linux-kernel - Re: Silence compiler warning in arch/x86/kvm/emulate.c

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Date:	Fri, 19 Feb 2016 12:11:39 +0100
From:	Aurelien Jarno <aurelien@...el32.net>
To:	Valdis Kletnieks <Valdis.Kletnieks@...edu>
Cc:	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Aruna Hewapathirane <aruna.hewapathirane@...il.com>
Subject: Re: Silence compiler warning in  arch/x86/kvm/emulate.c

On 2015-08-29 17:49, Valdis Kletnieks wrote:
> Compiler warning:
> 
>  CC [M]  arch/x86/kvm/emulate.o
> arch/x86/kvm/emulate.c: In function "__do_insn_fetch_bytes":
> arch/x86/kvm/emulate.c:814:9: warning: "linear" may be used uninitialized in this function [-Wmaybe-uninitialized]
> 
> GCC is smart enough to realize that the inlined __linearize may return before
> setting the value of linear, but not smart enough to realize the same
> X86EMU_CONTINUE blocks actual use of the value.  However, the value of
> 'linear' can only be set to one value, so hoisting the one line of code
> upwards makes GCC happy with the code.
> 
> Reported-by: Aruna Hewapathirane <aruna.hewapathirane@...il.com>
> Tested-by: Aruna Hewapathirane <aruna.hewapathirane@...il.com>
> Signed-off-by: Valdis Kletnieks <valdis.kletnieks@...edu>
> 
> --- a/arch/x86/kvm/emulate.c.dist	2015-08-11 14:10:05.366061993 -0400
> +++ b/arch/x86/kvm/emulate.c	2015-08-29 13:43:13.014163958 -0400
> @@ -650,6 +650,7 @@ static __always_inline int __linearize(s
>  	u16 sel;
>  
>  	la = seg_base(ctxt, addr.seg) + addr.ea;
> +	*linear = la;
>  	*max_size = 0;
>  	switch (mode) {
>  	case X86EMUL_MODE_PROT64:
> @@ -693,7 +694,6 @@ static __always_inline int __linearize(s
>  	}
>  	if (insn_aligned(ctxt, size) && ((la & (size - 1)) != 0))
>  		return emulate_gp(ctxt, 0);
> -	*linear = la;
>  	return X86EMUL_CONTINUE;
>  bad:
>  	if (addr.seg == VCPU_SREG_SS)
> 

Unfortunately this patch broke GNU/Hurd when running under KVM. It fails
to boot almost immediately. I haven't debug it more, but it looks like
*linear should not always be written. This can easily be reproduced by
trying to boot Debian Installer from this ISO:

http://ftp.debian-ports.org/debian-cd/hurd-i386/debian-hurd-2015/debian-hurd-2015-i386-CD-1.iso

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@...el32.net                 http://www.aurel32.net