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@xxxxxxxxx> 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@xxxxxxxxx>
> > ---
> > 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
