| Message ID | 20190415183729.170980546@linuxfoundation.org |
|---|---|
| State | Superseded |
| Headers | show |
| Series | None | expand |
On Tue, Apr 16, 2019 at 09:47:51AM -0700, Nathan Chancellor wrote: > 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 Great, so it seems this needs to be fixed in Linus's tree first, before I can backport it everywhere. Want me to send a patch for this or can you? thanks, greg k-h
--- 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: