Messages in this thread | | | From | Ard Biesheuvel <> | Date | Mon, 11 Mar 2019 15:36:21 +0100 | Subject | Re: [PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable |
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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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