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

[Björn Töpel]

Guo Ren <guoren@xxxxxxxxxx> 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).

If anything it would be more likely that the existing
"stop_machine()-to-replace-with-ebreak" works (again, replacing one
instruction does not have the !PREEMPTION issues). Then again, no spec,
so mostly guessing from my side. :-(

Oh, but the existing "ebreak replace" might be broken like [3].


Björn


[1] https://lore.kernel.org/linux-riscv/Y7%2F6AtX5X0+5qF6Y@FVFF77S0Q05N/
[2] https://developer.arm.com/documentation/ddi0487/latest
[3] https://lore.kernel.org/linux-riscv/20230126170607.1489141-2-guoren@xxxxxxxxxx/