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

[Richard Weinberger <richard@nod.at>]

Eli,

Am 18.03.2016 um 02:41 schrieb Eli Cooper:
> Hi Richard,
> 
> On 2016/3/18 6:21, Richard Weinberger wrote:
>> Where exactly are the FPU regs restored in the sigregturn case?
>> Not sure if I fully understand the error scenario.
> 
> Well, sys_sigreturn() or sys_rt_sigreturn() calls copy_sc_from_user(),
> and the latter copies fpstate, which is the saved FPU state before the
> signal handler was invoked, from sigframe and restores it.
> That is correct: after returning from the signal handler, the process is
> in the same FPU state before it was invoked.
> 
> However, userspace() saves the FPU state before a system call and
> restores it after. In the sigreturn case, after sys_sigreturn() returns,
> which has already made FPU in the right state, userspace() overwrites it
> by making it in the state prior to the sigreturn was called (i.e., the
> signal handler's state). That leaves the process in question a corrupted
> FPU state.

Okay. That's what I thought/feared.
I wonder how other architectures handle this case?
Ideally I'd like to avoid as much extra code as possible in userspace().

Thanks,
//richard

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