Re: [PATCH] riscv: kprobe: Optimize kprobe with accurate atomicity

["Björn Töpel" <bjorn@kernel.org>]

Guo Ren <guoren@kernel.org> writes:

> On Mon, Jan 30, 2023 at 11:28 PM Björn Töpel <bjorn@kernel.org> wrote:
>>
>> Guo Ren <guoren@kernel.org> writes:
>>
>> >> In the serie of RISCV OPTPROBES [1], it patches a long-jump instructions pair
>> >> AUIPC/JALR in kernel text, so in order to ensure other CPUs does not execute
>> >> in the instructions that will be modified, it is still need to stop other CPUs
>> >> via patch_text API, or you have any better solution to achieve the purpose?
>> >  - The stop_machine is an expensive way all architectures should
>> > avoid, and you could keep that in your OPTPROBES implementation files
>> > with static functions.
>> >  - The stop_machine couldn't work with PREEMPTION, so your
>> > implementation needs to work with !PREEMPTION.
>>
>> ...and stop_machine() with !PREEMPTION is broken as well, when you're
>> replacing multiple instructions (see Mark's post at [1]). The
>> stop_machine() dance might work when you're replacing *one* instruction,
>> not multiple as in the RISC-V case. I'll expand on this in a comment in
>> the OPTPROBES v6 series.
>>
>> >> >  static void __kprobes arch_prepare_simulate(struct kprobe *p)
>> >> > @@ -114,16 +120,23 @@ void *alloc_insn_page(void)
>> >> >  /* install breakpoint in text */
>> >> >  void __kprobes arch_arm_kprobe(struct kprobe *p)
>> >> >  {
>> >> > -     if ((p->opcode & __INSN_LENGTH_MASK) == __INSN_LENGTH_32)
>> >> > -             patch_text(p->addr, __BUG_INSN_32);
>> >> > -     else
>> >> > -             patch_text(p->addr, __BUG_INSN_16);
>> >> > +#ifdef CONFIG_RISCV_ISA_C
>> >> > +     u32 opcode = __BUG_INSN_16;
>> >> > +#else
>> >> > +     u32 opcode = __BUG_INSN_32;
>> >> > +#endif
>> >> > +     patch_text_nosync(p->addr, &opcode, GET_INSN_LENGTH(opcode));
>> >>
>> >> Sounds good, but it will leave some RVI instruction truncated in kernel text,
>> >> i doubt kernel behavior depends on the rest of the truncated instruction, well,
>> >> it needs more strict testing to prove my concern :)
>> > I do this on purpose, and it doesn't cause any problems. Don't worry;
>> > IFU hw must enforce the fetch sequence, and there is no way to execute
>> > broken instructions even in the speculative execution path.
>>
>> This is stretching reality a bit much. ARMv8, e.g., has a chapter in the
>> Arm ARM [2] Appendix B "Concurrent modification and execution of
>> instructions" (CMODX). *Some* instructions can be replaced concurrently,
>> and others cannot without caution. Assuming that that all RISC-V
>> implementations can, is a stretch. RISC-V hasn't even specified the
>> behavior of CMODX (which is problematic).
> Here we only use one sw/sh instruction to store a 32bit/16bit aligned element:
>
> INSN_0 <- ebreak (16bit/32bit aligned)
> INSN_1
> INSN_2
>
> The ebreak would cause an exception which implies a huge fence here.
> No machine could give a speculative execution for the ebreak path.

It's the concurrent modification that I was referring to (removing
stop_machine()). You're saying "it'll always work", I'm saying "I'm not
so sure". :-) E.g., writing c.ebreak on an 32b insn. Can you say that
will work on all RISC-V implementations? Do you have examples of
hardware where it will work?


Björn

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