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Re: mips:allmodconfig build failure in 3.16-rc1 due to bpf_jit code

To: Daniel Borkmann <>
Subject: Re: mips:allmodconfig build failure in 3.16-rc1 due to bpf_jit code
From: Alexei Starovoitov <>
Date: Tue, 17 Jun 2014 12:38:25 -0700
Cc: Markos Chandras <>, Guenter Roeck <>, "" <>, Linux MIPS Mailing List <>, Ralf Baechle <>, Network Development <>, "David S. Miller" <>
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On Tue, Jun 17, 2014 at 4:21 AM, Daniel Borkmann <> wrote:
> On 06/17/2014 01:09 PM, Markos Chandras wrote:
> ...
>> Thanks for these instructions. I will try them myself once I find some
>> time since I don't think bpf_jit for MIPS has ever been tested with all
>> the opcodes.
> Sounds great! If you find some tests are missing, please feel free to
> submit them as well via netdev.
> Best,
> Daniel


thank you for taking care of it so quickly :)
from the BPF perspective the fix looks good:
Acked-by: Alexei Starovoitov <>


please do run the testsuite.
Doing quick code review of mips jit, it looks like:

- your version of pkt_type_offset() will work for little endian only.
  (we've recently fixed it in net/core/filter.c)

- vlan tag handling is incorrect, since it's missing shifts.
  classic BPF standard for vlan_tag_present has to return 1 or 0
  and not just emit_and(r_A, r_s0, VLAN_TAG_PRESENT, ctx);

- pr_warn("%s: Unhandled opcode: 0x%02x\n", __FILE__,
  is way too heavy, since when jit is on, unprivileged user can spam log.

- /* sa is 5-bits long */
  BUG_ON(sa >= BIT(5));
is wrong too. Malicious user can cause kernel crash…
Also shift A>>=33 was always allowed by classic BPF checker, so
JITs have to silently do C-equivalent version of such shift.

- /* Determine if immediate is within the 16-bit signed range */
static inline bool is_range16(s32 imm)
        if (imm >= SBIT(15) || imm < -SBIT(15))
                return true;
the function name and comment are doing the opposite of
actual code, which makes harder to follow.

- the rest looks pretty good!

Also you'll get a lot more mileage out of mips jit if you use eBPF
instruction set as a base for JITing. You wouldn't need to worry
about vlan, pkt_type and other classic extensions. You'll get all
extensions for free, plus seccomp, tracing, etc.


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