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

Geert Uytterhoeven
2018-09-28T07:12:10Z

Hi Thomas,

On Fri, Sep 28, 2018 at 8:21 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> 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.

Not sure. My "[PATCH] futex: Switch to USER_DS for futex test"
(https://www.spinics.net/lists/stable/msg28846.html), which is
basically the same
as this patch, broke s390, so it was never merged.

See "[BUG -next] "futex: switch to USER_DS for futex test" breaks s390"
(https://www.spinics.net/lists/linux-next/msg27902.html)

Heiko said:
| Martin and I discussed this today and we will change the s390 code so that
| it will also survive very early USER_DS accesses (without valid current->mm)
| since we also discovered a couple of other oddities in our code.

I don't know if that has happened, and whether it would work on s390 now.

> > 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
> > }

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds