Re: [PATCH bpf-next v4 06/10] bpf: Track provenance for pointers formed from referenced PTR_TO_BTF_ID

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Sun, Dec 19, 2021 at 10:35:18AM IST, Alexei Starovoitov wrote:
> On Sat, Dec 18, 2021 at 8:33 PM Kumar Kartikeya Dwivedi
> <memxor@gmail.com> wrote:
> >
> > It is, but into parent_ref_obj_id, to match during release_reference.
> >
> > > Shouldn't r2 get a different ref_obj_id after r2 = r1->next ?
> >
> > It's ref_obj_id is still 0.
> >
> > Thinking about this more, we actually only need 1 extra bit of information in
> > reg_state, not even a new member. We can simply copy ref_obj_id and set this
> > bit, then we can reject this register during release but consider it during
> > release_reference.
>
> It seems to me that this patch created the problem and it's trying
> to fix it at the same time.
>

Yes, sort of. Maybe I need to improve the commit message? I give an example
below, and the first half of commit explains that if we simply did copy
ref_obj_id, it would lead to the case in the previous mail (same BTF ID ptr can
be passed), so we need to do something different.

Maybe that is what is confusing you.

> mark_btf_ld_reg() shouldn't be copying ref_obj_id.
> If it keeps it as zero the problem will not happen, no?

It is copying it but writing it to parent_ref_obj_id. It keeps ref_obj_id as 0
for all deref pointers.

r1 = acq(); // r1.ref = acquire_reference_state();
 ref = N
r2 = r1->a; // mark_btf_ld_reg -> copy r1.(ref ?: parent_ref) -> so r2.parent_ref = r1.ref
r3 = r2->b; // mark_btf_ld_reg -> copy r2.(ref ?: parent_ref) -> so r3.parent_ref = r2.parent_ref
r4 = r3->c; // mark_btf_ld_reg -> copy r3.(ref ?: parent_ref) -> so r4.parent_ref = r3.parent_ref
rel(r1);    // if (reg.ref == r1.ref || reg.parent_ref == r1.ref) invalidate(reg)

As you see, mark_btf_ld_reg only ever writes to parent_ref_obj_id, not
ref_obj_id. It just copies ref_obj_id when it is set, over parent_ref_obj_id,
and only one of two can be set.

--
Kartikeya