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

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Message-ID: <8a7f5bd7-2bb6-d187-cc6e-87ff01c440ce@oracle.com>
Date:   Thu, 20 Feb 2020 11:33:41 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Jürgen Groß <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>
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 2/20/20 1:37 AM, Jürgen Groß wrote:
> 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>
>

Applied to for-linus-5.6.

(I replaced 'unsigned' with 'unsigned int' to quiet down checkpatch )


-boris