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

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Date:   Tue, 18 Sep 2018 11:46:30 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthias Kaehlcke' <mka@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
CC:     "linux-kernel@...r.kernel.org" <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] tty/sysrq: Make local variable 'killer' in
 sysrq_handle_crash() global

From: Matthias Kaehlcke
> Sent: 17 September 2018 22:33
> 
> 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.

You need an explicit initialiser in order to make it global data
rather than a common section.

	David

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