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

[Aurelien Jarno <aurelien@aurel32.net>]

On 2016-02-19 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@gmail.com>
> > Tested-by: Aruna Hewapathirane <aruna.hewapathirane@gmail.com>
> > Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.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. 

Actually the same patch with a bit more context shows the issue:

> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index e7a4fde..b372a75 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -647,12 +647,13 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
>  	bool usable;
>  	ulong la;
>  	u32 lim;
>  	u16 sel;
>  
>  	la = seg_base(ctxt, addr.seg) + addr.ea;
> +	*linear = la;

The assignation is moved here...

>  	*max_size = 0;
>  	switch (mode) {
>  	case X86EMUL_MODE_PROT64:
>  		if (is_noncanonical_address(la))
>  			goto bad;
>  
> @@ -690,13 +691,12 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
>  		}
>  		la &= (u32)-1;

... while the value of la might be modified in between.


>  		break;
>  	}
>  	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)
>  		return emulate_ss(ctxt, 0);
>  	else
>  		return emulate_gp(ctxt, 0);

One possibility would be to assign it both at the beginning of the
function and at the original location should fix the bug and prevent GCC
to issue a warning.

Aurelien

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