linux-kernel - Re: [PATCH v2] sysrq: Use panic() to force a crash

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Message-ID: <20181114225718.GN22824@google.com>
Date:   Wed, 14 Nov 2018 14:57:18 -0800
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Cc:     linux-kernel@...r.kernel.org, Evan Green <evgreen@...omium.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Douglas Anderson <dianders@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Manoj Gupta <manojgupta@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2] sysrq: Use panic() to force a crash

On Thu, Sep 20, 2018 at 10:12:53AM -0700, Matthias Kaehlcke wrote:
> sysrq_handle_crash() currently forces a crash by dereferencing a
> NULL pointer, which is undefined behavior in C. Just call panic()
> instead, which is simpler and doesn't depend on compiler specific
> handling of the undefined behavior.
> 
> Remove the comment on why the RCU lock needs to be released, it isn't
> accurate anymore since the crash now isn't handled by the page fault
> handler (for reference: the comment was added by commit 984cf355aeaa
> ("sysrq: Fix warning in sysrq generated crash.")). Releasing the lock
> is still good practice though.
> 
> Suggested-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> ---
>  drivers/tty/sysrq.c | 13 +++----------
>  1 file changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 06ed20dd01ba..d779a51499a0 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -134,17 +134,10 @@ static struct sysrq_key_op sysrq_unraw_op = {
>  
>  static void sysrq_handle_crash(int key)
>  {
> -	char *killer = NULL;
> -
> -	/* we need to release the RCU read lock here,
> -	 * otherwise we get an annoying
> -	 * 'BUG: sleeping function called from invalid context'
> -	 * complaint from the kernel before the panic.
> -	 */
> +	/* release the RCU read lock before crashing */
>  	rcu_read_unlock();
> -	panic_on_oops = 1;	/* force panic */
> -	wmb();
> -	*killer = 1;
> +
> +	panic("sysrq triggered crash\n");
>  }
>  static struct sysrq_key_op sysrq_crash_op = {
>  	.handler	= sysrq_handle_crash,

Does this need further changes or is it ready to land?