Subject: [PATCH v2] arm: eBPF JIT compiler

From: [email-masked] (Daniel Borkmann)
Date: Mon, 12 Jun 2017 12:21:03 +0200

On 05/30/2017 09:19 PM, Kees Cook wrote:
> Forwarding this to net-dev and eBPF folks, who weren't on CC...

Sorry for being late. Some comments below from a cursory look ...

> -Kees
>
> On Thu, May 25, 2017 at 4:13 PM, Shubham Bansal
> <[email-masked]> wrote:
>> The JIT compiler emits ARM 32 bit instructions. Currently, It supports
>> eBPF only. Classic BPF is supported because of the conversion by BPF
>> core.
>>
>> This patch is essentially changing the current implementation of JIT
>> compiler of Berkeley Packet Filter from classic to internal with almost
>> all instructions from eBPF ISA supported except the following
>>          BPF_ALU64 | BPF_DIV | BPF_K
>>          BPF_ALU64 | BPF_DIV | BPF_X
>>          BPF_ALU64 | BPF_MOD | BPF_K
>>          BPF_ALU64 | BPF_MOD | BPF_X
>>          BPF_STX | BPF_XADD | BPF_W
>>          BPF_STX | BPF_XADD | BPF_DW
>>          BPF_JMP | BPF_CALL

Any plans to implement above especially BPF_JMP | BPF_CALL in near future?
Reason why I'm asking is that i) currently the arm32 cBPF JIT implements
all of the cBPF extensions (except SKF_AD_RANDOM and SKF_AD_VLAN_TPID).
Some of the programs that were JITed before e.g. using SKF_AD_CPU would now
fall back to the eBPF interpreter due to lack of translation in JIT, but
also ii) that probably most (if not all) of eBPF programs use BPF helper
calls heavily, which will still redirect them to the interpreter right now
due to lack of BPF_JMP | BPF_CALL support, so it's really quite essential
to have it implemented.

>> Implementation is using scratch space to emulate 64 bit eBPF ISA on 32 bit
>> ARM because of deficiency of general purpose registers on ARM. Currently,
>> only LITTLE ENDIAN machines are supported in this eBPF JIT Compiler.
>>
>> Tested on ARMv7 with QEMU by me (Shubham Bansal).
>> Tested on ARMv5 by Andrew Lunn (andrew at lunn.ch).
>> Expected to work on ARMv6 as well, as its a part ARMv7 and part ARMv5.
>> Although, a proper testing is not done for ARMv6.
>>
>> Both of these testing are done with and without CONFIG_FRAME_POINTER
>> separately for LITTLE ENDIAN machine.
>>
>> For testing:
>>
>> 1. JIT is enabled with
>>          echo 1 > /proc/sys/net/core/bpf_jit_enable
>> 2. Constant Blinding can be enabled along with JIT using
>>          echo 1 > /proc/sys/net/core/bpf_jit_enable
>>          echo 2 > /proc/sys/net/core/bpf_jit_harden
>>
>> See Documentation/networking/filter.txt for more information.
>>
>> Result : test_bpf: Summary: 314 PASSED, 0 FAILED, [278/306 JIT'ed]

Did you also manage to get the BPF selftest suite running in the meantime
(tools/testing/selftests/bpf/)? There are a couple of programs that clang
will compile (test_pkt_access.o, test_xdp.o, test_l4lb.o, test_tcp_estats.o)
and then test run.

Did you manage to get tail calls tested as well (I assume so since you
implemented emit_bpf_tail_call() in the patch but just out of curiosity)?

>> Signed-off-by: Shubham Bansal <[email-masked]>
>> ---
>>   Documentation/networking/filter.txt |    4 +-
>>   arch/arm/Kconfig                    |    2 +-
>>   arch/arm/net/bpf_jit_32.c           | 2404 ++++++++++++++++++++++++-----------
>>   arch/arm/net/bpf_jit_32.h           |  108 +-
>>   4 files changed, 1713 insertions(+), 805 deletions(-)
>>
[...]

If arm folks take the patch, there will be two minor (silent) merge
conflicts with net-next:

1) In bpf_int_jit_compile(), below the jited = 1 assignment, there
    needs to come a prog->jited_len = image_size.
2) The internal tail call opcode changed from BPF_JMP | BPF_CALL | BPF_X
    into BPF_JMP | BPF_TAIL_CALL.

Two minor things below, could probably also be as follow-up.

[...]
>> +       /* dst = imm64 */
>> +       case BPF_LD | BPF_IMM | BPF_DW:
>> +       {
>> +               const struct bpf_insn insn1 = insn[1];
>> +               u32 hi, lo = imm;
>> +
>> +               if (insn1.code != 0 || insn1.src_reg != 0 ||
>> +                   insn1.dst_reg != 0 || insn1.off != 0) {
>> +                       /* Note: verifier in BPF core must catch invalid
>> +                        * instruction.
>> +                        */
>> +                       pr_err_once("Invalid BPF_LD_IMM64 instruction\n");
>> +                       return -EINVAL;
>> +               }

Nit: this check can be removed as verifier already takes care
of it. (No JIT checks for this anymore.)

>> +               hi = insn1.imm;
>> +               emit_a32_mov_i(dst_lo, lo, dstk, ctx);
>> +               emit_a32_mov_i(dst_hi, hi, dstk, ctx);
>> +
>> +               return 1;
>> +       }
[...]
>> -       /* compute offsets only during the first pass */
>> -       if (ctx->target == NULL)
>> -               ctx->offsets[i] = ctx->idx * 4;
>> +static int validate_code(struct jit_ctx *ctx)
>> +{
>> +       int i;
>> +
>> +       for (i = 0; i < ctx->idx; i++) {
>> +               u32 a32_insn = le32_to_cpu(ctx->target[i]);

Given __opcode_to_mem_arm(ARM_INST_UDF) is used to fill the image,
perhaps use the __mem_to_opcode_arm() helper for the check?

>> +               if (a32_insn == ARM_INST_UDF)
>> +                       return -1;
>> +       }
>>
>>          return 0;
>>   }
>>
>> +void bpf_jit_compile(struct bpf_prog *prog)
>> +{
>> +       /* Nothing to do here. We support Internal BPF. */
>> +}