Re: [uml-devel] [PATCH] um: fix FPU register double-restore after sigreturn

[Richard Weinberger <richard@nod.at>]

Jeff, Eli,

Am 18.03.2016 um 17:42 schrieb Jeff Dike:
> On Sat, Mar 19, 2016 at 12:13:47AM +0800, Eli Cooper wrote:
>> I agree that we should keep userspace() minimal. So what in the first
>> place is the FPU state save/restore code for? Or, what (except for
>> sigreturn) could possibly mess the FPU state of the ptrace'd process
>> without it?
> 
> My (vague) recollection was that with libc in there, I couldn't be
> confident that it wouldn't unexpectedly use FP for something.  So,
> saving and restoring FP state was a hedge against that happening.
> 
> 				Jeff
> 

git blame points to:
commit fbfe9c847edf57ac8232aeafb290f272289893a3
Author: Ingo van Lil <inguin@gmx.de>
Date:   Wed Sep 14 16:21:23 2011 -0700

    um: Save FPU registers between task switches

    Some time ago Jeff prepared 42daba316557 ("uml: stop saving process FP
    state") for UML to stop saving the process FP state between task
    switches.  The assumption was that since with SKAS0 every guest process
    runs inside a host process context the host OS will take care of keeping
    the proper FP state.

    Unfortunately this is not true for multi-threaded applications, where
    all guest threads share a single host process context yet all may use
    the FPU on their own.  Although I haven't verified it I suspect things
    to be even worse in SKAS3 mode where all guest processes run inside a
    single host process.

    The patch reintroduces the saving and restoring of the FP context
    between task switches.

    [richard@nod.at: Ingo posted this patch in 2009, sadly it was never applied
    and got lost. Now in 2011 the problem was reported by Gunnar.]

    Signed-off-by: Ingo van Lil <inguin@gmx.de>
    Signed-off-by: Richard Weinberger <richard@nod.at>
    Reported-by: <gunnarlindroth@hotmail.com>
    Tested-by: <gunnarlindroth@hotmail.com>
    Cc: Stanislav Meduna <stano@meduna.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

So, it is not that easy. :=)

Thanks,
//richard

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