linux-kernel - Re: [PATCH 1/2] kprobes: Remove kprobe::fault

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Message-Id: <1622025445.6q8nl3t4ap.naveen@linux.ibm.com>
Date:   Wed, 26 May 2021 16:20:25 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     mhiramat@...nel.org, Peter Zijlstra <peterz@...radead.org>
Cc:     ananth@...ux.ibm.com, Christoph Hellwig <hch@....de>,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        rostedt@...dmis.org, x86@...nel.org
Subject: Re: [PATCH 1/2] kprobes: Remove kprobe::fault_handler

Peter Zijlstra wrote:
> The reason for kprobe::fault_handler(), as given by their comment:
> 
>  * We come here because instructions in the pre/post
>  * handler caused the page_fault, this could happen
>  * if handler tries to access user space by
>  * copy_from_user(), get_user() etc. Let the
>  * user-specified handler try to fix it first.
> 
> Is just plain bad. Those other handlers are ran from non-preemptible
> context and had better use _nofault() functions. Also, there is no
> upstream usage of this.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Christoph Hellwig <hch@....de>
> ---
>  Documentation/trace/kprobes.rst    |   24 +++++-------------------
>  arch/arc/kernel/kprobes.c          |   10 ----------
>  arch/arm/probes/kprobes/core.c     |    9 ---------
>  arch/arm64/kernel/probes/kprobes.c |   10 ----------
>  arch/csky/kernel/probes/kprobes.c  |   10 ----------
>  arch/ia64/kernel/kprobes.c         |    9 ---------
>  arch/mips/kernel/kprobes.c         |    3 ---
>  arch/powerpc/kernel/kprobes.c      |   10 ----------
>  arch/riscv/kernel/probes/kprobes.c |   10 ----------
>  arch/s390/kernel/kprobes.c         |   10 ----------
>  arch/sh/kernel/kprobes.c           |   10 ----------
>  arch/sparc/kernel/kprobes.c        |   10 ----------
>  arch/x86/kernel/kprobes/core.c     |   10 ----------
>  include/linux/kprobes.h            |    8 --------
>  kernel/kprobes.c                   |   19 -------------------
>  samples/kprobes/kprobe_example.c   |   15 ---------------
>  16 files changed, 5 insertions(+), 172 deletions(-)
> 

<snip>

> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -947,16 +947,6 @@ int kprobe_fault_handler(struct pt_regs
>  		 * these specific fault cases.
>  		 */
>  		kprobes_inc_nmissed_count(cur);

Not necessarily related, but I'm wondering why we're incrementing the 
probe miss count here. Unlike what the comment above indicates, this is 
not a 'fault' counter, but just a count of the number of times the probe 
handler wasn't called.

> -
> -		/*
> -		 * We come here because instructions in the pre/post
> -		 * handler caused the page_fault, this could happen
> -		 * if handler tries to access user space by
> -		 * copy_from_user(), get_user() etc. Let the
> -		 * user-specified handler try to fix it first.
> -		 */
> -		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
> -			return 1;
>  	}


- Naveen