linux-kernel - Re: [PATCH] tty/sysrq: Make local variable 'killer' in sysrq_handle

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Message-ID: <a75e8a5d-25d0-83b8-6f7c-b80e69985b60@suse.cz>
Date:   Tue, 18 Sep 2018 08:11:33 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Matthias Kaehlcke <mka@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Douglas Anderson <dianders@...omium.org>,
        Evan Green <evgreen@...omium.org>,
        Manoj Gupta <manojgupta@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty/sysrq: Make local variable 'killer' in
 sysrq_handle_crash() global

On 09/17/2018, 11:33 PM, Matthias Kaehlcke wrote:
> sysrq_handle_crash() dereferences a NULL pointer on purpose to force
> an exception, the local variable 'killer' is assigned to NULL and
> dereferenced later. Clang detects the NULL pointer dereference at compile
> time and emits a BRK instruction (on arm64) instead of the expected NULL
> pointer exception. Change 'killer' to a global variable (and rename it
> to 'sysrq_killer' to avoid possible clashes) to prevent Clang from
> detecting the condition. By default global variables are initialized
> with zero/NULL in C, therefore an explicit initialization is not needed.
> 
> Reported-by: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
> Suggested-by: Evan Green <evgreen@...omium.org>
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> ---
>  drivers/tty/sysrq.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 06ed20dd01ba..49fa8e758690 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -132,10 +132,10 @@ static struct sysrq_key_op sysrq_unraw_op = {
>  #define sysrq_unraw_op (*(struct sysrq_key_op *)NULL)
>  #endif /* CONFIG_VT */
>  
> +char *sysrq_killer;
> +
>  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'
> @@ -144,7 +144,7 @@ static void sysrq_handle_crash(int key)
>  	rcu_read_unlock();
>  	panic_on_oops = 1;	/* force panic */
>  	wmb();
> -	*killer = 1;
> +	*sysrq_killer = 1;

Just because a static analyzer is wrong? Oh wait, even compiler is
wrong. At least make it a static global. Or what about OPTIMIZER_HIDE_VAR?

thanks,
-- 
js
suse labs