linux-kernel - Re: [PATCH 1/2] x86: kvm: avoid -Wsometimes-uninitized warning

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Date:   Fri, 12 Jul 2019 12:02:53 +0000
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 1/2] x86: kvm: avoid -Wsometimes-uninitized warning

On Fri, Jul 12, 2019 at 11:12:29AM +0200, Arnd Bergmann wrote:
> clang points out that running a 64-bit guest on a 32-bit host
> would lead to uninitialized variables:
> 
> arch/x86/kvm/hyperv.c:1610:6: error: variable 'ingpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
>         if (!longmode) {
>             ^~~~~~~~~
> arch/x86/kvm/hyperv.c:1632:55: note: uninitialized use occurs here
>         trace_kvm_hv_hypercall(code, fast, rep_cnt, rep_idx, ingpa, outgpa);
>                                                              ^~~~~
> arch/x86/kvm/hyperv.c:1610:2: note: remove the 'if' if its condition is always true
>         if (!longmode) {
>         ^~~~~~~~~~~~~~~
> arch/x86/kvm/hyperv.c:1595:18: note: initialize the variable 'ingpa' to silence this warning
>         u64 param, ingpa, outgpa, ret = HV_STATUS_SUCCESS;
>                         ^
>                          = 0
> arch/x86/kvm/hyperv.c:1610:6: error: variable 'outgpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> arch/x86/kvm/hyperv.c:1610:6: error: variable 'param' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> 
> Since that combination is not supported anyway, change the condition
> to tell the compiler how the code is actually executed.

Hmm, the compiler *is* told all it needs:


arch/x86/kvm/x86.h:
...
static inline int is_long_mode(struct kvm_vcpu *vcpu)
{
#ifdef CONFIG_X86_64
	return vcpu->arch.efer & EFER_LMA;
#else
	return 0;
#endif
}

static inline bool is_64_bit_mode(struct kvm_vcpu *vcpu)
{
	int cs_db, cs_l;

	if (!is_long_mode(vcpu))
		return false;
	kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
	return cs_l;
}
...

so in !CONFIG_X86_64 case is_64_bit_mode() unconditionally returns
false, and the branch setting the values of the variables is always
taken.

> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/x86/kvm/hyperv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index a39e38f13029..950436c502ba 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1607,7 +1607,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>  
>  	longmode = is_64_bit_mode(vcpu);
>  
> -	if (!longmode) {
> +	if (!IS_ENABLED(CONFIG_X86_64) || !longmode) {
>  		param = ((u64)kvm_rdx_read(vcpu) << 32) |
>  			(kvm_rax_read(vcpu) & 0xffffffff);
>  		ingpa = ((u64)kvm_rbx_read(vcpu) << 32) |

So this is rather a workaround for the compiler giving false positive.
I suggest to at least rephrase the log message to inidcate this.

Roman.