netdev - Re: [PATCH bpf-next 1/2] selftests/bpf: add loop test 4

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Date:   Mon, 5 Aug 2019 14:37:55 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <ast@...com>
Cc:     Yonghong Song <yhs@...com>, Alexei Starovoitov <ast@...nel.org>,
        "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 1/2] selftests/bpf: add loop test 4

On Mon, Aug 5, 2019 at 1:53 PM Alexei Starovoitov <ast@...com> wrote:
>
> On 8/5/19 1:04 PM, Yonghong Song wrote:
> >
> >
> > On 8/5/19 12:45 PM, Andrii Nakryiko wrote:
> >> On Sat, Aug 3, 2019 at 8:19 PM Alexei Starovoitov <ast@...nel.org> wrote:
> >>>
> >>> Add a test that returns a 'random' number between [0, 2^20)
> >>> If state pruning is not working correctly for loop body the number of
> >>> processed insns will be 2^20 * num_of_insns_in_loop_body and the program
> >>> will be rejected.
> >>>
> >>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> >>> ---
> >>>    .../bpf/prog_tests/bpf_verif_scale.c          |  1 +
> >>>    tools/testing/selftests/bpf/progs/loop4.c     | 23 +++++++++++++++++++
> >>>    2 files changed, 24 insertions(+)
> >>>    create mode 100644 tools/testing/selftests/bpf/progs/loop4.c
> >>>
> >>> diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c b/tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c
> >>> index b4be96162ff4..757e39540eda 100644
> >>> --- a/tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c
> >>> +++ b/tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c
> >>> @@ -71,6 +71,7 @@ void test_bpf_verif_scale(void)
> >>>
> >>>                   { "loop1.o", BPF_PROG_TYPE_RAW_TRACEPOINT },
> >>>                   { "loop2.o", BPF_PROG_TYPE_RAW_TRACEPOINT },
> >>> +               { "loop4.o", BPF_PROG_TYPE_RAW_TRACEPOINT },
> >>>
> >>>                   /* partial unroll. 19k insn in a loop.
> >>>                    * Total program size 20.8k insn.
> >>> diff --git a/tools/testing/selftests/bpf/progs/loop4.c b/tools/testing/selftests/bpf/progs/loop4.c
> >>> new file mode 100644
> >>> index 000000000000..3e7ee14fddbd
> >>> --- /dev/null
> >>> +++ b/tools/testing/selftests/bpf/progs/loop4.c
> >>> @@ -0,0 +1,23 @@
> >>> +// SPDX-License-Identifier: GPL-2.0
> >>> +// Copyright (c) 2019 Facebook
> >>> +#include <linux/sched.h>
> >>> +#include <linux/ptrace.h>
> >>> +#include <stdint.h>
> >>> +#include <stddef.h>
> >>> +#include <stdbool.h>
> >>> +#include <linux/bpf.h>
> >>> +#include "bpf_helpers.h"
> >>> +
> >>> +char _license[] SEC("license") = "GPL";
> >>> +
> >>> +SEC("socket")
> >>> +int combinations(volatile struct __sk_buff* skb)
> >>> +{
> >>> +       int ret = 0, i;
> >>> +
> >>> +#pragma nounroll
> >>> +       for (i = 0; i < 20; i++)
> >>> +               if (skb->len)
> >>> +                       ret |= 1 << i;
> >>
> >> So I think the idea is that because verifier shouldn't know whether
> >> skb->len is zero or not, then you have two outcomes on every iteration
> >> leading to 2^20 states, right?
> >>
> >> But I'm afraid that verifier can eventually be smart enough (if it's
> >> not already, btw), to figure out that ret can be either 0 or ((1 <<
> >> 21) - 1), actually. If skb->len is put into separate register, then
> >> that register's bounds will be established on first loop iteration as
> >> either == 0 on one branch or (0, inf) on another branch, after which
> >> all subsequent iterations will not branch at all (one or the other
> >> branch will be always taken).
> >>
> >> It's also possible that LLVM/Clang is smart enough already to figure
> >> this out on its own and optimize loop into.
> >>
> >>
> >> if (skb->len) {
> >>       for (i = 0; i < 20; i++)
> >>           ret |= 1 << i;
> >> }
> >
> > We have
> >      volatile struct __sk_buff* skb
> >
> > So from the source code, skb->len could be different for each
> > iteration. The compiler cannot do the above optimization.
>
> yep.
> Without volatile llvm optimizes it even more than Andrii predicted :)

My bad, completely missed volatile.

>
> >>
> >>
> >> So two complains:
> >>
> >> 1. Let's obfuscate this a bit more, e.g., with testing (skb->len &
> >> (1<<i)) instead, so that result really depends on actual length of the
> >> packet.
> >> 2. Is it possible to somehow turn off this precision tracking (e.g.,
> >> running not under root, maybe?) and see that this same program fails
> >> in that case? That way we'll know test actually validates what we
> >> think it validates.
>
> that's on my todo list already.
> To do proper unit tests for all this stuff there should be a way
> to turn off not only precision, but heuristics too.
> All magic numbers in is_state_visited() need to be switchable.
> I'm still thinking on the way to expose it to tests infra.

Yep, that would be great.

I have nothing beyond what Yonghong suggested.

Acked-by: Andrii Nakryiko <andriin@...com>