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

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Message-ID: <CAGXu5j+k+NMQGmmhuW-4pHFd6agfndJfxQ9k--h+ZpsDBVB8ig@mail.gmail.com>
Date:   Mon, 22 May 2017 11:58:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Shubham Bansal <illusionist.neo@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Mircea Gherzan <mgherzan@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, ast@...com
Subject: Re: arch: arm: bpf: Converting cBPF to eBPF for arm 32 bit

On Sat, May 20, 2017 at 1:01 PM, Shubham Bansal
<illusionist.neo@...il.com> wrote:
> Hi Daniel and Kees,
>
> Before I send the patch, I have tested the JIT compiler on ARMv7 but
> not on ARMv5 or ARMv6. So can you tell me which arch versions I should
> test it for?
> Also for my testing, CONFIG_FRAME_POINTER and CONFIG_CPU_BIG_ENDIAN
> are both disabled. But I need to test JIT with these flags as well.
> Whenever I put these flags in .config file, the arm kernel is not
> getting compiler with these flags. Can you tell me why? If you need
> more information regarding this, please let me know.

I think it is fine to only target ARMv7. It is harder and harder to
find devices on v5 or v6 CPUs that would want to be using BPF JIT,
IMO.

When they "disappear", it's because there isn't a prerequisite met. I
either read the Kconfig files or use "make menuconfig" and "search" to
tell me where a config is defined and what is needed to meet the
prerequisites.

In the case of CPU_BIG_ENDIAN, you need ARCH_SUPPORTS_BIG_ENDIAN,
which appears to be only ARCH_IXP4XX. I don't think you're going to
find an emulator that will handle this, so I'd suggest ignoring this
config for now unless you can find someone with that hardware that you
can work with to test it.

In the case of CONFIG_FRAME_POINTER, I assume you built a
THUMB2_KERNEL? I'd read the notes in arch/arm/Kconfig.debug for
'config FRAME_POINTER'.

-Kees

-- 
Kees Cook
Pixel Security