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

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 [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) */

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).

quoted

+       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.

quoted

        __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