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    SubjectRe: [PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable
    On Mon, 11 Mar 2019 at 15:34, Arnd Bergmann <arnd@arndb.de> wrote:
    >
    > On Fri, Mar 8, 2019 at 12:56 PM Ard Biesheuvel
    > <ard.biesheuvel@linaro.org> wrote:
    > > On Fri, 8 Mar 2019 at 11:58, Russell King - ARM Linux admin <linux@armlinux.org.uk> wrote:
    > > > On Fri, Mar 08, 2019 at 11:45:21AM +0100, Ard Biesheuvel wrote:
    >
    > >
    > > Perhaps. So let me summarize what I do understand.
    > >
    > > 1) if futex_atomic_cmpxchg_inatomic() is instantiated *and executed*
    > > with the same compile time constant value of 0x0 for newval and uaddr,
    > > we end up with an opcode for the STRT instruction that is CONSTRAINED
    > > UNPREDICTABLE, but we will never execute it since the preceding load
    > > will fault and enter the fixup handler.
    > > 2) such occurrences of futex_atomic_cmpxchg_inatomic() are unlikely to
    > > occur in the code, but may be instantiated by the compiler when doing
    > > constant propagation (like in the ilog2() case I quoted), but these
    > > instantiations will never be called
    > > 3) both clang and gcc may map different inline asm input operands onto
    > > the same register if the value is guaranteed to be the same (i.e.,
    > > they are both compile time constants)
    > >
    > > My statement about uaddr was slightly misguided, in the sense that our
    > > invocation of STRT does use the post-index variant, but with an
    > > increment of zero, so the value written back to the register equals
    > > the original value. But it does explain why this opcode is CONSTRAINED
    > > UNPREDICTABLE in the first place.
    > >
    > > Given 2) above, I wonder if anyone could confirm whether adding
    > > 'BUG_ON(__builtin_constant_p(uaddr))' silences the warning as well.
    >
    > Like this?
    >
    > diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
    > index 0a46676b4245..e6e9b403cd61 100644
    > --- a/arch/arm/include/asm/futex.h
    > +++ b/arch/arm/include/asm/futex.h
    > @@ -57,6 +57,7 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
    > /* Prefetching cannot fault */
    > prefetchw(uaddr);
    > __ua_flags = uaccess_save_and_enable();
    > + BUG_ON(__builtin_constant_p(uaddr) || !uaddr);
    > __asm__ __volatile__("@futex_atomic_cmpxchg_inatomic\n"
    > "1: ldrex %1, [%4]\n"
    > " teq %1, %2\n"
    >
    > This had no effect here.
    >
    > My first attempt (before finding the original patch from Mikael Pettersson)
    > was to change the probe to pass '1' as the value instead of '0', that
    > worked fine.
    >

    Which probe is that?

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