netdev - Re: arch: arm: bpf: Converting cBPF to eBPF for arm 32 bit

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Message-ID: <CAHgaXdLsUYEmY86Nz3rC50repRA_rz-CVYx_i90k1Q5G1E-mCw@mail.gmail.com>
Date:   Sun, 7 May 2017 03:47:08 +0530
From:   Shubham Bansal <illusionist.neo@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Kees Cook <keescook@...omium.org>,
        Mircea Gherzan <mgherzan@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, ast@...com
Subject: Re: arch: arm: bpf: Converting cBPF to eBPF for arm 32 bit

Okay. My mistake. I just checked the verify function.

Apologies.
Best,
Shubham Bansal


On Sun, May 7, 2017 at 1:57 AM, Shubham Bansal
<illusionist.neo@...il.com> wrote:
> Thanks David.
>
> Hi all,
>
> I have two questions about the code at arch/arm64/net/bpf_jit_comp.c.
>
> 1. At line 708, " const u8 r1 = bpf2a64[BPF_REG_1]; /* r1: struct
> sk_buff *skb */ ".
>     Why is this code using BPF_REG_1 before saving it? As far as I
> know, BPF_REG_1 has pointer to bpf program context and this code
> clearly is overwriting that pointer which makes that pointer useless
> for future usage. It clearly looks like a bug.
>
> 2. At line 256, " emit(A64_LDR64(prg, tmp, r3), ctx); ".
>     This line of code is used to load an array( of pointers ) element,
> where r3 is used as an index of that array. Shouldn't it be be
> arithmetic left shifted by 3 or multiplied by 8 to get the right
> address in that array of pointers ?
>
> Apologies if any of the above question is stupid to ask.
>
> Best,
> Shubham
> Best,
> Shubham Bansal
>
>
> On Sun, May 7, 2017 at 12:08 AM, David Miller <davem@...emloft.net> wrote:
>> From: Shubham Bansal <illusionist.neo@...il.com>
>> Date: Sat, 6 May 2017 22:18:16 +0530
>>
>>> Hi Daniel,
>>>
>>> Thanks for the last reply about the testing of eBPF JIT.
>>>
>>> I have one issue though, I am not able to find what BPF_ABS and
>>> BPF_IND instruction does exactly.
>>
>> They are not instructions, they are modifiers for the BPF_LD
>> instruction which indicate an SKB load is to be performed.
>>
>> You never need to ask what a BPF instruction does, it is clear
>> defined in the BPF interperter found in kernel/bpf/core.c
>>
>> Look for the case statement LD_ABS_W and friends in __bpf_prog_run().