1 for the !tnum_is_const(reg->var_off) case).

if (arg_type & ARG_CONST)
	meta->mem_size = reg->var_off.value;
	if (!(arg_type & ARG_ZERO) && !meta->mem_size)
		// error

The check_mem_size_reg call is only made when we see that previous reg was
ARG_PTR_TO_MEM. When preceding argument is not ARG_PTR_TO_MEM, we error if
ARG_CONST is not set for ARG_MEM_SIZE (so that either the mem size is for
previous parameter, or otherwise a constant size for the returned pointer).
We can also only allow certain pointer return types for that case.

If that is too much automagic, we can also discern using ARG_MEM_SIZE vs
ARG_RET_MEM_SIZE, but I think the above is fine.

ARG_CONST ofcourse only applies to args taking scalar type.

>
>
> >         __BPF_ARG_TYPE_MAX,
> >
> >         /* 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;
> > +}
> > +
> >  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