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 [off-list ref] wrote:

quoted

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) */
+       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.

quoted

        /* 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.

quoted

 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