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

[Daniel Xu]

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.

> 
> > +               return -1;
> > +       else if (slot >= state->allocated_stack)
> > +               /* Stack uninitialized */
> > +               return -1;
> > +
> > +       spi = slot / BPF_REG_SIZE;
> > +       reg = &state->stack[spi].spilled_ptr;
> > +       if (!tnum_is_const(reg->var_off))
> > +               /* Stack value not statically known */
> > +               return -1;
> > +
> > +       return reg->var_off.value;
> > +}
> 
> Looks like the code is more subtle than it looks.
> 
> I think it's better to guard it all with CAP_BPF.

Ack.