Re: [PATCH bpf-next v2 14/15] bpf: Optimize state pruning for spilled scalars

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2024-01-09 at 16:22 -0800, Andrii Nakryiko wrote:
[...]
> >  static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
> >                       struct bpf_func_state *cur, struct bpf_idmap *idmap, bool exact)
> >  {
> > +       struct bpf_reg_state unbound_reg = {};
> > +       struct bpf_reg_state zero_reg = {};
> >         int i, spi;
> > 
> > +       __mark_reg_unknown(env, &unbound_reg);
> > +       __mark_reg_const_zero(env, &zero_reg);
> > +       zero_reg.precise = true;
> 
> these are immutable, right? Would it make sense to set them up just
> once as static variables instead of initializing on each check?

Should be possible.

> > +
> >         /* walk slots of the explored stack and ignore any additional
> >          * slots in the current stack, since explored(safe) state
> >          * didn't use them
> > @@ -16484,6 +16524,49 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
> >                         continue;
> >                 }
> > 
> 
> we didn't check that cur->stack[spi] is ok to access yet, it's done a
> bit later with `if (i >= cur->allocated_stack)`, if I'm not mistaken.
> So these checks would need to be moved a bit lower, probably.

Right. And it seems the issue is already present:

		if (exact &&
		    old->stack[spi].slot_type[i % BPF_REG_SIZE] !=
		    cur->stack[spi].slot_type[i % BPF_REG_SIZE])
			return false;

This is currently executed before `if (i >= cur->allocated_stack)` check as well.
Introduced by another commit of mine :(

> > +               /* load of stack value with all MISC and ZERO slots produces unbounded
> > +                * scalar value, call regsafe to ensure scalar ids are compared.
> > +                */
> > +               if (is_spilled_unbound_scalar_reg64(&old->stack[spi]) &&
> > +                   is_stack_unbound_slot64(env, &cur->stack[spi])) {
> > +                       i += BPF_REG_SIZE - 1;
> > +                       if (!regsafe(env, &old->stack[spi].spilled_ptr, &unbound_reg,
> > +                                    idmap, exact))
> > +                               return false;
> > +                       continue;
> > +               }
> > +
> > +               if (is_stack_unbound_slot64(env, &old->stack[spi]) &&
> > +                   is_spilled_unbound_scalar_reg64(&cur->stack[spi])) {
> > +                       i += BPF_REG_SIZE - 1;
> > +                       if (!regsafe(env,  &unbound_reg, &cur->stack[spi].spilled_ptr,
> > +                                    idmap, exact))
> > +                               return false;
> > +                       continue;
> > +               }
> 
> scalar_old = scalar_cur = NULL;
> if (is_spilled_unbound64(&old->..))
>     scalar_old = old->stack[spi].slot_type[0] == STACK_SPILL ?
> &old->stack[spi].spilled_ptr : &unbound_reg;
> if (is_spilled_unbound64(&cur->..))
>     scalar_cur = cur->stack[spi].slot_type[0] == STACK_SPILL ?
> &cur->stack[spi].spilled_ptr : &unbound_reg;
> if (scalar_old && scalar_cur) {
>     if (!regsafe(env, scalar_old, scalar_new, idmap, exact)
>         return false;
>     i += BPF_REG_SIZE - 1;
>     continue;
> }

Ok, I'll switch to this.
(Although, I think old variant is a bit simpler to follow).

> where is_spilled_unbound64() would be basically `return
> is_spilled_unbound_scalar_reg64(&old->..) ||
> is_stack_unbound_slot64(&old->...)`;
> 
> Similarly for zero case? Though I'm wondering if zero case should be
> checked first, as it's actually a subset of is_spilled_unbound64 when
> it comes to STACK_ZERO/STACK_MISC mixes, no?

Yes, makes sense.

[...]