arch: arm: bpf: Converting cBPF to eBPF for arm 32 bit

[Kees Cook]

On Mon, Jan 30, 2017 at 2:38 AM, Shubham Bansal
<illusionist.neo at gmail.com> wrote:
> Hi all,
>
> Please ignore last copy of this mail. Kernel mailing lists bounced my
> last mail back because of HTML content.
>
> Just starting a new thread with proper heading on the main kernel
> hardening and net-dev mailing list so that other people can be involved
> in this. Please don't take this as a personal mail.
>
> I am working on conversion of arm32 cBPF into eBPF JIT. I wanted some
> help, regarding understanding of kernel code, from the dev available on
> the mailing list. If you look at the ./arch/arm/net/bpf_jit_32.c code,
> you will see jit_ctx structure. If anybody could help me understand what
> each fields of this structure represent then it would be great.
>
> Also, currently I am mapping the eBPF registers to arm 32 bit registers
> in the following way.
>
>> static const int bpf2a32[] = {
>>
>>         /* return value from in-kernel function, and exit value from
>>         eBPF
>> */
>>         [BPF_REG_0] = ARM_R0,
>>
>>         /* arguments from eBPF program to in-kernel function */
>>
>>         [BPF_REG_1] = ARM_R1,
>>
>>         [BPF_REG_2] = ARM_R2,
>>
>>         [BPF_REG_3] = ARM_R3,
>>
>>         [BPF_REG_4] = ARM_R4,
>>
>>         [BPF_REG_5] = ARM_R5,
>>
>>         /* callee saved registers that in-kernel function will
>>         preserve */
>>
>>         [BPF_REG_6] = ARM_R6,
>>
>>         [BPF_REG_7] = ARM_R7,
>>
>>         [BPF_REG_8] = ARM_R8,
>>
>>         [BPF_REG_9] = ARM_R9,
>>
>>         /* Read only Frame Pointer to access Stack */
>>
>>         [BPF_REG_FP] = ARM_FP,
>>
>>         /* Temperory Register for internal BPF JIT */
>>
>>         [TMP_REG_1] = ARM_R11,
>>
>>         /* temporary register for blinding constants */
>>
>>         [BPF_REG_AX] = ARM_R10,
>>
>> };
>
> But I have some question if anybody could help with those.
>
> 1.) Currently, as eBPF uses 64 bit registers, I am mapping 64 bit eBPF
> registers with 32 bit arm registers which looks wrong to me. Do anybody
> have some idea about how to map eBPF->arm 32 bit registers ?

I was going to say "look at the x86 32-bit implementation." ... But
there isn't one. :( I'm going to guess that there isn't a very good
answer here. I assume you'll have to build some kind of stack scratch
space to load/save.

> 2.) Also, is my current mapping good enough to make the JIT fast enough ?
> because as you might know, eBPF JIT mostly depends on 1-to-1 mapping of
> its instructions with native instructions.

I don't know -- it might be tricky with needing to deal with 64-bit
registers. But if you can make it faster than the non-JIT, it should
be a win. :) Yay assembly.

-Kees

-- 
Kees Cook
Pixel Security