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

[Nathan Chancellor]

On Tue, Apr 16, 2019 at 10:13:40AM +0100, Will Deacon wrote:
> 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

Yes, it does and I sure can. Greg, let me know if that is okay.