netdev - Re: [PATCH v7 bpf-next 7/7] selftests: bpf: add dummy prog for bpf2bpf with tailcall

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Date:   Tue, 15 Sep 2020 08:48:19 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: [PATCH v7 bpf-next 7/7] selftests: bpf: add dummy prog for
 bpf2bpf with tailcall

On Tue, Sep 15, 2020 at 8:03 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 9/15/20 6:39 AM, Alexei Starovoitov wrote:
> > On Fri, Sep 11, 2020 at 08:59:27PM +0200, Maciej Fijalkowski wrote:
> >> On Thu, Sep 03, 2020 at 12:51:14PM -0700, Alexei Starovoitov wrote:
> >>> On Wed, Sep 02, 2020 at 10:08:15PM +0200, Maciej Fijalkowski wrote:
> [...]
> >>> Could you add few more tests to exercise the new feature more thoroughly?
> >>> Something like tailcall3.c that checks 32 limit, but doing tail_call from subprog.
> >>> And another test that consume non-trival amount of stack in each function.
> >>> Adding 'volatile char arr[128] = {};' would do the trick.
> >>
> >> Yet another prolonged silence from my side, but not without a reason -
> >> this request opened up a Pandora's box.
> >
> > Great catch and thanks to our development practices! As a community we should
> > remember this lesson and request selftests more often than not.
>
> +1, speaking of pandora ... ;-) I recently noticed that we also have the legacy
> ld_abs/ld_ind instructions. Right now check_ld_abs() gates them by bailing out
> if env->subprog_cnt > 1, but that doesn't solve everything given the prog itself
> may not have bpf2bpf calls, but it could get tail-called out of a subprog. We
> need to reject such cases (& add selftests for it), otherwise this would be a
> verifier bypass given they may implicitly exit the program (and then mismatch
> the return type that the verifier was expecting).

Good point. I think it's easier to allow ld_abs though.
The comment in check_ld_abs() is obsolete after gen_ld_abs() was added.
The verifier needs to check that subprog that is doing ld_abs or tail_call
has 'int' return type and check_reference_leak() doesn't error before
ld_abs and before bpf_tail_call.
In that sense doing bpf_tail_call from subprog has the same issues as ld_abs
(reference leaks and int return requirement)