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@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) */ > > 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