From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Darren Hart <dvhart@infradead.org>,
Mikael Pettersson <mikpe@it.uu.se>,
Mikael Pettersson <mikpelinux@gmail.com>,
Dave Martin <Dave.Martin@arm.com>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable
Date: Thu, 7 Mar 2019 23:48:50 +0000 [thread overview]
Message-ID: <20190307234850.nsbpkfcit3lnmytu@shell.armlinux.org.uk> (raw)
In-Reply-To: <CAKwvOdk=9j2HvtWOpdOEw9sm0Nh2oU3c1c6+hJJ5WMweAM=4EQ@mail.gmail.com>
On Thu, Mar 07, 2019 at 11:39:08AM -0800, Nick Desaulniers wrote:
> On Thu, Mar 7, 2019 at 1:15 AM Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > Passing registers containing zero as both the address (NULL pointer)
> > and data into cmpxchg_futex_value_locked() leads clang to assign
> > the same register for both inputs on ARM, which triggers a warning
> > explaining that this instruction has unpredictable behavior on ARMv5.
> >
> > /tmp/futex-7e740e.s: Assembler messages:
> > /tmp/futex-7e740e.s:12713: Warning: source register same as write-back base
> >
> > This patch was suggested by Mikael Pettersson back in 2011 (!) with gcc-4.4,
> > as Mikael wrote:
> > "One way of fixing this is to make uaddr an input/output register, since
> > "that prevents it from overlapping any other input or output."
> >
> > but then withdrawn as the warning was determined to be harmless, and it
> > apparently never showed up again with later gcc versions.
> >
> > Now the same problem is back when compiling with clang, and we are trying
> > to get clang to build the kernel without warnings, as gcc normally does.
> >
> > Cc: Mikael Pettersson <mikpe@it.uu.se>
> > Cc: Mikael Pettersson <mikpelinux@gmail.com>
> > Cc: Dave Martin <Dave.Martin@arm.com>
> > Link: https://lore.kernel.org/linux-arm-kernel/20009.45690.158286.161591@pilspetsen.it.uu.se/
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> > arch/arm/include/asm/futex.h | 10 +++++-----
> > 1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
> > index 0a46676b4245..79790912974e 100644
> > --- a/arch/arm/include/asm/futex.h
> > +++ b/arch/arm/include/asm/futex.h
> > @@ -110,13 +110,13 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
> > preempt_disable();
> > __ua_flags = uaccess_save_and_enable();
> > __asm__ __volatile__("@futex_atomic_cmpxchg_inatomic\n"
> > - "1: " TUSER(ldr) " %1, [%4]\n"
> > - " teq %1, %2\n"
> > + "1: " TUSER(ldr) " %1, [%2]\n"
> > + " teq %1, %3\n"
> > " it eq @ explicit IT needed for the 2b label\n"
> > - "2: " TUSER(streq) " %3, [%4]\n"
> > + "2: " TUSER(streq) " %4, [%2]\n"
> > __futex_atomic_ex_table("%5")
> > - : "+r" (ret), "=&r" (val)
> > - : "r" (oldval), "r" (newval), "r" (uaddr), "Ir" (-EFAULT)
> > + : "+&r" (ret), "=&r" (val), "+&r" (uaddr)
> > + : "r" (oldval), "r" (newval), "Ir" (-EFAULT)
> > : "cc", "memory");
> > uaccess_restore(__ua_flags);
>
> Underspecification of constraints to extended inline assembly is a
> common issue exposed by other compilers (and possibly but in-effect
> infrequently compiler upgrades).
> So the reordering of the constraints means the in the assembly (notes
> for other reviewers):
> %2 -> %3
> %3 -> %4
> %4 -> %2
> Yep, looks good to me, thanks for finding this old patch and resending, Arnd!
I don't see what is "underspecified" in the original constraints.
Please explain.
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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next prev parent reply other threads:[~2019-03-07 23:49 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-07 9:14 [PATCH 1/2] futex: mark futex_detect_cmpxchg() as 'noinline' Arnd Bergmann
2019-03-07 9:14 ` [PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable Arnd Bergmann
2019-03-07 19:39 ` Nick Desaulniers
2019-03-07 23:48 ` Russell King - ARM Linux admin [this message]
2019-03-08 0:04 ` Nick Desaulniers
2019-03-08 9:54 ` Russell King - ARM Linux admin
2019-03-08 8:57 ` Ard Biesheuvel
2019-03-08 9:53 ` Russell King - ARM Linux admin
2019-03-08 10:08 ` Ard Biesheuvel
2019-03-08 10:16 ` Ard Biesheuvel
2019-03-08 10:56 ` Russell King - ARM Linux admin
2019-03-08 10:34 ` Russell King - ARM Linux admin
2019-03-08 10:45 ` Ard Biesheuvel
2019-03-08 10:58 ` Russell King - ARM Linux admin
2019-03-08 11:55 ` Ard Biesheuvel
2019-03-11 14:34 ` Arnd Bergmann
2019-03-11 14:36 ` Ard Biesheuvel
2019-03-11 16:29 ` Arnd Bergmann
2019-03-11 16:36 ` Ard Biesheuvel
2019-03-11 20:58 ` Arnd Bergmann
2019-03-08 11:55 ` Dave Martin
2019-03-07 17:19 ` [PATCH 1/2] futex: mark futex_detect_cmpxchg() as 'noinline' Joe Perches
2019-03-07 17:25 ` Russell King - ARM Linux admin
2019-03-07 17:42 ` Joe Perches
2019-03-07 18:07 ` Russell King - ARM Linux admin
2019-03-07 18:12 ` Nick Desaulniers
2019-03-07 18:21 ` Nathan Chancellor
2019-03-07 22:24 ` Arnd Bergmann
2020-12-12 12:26 ` Marco Elver
2020-12-12 20:01 ` Thomas Gleixner
2020-12-14 10:22 ` Marco Elver
2020-12-14 13:15 ` Arnd Bergmann
2020-12-15 6:09 ` Guo Ren
2020-12-15 11:26 ` Arnd Bergmann
2020-12-15 19:38 ` Sam Ravnborg
2020-12-15 23:24 ` Arnd Bergmann
2020-12-17 15:32 ` Andreas Larsson
2020-12-17 16:43 ` Arnd Bergmann
2020-12-18 11:08 ` Andreas Larsson
2020-12-17 20:03 ` Sam Ravnborg
2020-12-16 10:07 ` David Laight
2020-12-16 11:40 ` Peter Zijlstra
2020-12-20 15:44 ` Guo Ren
2020-12-20 17:49 ` Arnd Bergmann
2020-12-21 2:58 ` Guo Ren
2021-07-22 20:05 ` Nathan Chancellor
2021-10-25 13:52 ` Arnd Bergmann
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