linux-kernel - Re: [PATCH 09/10] treewide: Remove uninitialized

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Date:   Wed, 3 Jun 2020 21:02:31 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Joe Perches <joe@...ches.com>,
        Andy Whitcroft <apw@...onical.com>, x86@...nel.org,
        drbd-dev@...ts.linbit.com, linux-block@...r.kernel.org,
        b43-dev@...ts.infradead.org, netdev@...r.kernel.org,
        linux-wireless@...r.kernel.org, linux-ide@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-spi@...r.kernel.org,
        linux-mm@...ck.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH 09/10] treewide: Remove uninitialized_var() usage

On Wed, Jun 03, 2020 at 08:33:15PM -0700, Nathan Chancellor wrote:
> On Wed, Jun 03, 2020 at 04:32:02PM -0700, Kees Cook wrote:
> > Using uninitialized_var() is dangerous as it papers over real bugs[1]
> > (or can in the future), and suppresses unrelated compiler warnings
> > (e.g. "unused variable"). If the compiler thinks it is uninitialized,
> > either simply initialize the variable or make compiler changes.
> > 
> > I preparation for removing[2] the[3] macro[4], remove all remaining
> > needless uses with the following script:
> > 
> > git grep '\buninitialized_var\b' | cut -d: -f1 | sort -u | \
> > 	xargs perl -pi -e \
> > 		's/\buninitialized_var\(([^\)]+)\)/\1/g;
> > 		 s:\s*/\* (GCC be quiet|to make compiler happy) \*/$::g;'
> > 
> > drivers/video/fbdev/riva/riva_hw.c was manually tweaked to avoid
> > pathological white-space.
> > 
> > No outstanding warnings were found building allmodconfig with GCC 9.3.0
> > for x86_64, i386, arm64, arm, powerpc, powerpc64le, s390x, mips, sparc64,
> > alpha, and m68k.
> > 
> > [1] https://lore.kernel.org/lkml/20200603174714.192027-1-glider@google.com/
> > [2] https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=1TGqCR5vQkCzWJ0QxK6CernOU6eedsudAixw@mail.gmail.com/
> > [3] https://lore.kernel.org/lkml/CA+55aFwgbgqhbp1fkxvRKEpzyR5J8n1vKT1VZdz9knmPuXhOeg@mail.gmail.com/
> > [4] https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=yVJu65TpLgN_ybYNv0VEOKA@mail.gmail.com/
> > 
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> 
> <snip>
> 
> > diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
> > index a0f6813f4560..a71fa7204882 100644
> > --- a/arch/powerpc/kvm/book3s_pr.c
> > +++ b/arch/powerpc/kvm/book3s_pr.c
> > @@ -1829,7 +1829,7 @@ static int kvmppc_vcpu_run_pr(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
> >  {
> >  	int ret;
> >  #ifdef CONFIG_ALTIVEC
> > -	unsigned long uninitialized_var(vrsave);
> > +	unsigned long vrsave;
> >  #endif
> 
> This variable is actually unused:
> 
> ../arch/powerpc/kvm/book3s_pr.c:1832:16: warning: unused variable 'vrsave' [-Wunused-variable]
>         unsigned long vrsave;
>                       ^
> 1 warning generated.
> 
> It has been unused since commit 99dae3bad28d ("KVM: PPC: Load/save
> FP/VMX/VSX state directly to/from vcpu struct").
> 
> $ git grep vrsave 99dae3bad28d8fdd32b7bfdd5e2ec7bb2d4d019d arch/powerpc/kvm/book3s_pr.c
> 99dae3bad28d8fdd32b7bfdd5e2ec7bb2d4d019d:arch/powerpc/kvm/book3s_pr.c:  unsigned long uninitialized_var(vrsave);
> 
> I would nuke the whole '#ifdef' block.

Ah, thanks! I wonder why I don't have CONFIG_ALTIVEC in any of my ppc
builds. Hmmm.

-Kees

-- 
Kees Cook