Re: [PATCH] futex: Set USER_DS for the futex_detect_cmpxchg() test

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 27 Sep 2018, Andy Lutomirski wrote:
> I have a couple questions here:
> 
>  - Is this actually okay on all architectures?  That is, are there
>    cases where we'll screw up if we fail a USER_DS access this early?
>    s390 stands out as the obvious special case (where USER_DS is not
>    than just a subset of KERNEL_DS), but s390 opts out.
>
>  - Why doesn't x86 set HAVE_FUTEX_CMPXCHG?  Or do we still support
>    some 32-bit configurations that don't have cmpxchg and don't know
>    about it at compile time?

I'm not entirely sure. Have to dig into the details. I assume S390 just can
set it though.

Thanks,

	tglx

>  kernel/futex.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 11fc3bb456d6..16bd3e72602a 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -3593,6 +3593,7 @@ static void __init futex_detect_cmpxchg(void)
>  {
>  #ifndef CONFIG_HAVE_FUTEX_CMPXCHG
>  	u32 curval;
> +	mm_segment_t old_seg;
>  
>  	/*
>  	 * This will fail and we want it. Some arch implementations do
> @@ -3604,8 +3605,11 @@ static void __init futex_detect_cmpxchg(void)
>  	 * implementation, the non-functional ones will return
>  	 * -ENOSYS.
>  	 */
> +	old_seg = get_fs();
> +	set_fs(USER_DS);
>  	if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
>  		futex_cmpxchg_enabled = 1;
> +	set_fs(old_seg);
>  #endif
>  }
>  
> -- 
> 2.17.1
> 
>