Messages in this thread | | | From | Arnd Bergmann <> | Date | Mon, 11 Mar 2019 21:58:55 +0100 | Subject | Re: [PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable |
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On Mon, Mar 11, 2019 at 5:36 PM Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: > > On Mon, 11 Mar 2019 at 17:30, Arnd Bergmann <arnd@arndb.de> wrote: > > > > On Mon, Mar 11, 2019 at 3:36 PM Ard Biesheuvel > > <ard.biesheuvel@linaro.org> wrote: > > > 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: > > > > > > > > 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? > > > > diff --git a/kernel/futex.c b/kernel/futex.c > > index c3b73b0311bc..19615ad3c4f7 100644 > > --- a/kernel/futex.c > > +++ b/kernel/futex.c > > @@ -3864,7 +3864,7 @@ static void __init futex_detect_cmpxchg(void) > > * implementation, the non-functional ones will return > > * -ENOSYS. > > */ > > - if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT) > > + if (cmpxchg_futex_value_locked(&curval, NULL, 1, 1) == -EFAULT) > > futex_cmpxchg_enabled = 1; > > #endif > > } > > > > Ah ok. > > That explains a lot. > > Can't we just return -EFAULT if uaddr is NULL? Or does that defeat this check?
I think that would work here, it would just create a tiny overhead for each call to futex_atomic_cmpxchg_inatomic().
Semi-related side note: After I looked at access_ok() for a bit too long, I tried replacing it with
#define access_ok(addr, size) \ (((u64)(uintptr_t)addr + (u64)(size_t)size) >= current_thread_info()->addr_limit)
which interestingly seemed to improve the output with clang (it lets it combine multiple access_ok() checks and schedule the instructions better, compared to our inline asm implementation), but it unfortunately creates horrible code with gcc.
Arnd
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