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

On Fri, Mar 11, 2022 at 04:39:40AM IST, Andrii Nakryiko wrote:

On Thu, Mar 10, 2022 at 3:05 PM Andrii Nakryiko
[off-list ref] wrote:

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On Mon, Mar 7, 2022 at 10:26 PM Kumar Kartikeya Dwivedi
[off-list ref] wrote:

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On Tue, Mar 08, 2022 at 11:12:13AM IST, Andrii Nakryiko wrote:

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On Sun, Mar 6, 2022 at 3:43 PM Kumar Kartikeya Dwivedi [off-list ref] wrote:

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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).

Ah, ok, didn't realize that it's not always scalar for ARG_ANYTHING

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+       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

I think using "ARG_MEM_SIZE" as part of ARG_CONSTANT is backwards and
misleading. It makes more sense to me to have ARG_CONSTANT and use
ARG_ZERO (or rather ARG_MAYBE_ZERO?) and ARG_MEM_SIZE (to specify that
this constant is describing the size of memory of a pointer that is
passed in a previous argument).

Basically, something like:

ARG_CONST_SIZE => ARG_CONSTANT | ARG_MEM_SIZE
ARG_CONST_SIZE_OR_ZERO => ARG_CONSTANT | ARG_MEM_SIZE | ARG_MAYBE_ZERO

Then we can replace ARG_CONST_ALLOC_SIZE and
ARG_CONST_ALLOC_SIZE_OR_ZERO with ARG_CONSTANT  and ARG_CONSTANT |
ARG_MAYBE_ZERO and we'll have a bit of special case to handle
bpf_ringbuf_reserve.

For ARG_CONSTANT, verifier will remember the value in
bpf_call_arg_meta, and then we can use it as necessary (e.g., instead
of mem_size when ARG_MEM_SIZE is specified) depending on context,
helper being called, etc.

Adding ARG_CONST just makes no sense as we always want constant value,
otherwise it might as well be just ARG_ANYTHING, right?

Re-reading this, this paragraph is very confusing (especially taking
into account what I wrote above). My point was that in your table, you
have ARG_MEM_SIZE as a "base type" and ARG_CONST as "modifier". And
that makes little sense to me, because in all cases we have a
constant, but not in all cases we use that constant to describe the
size of memory passed in a previous argument. So I inverted that,
ARG_CONSTANT as "base type", ARG_MEM_SIZE and ARG_MAYBE_ZERO as
modifiers. And we then don't need 5 different resulting types because
"CONST_ALLOC_SIZE" handling is just a custom constant handling for
bpf_ringbuf_reserve. Just like for your use case you wanted to use
plain ARG_CONSTANT and add some extra logic for your
bpf_packet_pointer(). I hope this clarifies it a bit.

Makes sense, I'll split it out as a separate change. Thanks!

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I haven't spent much time thinking about this, though, so I'm probably
missing something.

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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.

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        __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

--
Kartikeya