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

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Message-ID: <202002201505.C3863B2D0@keescook>
Date:   Thu, 20 Feb 2020 15:05:36 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Jürgen Groß <jgross@...e.com>,
        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 Thu, Feb 20, 2020 at 11:33:41AM -0500, Boris Ostrovsky wrote:
> 
> 
> 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 )

Oh, good call. Sorry I missed that!

Thanks!

-- 
Kees Cook