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

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Message-ID: <CAADnVQ+EtYjnH+=tZCOYX+ioyx=d4NAxFFpRpN2PVfvye6thTA@mail.gmail.com>
Date:   Sun, 19 Dec 2021 09:43:18 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next v4 06/10] bpf: Track provenance for pointers
 formed from referenced PTR_TO_BTF_ID

On Sat, Dec 18, 2021 at 9:25 PM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> 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@...il.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.

I'm still confused.
Why does mark_btf_ld_reg() need to copy ref_obj_id ?
It should keep it as zero.
mark_btf_ld_reg() is used in deref only.
The ref_obj_id is assigned by check_helper_call().
r2 = r0; will copy it, but
r2 = r0->next; will keep r2->ref_obj_id as zero.

> > 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.

I don't understand why such logic is needed.