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Date:   Tue, 10 Sep 2019 16:14:17 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Subject: Re: [PATCH 2/2] powerpc/watchpoint: Disable watchpoint hit by
 larx/stcx instructions
To:     mikey@neuling.org, mpe@ellerman.id.au,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc:     benh@kernel.crashing.org, christophe.leroy@c-s.fr,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        paulus@samba.org
References: <20190910102422.23233-1-ravi.bangoria@linux.ibm.com>
        <20190910102422.23233-3-ravi.bangoria@linux.ibm.com>
In-Reply-To: <20190910102422.23233-3-ravi.bangoria@linux.ibm.com>
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Ravi Bangoria wrote:
> If watchpoint exception is generated by larx/stcx instructions, the
> reservation created by larx gets lost while handling exception, and
> thus stcx instruction always fails. Generally these instructions are
> used in a while(1) loop, for example spinlocks. And because stcx
> never succeeds, it loops forever and ultimately hangs the system.
> 
> Note that ptrace anyway works in one-shot mode and thus for ptrace
> we don't change the behaviour. It's up to ptrace user to take care
> of this.
> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
> ---
>  arch/powerpc/kernel/hw_breakpoint.c | 49 +++++++++++++++++++----------
>  1 file changed, 33 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
> index 28ad3171bb82..9fa496a598ce 100644
> --- a/arch/powerpc/kernel/hw_breakpoint.c
> +++ b/arch/powerpc/kernel/hw_breakpoint.c
> @@ -195,14 +195,32 @@ void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
>  	tsk->thread.last_hit_ubp = NULL;
>  }
>  
> +static bool is_larx_stcx_instr(struct pt_regs *regs, unsigned int instr)
> +{
> +	int ret, type;
> +	struct instruction_op op;
> +
> +	ret = analyse_instr(&op, regs, instr);
> +	type = GETTYPE(op.type);
> +	return (!ret && (type == LARX || type == STCX));
> +}
> +
>  /*
>   * Handle debug exception notifications.
>   */
>  static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,
>  			     unsigned long addr)
>  {
> -	int stepped;
> -	unsigned int instr;
> +	unsigned int instr = 0;
> +
> +	if (__get_user_inatomic(instr, (unsigned int *)regs->nip))
> +		goto fail;
> +
> +	if (is_larx_stcx_instr(regs, instr)) {
> +		printk_ratelimited("Watchpoint: Can't emulate/single-step larx/"
> +				   "stcx instructions. Disabling watchpoint.\n");

The below WARN() uses the term 'breakpoint'. Better to use consistent 
terminology. I would rewrite the above as:
	printk_ratelimited("Breakpoint hit on instruction that can't be emulated. "
				"Breakpoint at 0x%lx will be disabled.\n", addr);

Otherwise:
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>

- Naveen

> +		goto disable;
> +	}
>  
>  	/* Do not emulate user-space instructions, instead single-step them */
>  	if (user_mode(regs)) {
> @@ -211,23 +229,22 @@ static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,
>  		return false;
>  	}
>  
> -	stepped = 0;
> -	instr = 0;
> -	if (!__get_user_inatomic(instr, (unsigned int *)regs->nip))
> -		stepped = emulate_step(regs, instr);
> +	if (!emulate_step(regs, instr))
> +		goto fail;
>  
> +	return true;
> +
> +fail:
>  	/*
> -	 * emulate_step() could not execute it. We've failed in reliably
> -	 * handling the hw-breakpoint. Unregister it and throw a warning
> -	 * message to let the user know about it.
> +	 * We've failed in reliably handling the hw-breakpoint. Unregister
> +	 * it and throw a warning message to let the user know about it.
>  	 */
> -	if (!stepped) {
> -		WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
> -			"0x%lx will be disabled.", addr);
> -		perf_event_disable_inatomic(bp);
> -		return false;
> -	}
> -	return true;
> +	WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
> +		"0x%lx will be disabled.", addr);
> +
> +disable:
> +	perf_event_disable_inatomic(bp);
> +	return false;
>  }
>  
>  int hw_breakpoint_handler(struct die_args *args)
> -- 
> 2.21.0
> 
>