linux-kernel - Re: [PATCH] x86/debug: Handle warnings before the notifier chain

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Message-ID: <20170724102140.yjmruyecl7gxg3q7@hirez.programming.kicks-ass.net>
Date:   Mon, 24 Jul 2017 12:21:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Richard Weinberger <richard.weinberger@...il.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Jason Wessel <jason.wessel@...driver.com>
Subject: Re: [PATCH] x86/debug: Handle warnings before the notifier chain

On Mon, Jul 24, 2017 at 01:04:28PM +0300, Alexander Shishkin wrote:
> Commit
> 
>   9a93848fe787 ("x86/debug: Implement __WARN() using UD0")
> 
> turned warnings into UD0, but the fixup code only runs after the
> notify_die() chain. This is a problem, in particular, with kgdb,
> which kicks in as if it was a BUG().
> 
> Fix this by running the fixup code before the notifier chain in
> the invalid op handler path.

So arguable it might be nice to have kgdb trap on a WARN, but it
treating it as a BUG is obviously undesirable.

Jason, what would you want kgdb to do here?

> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Cc: Arjan van de Ven <arjan@...ux.intel.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Richard Weinberger <richard.weinberger@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: linux-kernel@...r.kernel.org
> ---
>  arch/x86/kernel/traps.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index bf54309b85..b2157d4a53 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -221,9 +221,6 @@ do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
>  		if (fixup_exception(regs, trapnr))
>  			return 0;
>  
> -		if (fixup_bug(regs, trapnr))
> -			return 0;
> -
>  		tsk->thread.error_code = error_code;
>  		tsk->thread.trap_nr = trapnr;
>  		die(str, regs, error_code);
> @@ -304,6 +301,13 @@ static void do_error_trap(struct pt_regs *regs, long error_code, char *str,
>  
>  	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
>  
> +	/*
> +	 * WARN*()s end up here; fix them up before we call the
> +	 * notifier chain.
> +	 */
> +	if (!user_mode(regs) && fixup_bug(regs, trapnr))
> +		return;
> +
>  	if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) !=
>  			NOTIFY_STOP) {
>  		cond_local_irq_enable(regs);
> -- 
> 2.13.2
>