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SubjectRe: [PATCH 4.9 72/76] arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value
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@arm.com>
> > >
> > > 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@kernel.org>
> > > Fixes: 6170a97460db ("arm64: Atomic operations")
> > > Signed-off-by: Will Deacon <will.deacon@arm.com>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > >
> > > ---
> > > 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

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