netdev - Re: [PATCH bpf-next] bpf: Fix register equivalence tracking.

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Date:   Wed, 14 Oct 2020 21:19:52 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next] bpf: Fix register equivalence tracking.

On Wed, Oct 14, 2020 at 09:04:23PM -0700, John Fastabend wrote:
> Andrii Nakryiko wrote:
> > On Wed, Oct 14, 2020 at 10:59 AM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > From: Alexei Starovoitov <ast@...nel.org>
> > >
> > > The 64-bit JEQ/JNE handling in reg_set_min_max() was clearing reg->id in either
> > > true or false branch. In the case 'if (reg->id)' check was done on the other
> > > branch the counter part register would have reg->id == 0 when called into
> > > find_equal_scalars(). In such case the helper would incorrectly identify other
> > > registers with id == 0 as equivalent and propagate the state incorrectly.
> 
> One thought. It seems we should never have reg->id=0 in find_equal_scalars()
> would it be worthwhile to add an additional check here? Something like,
> 
>   if (known_reg->id == 0)
> 	return
>
> Or even a WARN_ON_ONCE() there? Not sold either way, but maybe worth thinking
> about.

That cannot happen anymore due to
if (dst_reg->id && !WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id))
check in the caller.
I prefer not to repeat the same check twice. Also I really don't like defensive programming.
if (known_reg->id == 0)
       return;
is exactly that.
If we had that already, as Andrii argued in the original thread, we would have
never noticed this issue. <, >, <= ops would have worked, but == would be
sort-of working. It would mark one branch instead of both, and sometimes
neither of the branches. I'd rather have bugs like this one hurting and caught
quickly instead of warm feeling of being safe and sailing into unknown.