linux-kernel - Re: [PATCH] x86/xen: Distribute switch variables for initialization

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Date:   Thu, 20 Feb 2020 07:37:22 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Kees Cook <keescook@...omium.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/xen: Distribute switch variables for initialization

On 20.02.20 07:23, Kees Cook wrote:
> Variables declared in a switch statement before any case statements
> cannot be automatically initialized with compiler instrumentation (as
> they are not part of any execution flow). With GCC's proposed automatic
> stack variable initialization feature, this triggers a warning (and they
> don't get initialized). Clang's automatic stack variable initialization
> (via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
> doesn't initialize such variables[1]. Note that these warnings (or silent
> skipping) happen before the dead-store elimination optimization phase,
> so even when the automatic initializations are later elided in favor of
> direct initializations, the warnings remain.
> 
> To avoid these problems, move such variables into the "case" where
> they're used or lift them up into the main function body.
> 
> arch/x86/xen/enlighten_pv.c: In function ‘xen_write_msr_safe’:
> arch/x86/xen/enlighten_pv.c:904:12: warning: statement will never be executed [-Wswitch-unreachable]
>    904 |   unsigned which;
>        |            ^~~~~
> 
> [1] https://bugs.llvm.org/show_bug.cgi?id=44916
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen