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

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Date:   Wed, 3 Jun 2020 20:33:15 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Kees Cook <keescook@...omium.org>
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 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.

Cheers,
Nathan