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

[Aurelien Jarno]

On 2016-02-19 13:05, Paolo Bonzini wrote:
> 
> 
> On 19/02/2016 12:11, Aurelien Jarno wrote:
> > 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@xxxxxxxxx>
> >> Tested-by: Aruna Hewapathirane <aruna.hewapathirane@xxxxxxxxx>
> >> Signed-off-by: Valdis Kletnieks <valdis.kletnieks@xxxxxx>
> >>
> >> --- 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
> 
> The bug is that la can be changed by the "la &= (u32)-1" line.
> 
> So the fix could be like:
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 1505587d06e9..b9b09fec173b 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -650,10 +650,10 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
>  	u16 sel;
>  
>  	la = seg_base(ctxt, addr.seg) + addr.ea;
> -	*linear = la;
>  	*max_size = 0;
>  	switch (mode) {
>  	case X86EMUL_MODE_PROT64:
> +		*linear = la;
>  		if (is_noncanonical_address(la))
>  			goto bad;
>  
> @@ -662,6 +662,7 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
>  			goto bad;
>  		break;
>  	default:
> +		*linear = la = (u32)la;
>  		usable = ctxt->ops->get_segment(ctxt, &sel, &desc, NULL,
>  						addr.seg);
>  		if (!usable)
> @@ -689,7 +690,6 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
>  			if (size > *max_size)
>  				goto bad;
>  		}
> -		la &= (u32)-1;
>  		break;
>  	}
>  	if (insn_aligned(ctxt, size) && ((la & (size - 1)) != 0))
> 
> 
> Can you test it?

Sorry about the other mail, I missed this one. I have just tested you
patch above, and I confirm it works fine. Thanks.

Tested-by: Aurelien Jarno <aurelien@xxxxxxxxxxx>

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