netdev - Re: [PATCH net-next 3/4] bpf/verifier: when pruning a branch, ignore its write marks

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Date:   Tue, 22 Aug 2017 11:25:18 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Edward Cree <ecree@...arflare.com>, <davem@...emloft.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
CC:     <netdev@...r.kernel.org>,
        iovisor-dev <iovisor-dev@...ts.iovisor.org>
Subject: Re: [PATCH net-next 3/4] bpf/verifier: when pruning a branch, ignore
 its write marks

On 8/22/17 11:03 AM, Edward Cree wrote:
> On 22/08/17 16:50, Edward Cree wrote:
>> On 22/08/17 16:24, Alexei Starovoitov wrote:
>>> Do you have a test case for this by any chance?
>> I think something like
>>     if (cond)
>>         r0=0;
>>     if (cond)
>>         r0=0;
>>     return r0;
>> might tickle the bug, but I'm not sure.
> It turns out that (cond) has to be constructed not to alter our knowledge
>  of whatever register we're testing, but apart from that, this works.
>     {
>         "liveness pruning and write screening",
>         .insns = {
>             /* Get an unknown value */
>             BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
>             /* branch conditions teach us nothing about R2 */
>             BPF_JMP_IMM(BPF_JGE, BPF_REG_2, 0, 1),
>             BPF_MOV64_IMM(BPF_REG_0, 0),
>             BPF_JMP_IMM(BPF_JGE, BPF_REG_2, 0, 1),
>             BPF_MOV64_IMM(BPF_REG_0, 0),
>             BPF_EXIT_INSN(),
>         },
>         .errstr = "R0 !read_ok",
>         .result = REJECT,
>         .prog_type = BPF_PROG_TYPE_LWT_IN,
>     },
> This test fails on net-next, but passes with my patch.
> I'll include it in the next spin of the series.

nice. thanks for the test!