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 11:00:52AM +0200, Greg Kroah-Hartman 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>
> > > 
> > > commit 045afc24124d80c6998d9c770844c67912083506 upstream.
> > > 
> > > Rather embarrassingly, our futex() FUTEX_WAKE_OP implementation doesn't
> > > explicitly set the return value on the non-faulting path and instead
> > > leaves it holding the result of the underlying atomic operation. This
> > > means that any FUTEX_WAKE_OP atomic operation which computes a non-zero
> > > value will be reported as having failed. Regrettably, I wrote the buggy
> > > code back in 2011 and it was upstreamed as part of the initial arm64
> > > support in 2012.
> > > 
> > > The reasons we appear to get away with this are:
> > > 
> > >   1. FUTEX_WAKE_OP is rarely used and therefore doesn't appear to get
> > >      exercised by futex() test applications
> > > 
> > >   2. If the result of the atomic operation is zero, the system call
> > >      behaves correctly
> > > 
> > >   3. Prior to version 2.25, the only operation used by GLIBC set the
> > >      futex to zero, and therefore worked as expected. From 2.25 onwards,
> > >      FUTEX_WAKE_OP is not used by GLIBC at all.
> > > 
> > > Fix the implementation by ensuring that the return value is either 0
> > > to indicate that the atomic operation completed successfully, or -EFAULT
> > > if we encountered a fault when accessing the user mapping.
> > > 
> > > Cc: <stable@xxxxxxxxxx>
> > > Fixes: 6170a97460db ("arm64: Atomic operations")
> > > Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > 
> > > ---
> > >  arch/arm64/include/asm/futex.h |   16 ++++++++--------
> > >  1 file changed, 8 insertions(+), 8 deletions(-)
> > > 
> > > --- a/arch/arm64/include/asm/futex.h
> > > +++ b/arch/arm64/include/asm/futex.h
> > > @@ -33,8 +33,8 @@
> > >  "	prfm	pstl1strm, %2\n"					\
> > >  "1:	ldxr	%w1, %2\n"						\
> > >  	insn "\n"							\
> > > -"2:	stlxr	%w3, %w0, %2\n"						\
> > > -"	cbnz	%w3, 1b\n"						\
> > > +"2:	stlxr	%w0, %w3, %2\n"						\
> > > +"	cbnz	%w0, 1b\n"						\
> > >  "	dmb	ish\n"							\
> > >  "3:\n"									\
> > >  "	.pushsection .fixup,\"ax\"\n"					\
> > > @@ -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.
> 
> Argh, really?  That's a buggy compiler that you have there, as oldval
> will be set correctly if all is good, and if not, ret will be and the
> code will error out.
> 

Correct.

> Working around broken compilers is not something I really like doing :(
> 

Indeed, I wouldn't have brought it up if it wasn't the compiler for all
Android 4.9 kernels aside from the Pixel 3 (XL).

> That being said, does this also show up in the 4.19.y and 5.0.y tree
> right now?  If not, why not?
> 

It does.

$ make ARCH=arm64 CROSS_COMPILE=<path>/bin/aarch64-linux-gnu- defconfig kernel/futex.o

> thanks,
> 
> greg k-h