On Thu, Sep 8, 2022 at 2:37 PM Dave Marchevsky <davemarchevsky@xxxxxx> wrote:
>
> On 9/6/22 9:53 PM, Alexei Starovoitov wrote:
> > 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@xxxxxx> 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@xxxxxx>
> >>>> ---
> >>>> 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 ?
>
> tnum_range(0, 0) sounds good to me.
>
> Would you like me to send this separately with that change, so it can be applied
> independently of rest of these changes?
Whichever way is faster.
We can always apply a patch or a few patches out of a bigger set.
