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Date: Tue, 16 Apr 2019 10:13:40 +0100 From: Will Deacon <will.deacon@....com> To: Nathan Chancellor <natechancellor@...il.com> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, linux-kernel@...r.kernel.org, stable@...r.kernel.org, stable@...nel.org Subject: Re: [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@....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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