Re: [PATCH bpf-next v3 1/2] bpf: verifier: Support eliding map lookup nullness

[Daniel Xu]

Hit send too early.

On Tue, Oct 1, 2024, at 5:07 PM, Daniel Xu wrote:
> On Wed, Sep 25, 2024 at 10:24:01AM GMT, Alexei Starovoitov wrote:
>> On Tue, Sep 24, 2024 at 12:40 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>> >
>> > +
>> > +/* Returns constant key value if possible, else -1 */
>> > +static long get_constant_map_key(struct bpf_verifier_env *env,
>> > +                                struct bpf_reg_state *key)
>> > +{
>> > +       struct bpf_func_state *state = func(env, key);
>> > +       struct bpf_reg_state *reg;
>> > +       int stack_off;
>> > +       int slot;
>> > +       int spi;
>> > +
>> > +       if (key->type != PTR_TO_STACK)
>> > +               return -1;
>> > +       if (!tnum_is_const(key->var_off))
>> > +               return -1;
>> > +
>> > +       stack_off = key->off + key->var_off.value;
>> > +       slot = -stack_off - 1;
>> > +       if (slot < 0)
>> > +               /* Stack grew upwards */
>> 
>> The comment is misleading.
>> The verifier is supposed to catch this.
>> It's just this helper was called before the stack bounds
>> were checked?
>
> Yeah. Stack bounds checked in check_stack_access_within_bounds() as part
> of helper call argument checks.
>
>
>> Maybe the call can be done later?
>
> Maybe? The argument checking starts clobbering state so it'll probably
> be not very simple to pull information out after args are checked.
>
> I think the logic will probably be much easier to follow with current
> approach. But maybe I'm missing a simpler idea.

I can make the comment a bit more verbose. Maybe that's better than
trying to wire a bunch of logic through memory access checks.