Hi all,

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.

   - 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 ?
   - 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.


Appreciate the help from anybody from the mailing list.

Best,
Shubham Bansal