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

Thomas Gleixner
2018-09-28T06:20:52Z

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
>
>