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

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Message-ID: <f3ccc18f-7c25-a4e8-3d3d-c9f0bdf453ea@fb.com>
Date:   Mon, 5 Aug 2019 20:53:06 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...nel.org>
CC:     "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 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 :)

>>
>>
>> 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.