netdev - Re: More BPF verifier questions

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Date:   Tue, 6 Jun 2017 19:48:28 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Y Song <ys114321@...il.com>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        iovisor-dev <iovisor-dev@...ts.iovisor.org>
Subject: Re: More BPF verifier questions

On 05/06/17 08:06, Y Song wrote:
> On Fri, Jun 2, 2017 at 7:42 AM, Edward Cree <ecree@...arflare.com> wrote:
>> Test "helper access to variable memory: stack, bitwise AND + JMP, correct
>>  bounds" is listed as expected to pass, but it passes zero in the 'size'
>>  argument, an ARG_CONST_SIZE, to bpf_probe_read; I believe this should fail
>>  (and with my WIP patch it does).
> Probably a typo or mis-statement. "size" is not passed in with "zero", but
> with an unknown value. Hence, it probably should fail.
>
>       BPF_MOV64_IMM(BPF_REG_2, 16),
>       BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_2, -128),
>       BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1, -128),
>       BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 64),
>       BPF_MOV64_IMM(BPF_REG_4, 0),
>       BPF_JMP_REG(BPF_JGE, BPF_REG_4, BPF_REG_2, 2),
>       BPF_MOV64_IMM(BPF_REG_3, 0),
>       BPF_EMIT_CALL(BPF_FUNC_probe_read),
So, in fact this unknown value is really 16 & 64 == 0, but the verifier doesn't
 know that and concludes that it's either 0 or 64 (after the AND).  But then
 what I didn't spot before, and now have, is that the BPF_JGE tests if 0 >= size.
 Since we're in the false branch, that means size > 0, and so we're fine.
The test case is correct, and now that I've fixed the min/max tracking in my
 patches, the verifier accepts it again.

-Ed