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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 26 Jan 2023 11:15:59 -0500
guoren@kernel.org wrote:

> From: Guo Ren <guoren@linux.alibaba.com>
> 
> The previous implementation was based on the stop_matchine mechanism,
> which reduced the speed of arm/disarm_kprobe. Using minimum ebreak
> instruction would get accurate atomicity.
> 
> This patch removes the patch_text of riscv, which is based on
> stop_machine. Then riscv only reserved patch_text_nosync, and developers
> need to be more careful in dealing with patch_text atomicity.
> 
> When CONFIG_RISCV_ISA_C=n, the ebreak could replace the whole
> instruction. When CONFIG_RISCV_ISA_C=y, the patch uses 16-bit length
> c.ebreak instruction, which may occupy the first part of the 32-bit
> instruction and leave half the rest of the broken instruction. Because
> ebreak could detour the flow to skip it, leaving it in the kernel text
> memory is okay.
> 
> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@kernel.org>

I'm not sure how the RISCV specification ensures this type of self
code modification. But if you think calling the stop_machine() for
*each* probe arm/disarm is slow, there may be another way to avoid
ot by introducing a batch arming interface too. (reserve-commit way)

BTW, for the BPF usecase which is usually only for function
entry/exit, we will use Fprobes. Since that will use ftrace batch
text patching, I think we already avoid such slowdown.

FYI, for ftrace dynamic event usecase, there is another reason to slow
down the enable/disable dynamic event itself (to sync up the event
enabled status to ensure all event handler has been done, it waits
for rcu-sync for each operation.)

Thank you,

> ---
>  arch/riscv/include/asm/patch.h     |  1 -
>  arch/riscv/kernel/patch.c          | 33 ------------------------------
>  arch/riscv/kernel/probes/kprobes.c | 29 ++++++++++++++++++--------
>  3 files changed, 21 insertions(+), 42 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/patch.h b/arch/riscv/include/asm/patch.h
> index 9a7d7346001e..2500782e6f5b 100644
> --- a/arch/riscv/include/asm/patch.h
> +++ b/arch/riscv/include/asm/patch.h
> @@ -7,6 +7,5 @@
>  #define _ASM_RISCV_PATCH_H
>  
>  int patch_text_nosync(void *addr, const void *insns, size_t len);
> -int patch_text(void *addr, u32 insn);
>  
>  #endif /* _ASM_RISCV_PATCH_H */
> diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> index 765004b60513..8bd51ed8b806 100644
> --- a/arch/riscv/kernel/patch.c
> +++ b/arch/riscv/kernel/patch.c
> @@ -98,36 +98,3 @@ int patch_text_nosync(void *addr, const void *insns, size_t len)
>  	return ret;
>  }
>  NOKPROBE_SYMBOL(patch_text_nosync);
> -
> -static int patch_text_cb(void *data)
> -{
> -	struct patch_insn *patch = data;
> -	int ret = 0;
> -
> -	if (atomic_inc_return(&patch->cpu_count) == num_online_cpus()) {
> -		ret =
> -		    patch_text_nosync(patch->addr, &patch->insn,
> -					    GET_INSN_LENGTH(patch->insn));
> -		atomic_inc(&patch->cpu_count);
> -	} else {
> -		while (atomic_read(&patch->cpu_count) <= num_online_cpus())
> -			cpu_relax();
> -		smp_mb();
> -	}
> -
> -	return ret;
> -}
> -NOKPROBE_SYMBOL(patch_text_cb);
> -
> -int patch_text(void *addr, u32 insn)
> -{
> -	struct patch_insn patch = {
> -		.addr = addr,
> -		.insn = insn,
> -		.cpu_count = ATOMIC_INIT(0),
> -	};
> -
> -	return stop_machine_cpuslocked(patch_text_cb,
> -				       &patch, cpu_online_mask);
> -}
> -NOKPROBE_SYMBOL(patch_text);
> diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
> index 475989f06d6d..27f8960c321c 100644
> --- a/arch/riscv/kernel/probes/kprobes.c
> +++ b/arch/riscv/kernel/probes/kprobes.c
> @@ -24,12 +24,18 @@ post_kprobe_handler(struct kprobe *, struct kprobe_ctlblk *, struct pt_regs *);
>  static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
>  {
>  	unsigned long offset = GET_INSN_LENGTH(p->opcode);
> +#ifdef CONFIG_RISCV_ISA_C
> +	u32 opcode = __BUG_INSN_16;
> +#else
> +	u32 opcode = __BUG_INSN_32;
> +#endif
>  
>  	p->ainsn.api.restore = (unsigned long)p->addr + offset;
>  
> -	patch_text(p->ainsn.api.insn, p->opcode);
> -	patch_text((void *)((unsigned long)(p->ainsn.api.insn) + offset),
> -		   __BUG_INSN_32);
> +	patch_text_nosync(p->ainsn.api.insn, &p->opcode, offset);
> +	patch_text_nosync((void *)((unsigned long)(p->ainsn.api.insn) + offset),
> +			  &opcode, GET_INSN_LENGTH(opcode));
> +
>  }
>  
>  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));
>  }
>  
>  /* remove breakpoint from text */
>  void __kprobes arch_disarm_kprobe(struct kprobe *p)
>  {
> -	patch_text(p->addr, p->opcode);
> +#ifdef CONFIG_RISCV_ISA_C
> +	u32 opcode = __BUG_INSN_16;
> +#else
> +	u32 opcode = __BUG_INSN_32;
> +#endif
> +	patch_text_nosync(p->addr, &p->opcode, GET_INSN_LENGTH(opcode));
>  }
>  
>  void __kprobes arch_remove_kprobe(struct kprobe *p)
> -- 
> 2.36.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>