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

[...]

> > @@ -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

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