Re: [PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable

[Nick Desaulniers]

On Thu, Mar 7, 2019 at 1:15 AM Arnd Bergmann <arnd@xxxxxxxx> 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 but then withdrawn as the warning was determined to be harmless, and it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@xxxxxxxx>
> Cc: Mikael Pettersson <mikpelinux@xxxxxxxxx>
> Cc: Dave Martin <Dave.Martin@xxxxxxx>
> Link: https://lore.kernel.org/linux-arm-kernel/20009.45690.158286.161591@xxxxxxxxxxxxxxxxxxx/
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  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!
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

I think it would be good to further credit Mikael with reported by and
suggested by tags, but not sure which email address is preferred?

-- 
Thanks,
~Nick Desaulniers