Re: [PATCH 4.9 72/76] arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value

[Will Deacon]

On Mon, Apr 15, 2019 at 03:01:51PM -0700, Nathan Chancellor wrote:
> On Mon, Apr 15, 2019 at 08:44:36PM +0200, Greg Kroah-Hartman wrote:
> > From: Will Deacon <will.deacon@xxxxxxx>

[...]

> > @@ -53,29 +53,29 @@
> >  static inline int
> >  arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
> >  {
> > -	int oldval = 0, ret, tmp;
> > +	int oldval, ret, tmp;
> >  
> >  	pagefault_disable();
> >  
> >  	switch (op) {
> >  	case FUTEX_OP_SET:
> > -		__futex_atomic_op("mov	%w0, %w4",
> > +		__futex_atomic_op("mov	%w3, %w4",
> >  				  ret, oldval, uaddr, tmp, oparg);
> >  		break;
> >  	case FUTEX_OP_ADD:
> > -		__futex_atomic_op("add	%w0, %w1, %w4",
> > +		__futex_atomic_op("add	%w3, %w1, %w4",
> >  				  ret, oldval, uaddr, tmp, oparg);
> >  		break;
> >  	case FUTEX_OP_OR:
> > -		__futex_atomic_op("orr	%w0, %w1, %w4",
> > +		__futex_atomic_op("orr	%w3, %w1, %w4",
> >  				  ret, oldval, uaddr, tmp, oparg);
> >  		break;
> >  	case FUTEX_OP_ANDN:
> > -		__futex_atomic_op("and	%w0, %w1, %w4",
> > +		__futex_atomic_op("and	%w3, %w1, %w4",
> >  				  ret, oldval, uaddr, tmp, ~oparg);
> >  		break;
> >  	case FUTEX_OP_XOR:
> > -		__futex_atomic_op("eor	%w0, %w1, %w4",
> > +		__futex_atomic_op("eor	%w3, %w1, %w4",
> >  				  ret, oldval, uaddr, tmp, oparg);
> >  		break;
> >  	default:
> > 
> >
> 
> This causes a (false) build warning with AOSP's GCC 4.9.4 (which is
> used to build nearly all arm64 Android kernels before 4.14):
> 
>   CC      kernel/futex.o
> ../kernel/futex.c: In function 'do_futex':
> ../kernel/futex.c:1492:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
>    return oldval == cmparg;
>                  ^
> In file included from ../kernel/futex.c:69:0:
> ../arch/arm64/include/asm/futex.h:56:6: note: 'oldval' was declared here
>   int oldval, ret, tmp;
>       ^
> 
> The only reason I bring this up is Qualcomm based kernels have a Python
> script that emulates -Werror, meaning this will be fatal for a large
> number of kernels, when this eventually gets merged into them.

Thanks. Does restoring the initial assignment of 0 suppress the bogus
warning? If so, please could you send a patch on top for stable (assuming
Greg is ok with the simple change for this)?

Will