Re: [RFCv2 PATCH bpf-next 01/18] bpf: Add verifier support for custom callback return range

[Alexei Starovoitov <ast@fb.com>]

On 9/6/22 4:42 PM, Dave Marchevsky wrote:
> On 9/1/22 5:01 PM, Joanne Koong wrote:
>> On Tue, Aug 30, 2022 at 11:03 AM Dave Marchevsky <davemarchevsky@fb.com> wrote:
>>>
>>> Verifier logic to confirm that a callback function returns 0 or 1 was
>>> added in commit 69c087ba6225b ("bpf: Add bpf_for_each_map_elem() helper").
>>> At the time, callback return value was only used to continue or stop
>>> iteration.
>>>
>>> In order to support callbacks with a broader return value range, such as
>>> those added further in this series, add a callback_ret_range to
>>> bpf_func_state. Verifier's helpers which set in_callback_fn will also
>>> set the new field, which the verifier will later use to check return
>>> value bounds.
>>>
>>> Default to tnum_range(0, 1) instead of using tnum_unknown as a sentinel
>>> value as the latter would prevent the valid range (0, U64_MAX) being
>>> used.
>>>
>>> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
>>> ---
>>>   include/linux/bpf_verifier.h | 1 +
>>>   kernel/bpf/verifier.c        | 4 +++-
>>>   2 files changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>>> index 2e3bad8640dc..9c017575c034 100644
>>> --- a/include/linux/bpf_verifier.h
>>> +++ b/include/linux/bpf_verifier.h
>>> @@ -237,6 +237,7 @@ struct bpf_func_state {
>>>           */
>>>          u32 async_entry_cnt;
>>>          bool in_callback_fn;
>>> +       struct tnum callback_ret_range;
>>>          bool in_async_callback_fn;
>>>
>>>          /* The following fields should be last. See copy_func_state() */
>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>> index 9bef8b41e737..68bfa7c28048 100644
>>> --- a/kernel/bpf/verifier.c
>>> +++ b/kernel/bpf/verifier.c
>>> @@ -1745,6 +1745,7 @@ static void init_func_state(struct bpf_verifier_env *env,
>>>          state->callsite = callsite;
>>>          state->frameno = frameno;
>>>          state->subprogno = subprogno;
>>> +       state->callback_ret_range = tnum_range(0, 1);
>>>          init_reg_state(env, state);
>>>          mark_verifier_state_scratched(env);
>>>   }
>>> @@ -6879,6 +6880,7 @@ static int set_find_vma_callback_state(struct bpf_verifier_env *env,
>>>          __mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
>>>          __mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
>>>          callee->in_callback_fn = true;
>>> +       callee->callback_ret_range = tnum_range(0, 1);
>>
>> Thanks for removing this restriction for callback functions!
>>
>> One quick question: is this line above needed? I think in
>> __check_func_call, we always call init_func_state() first before
>> calling set_find_vma_callback_state(), so after the init_func_state()
>> call, the callee->callback_ret_range will already be set to
>> tnum_range(0,1).
>>
> 
> You're right, it's not strictly necessary. I think that the default range being
> tnum_range(0, 1) - although necessary for backwards compat - is unintuitive. So
> decided to be explicit with existing callbacks so folks didn't have to go
> searching for the default to understand what the ret_range is, and it's more
> obvious that callback_ret_range should be changed if existing helper code is
> reused.

Maybe then it's better to keep callback_ret_range as range(0,0)
in init_func_state() to nudge/force other places to set it explicitly ?