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

[Yonghong Song <yhs@fb.com>]

On 8/5/19 12:45 PM, Andrii Nakryiko wrote:
> On Sat, Aug 3, 2019 at 8:19 PM Alexei Starovoitov <ast@kernel.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@kernel.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.

> 
> 
> 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.
> 
> Thoughts?
> 
>> +       return ret;
>> +}
>> --
>> 2.20.0
>>