Hi Thomas, On Fri, Sep 28, 2018 at 8:21 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx 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
