linux-kernel - Re: [PATCH 1/1] i387: ptrace breaks the lazy-fpu-restore logic

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Date:	Tue, 17 Apr 2012 00:09:25 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <chuckebbert.lk@...il.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] i387: ptrace breaks the lazy-fpu-restore logic

Damn, I forgot to add

Cc: <stable@...r.kernel.org> # 3.3

On 04/16, Oleg Nesterov wrote:
> Starting from 7e16838d "i387: support lazy restore of FPU state"
> we assume that fpu_owner_task doesn't need restore_fpu_checking()
> on the context switch, its FPU state should match what we already
> have in the FPU on this CPU.
> 
> However, debugger can change the tracee's FPU state, in this case
> we should reset fpu.last_cpu to ensure fpu_lazy_restore() can't
> return true.
> 
> Change init_fpu() to do this, it is called by user_regset->set()
> methods.
> 
> Reported-by: Jan Kratochvil <jan.kratochvil@...hat.com>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  arch/x86/kernel/i387.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
> index 7734bcb..2d6e649 100644
> --- a/arch/x86/kernel/i387.c
> +++ b/arch/x86/kernel/i387.c
> @@ -235,6 +235,7 @@ int init_fpu(struct task_struct *tsk)
>  	if (tsk_used_math(tsk)) {
>  		if (HAVE_HWFP && tsk == current)
>  			unlazy_fpu(tsk);
> +		tsk->thread.fpu.last_cpu = ~0;
>  		return 0;
>  	}
>  
> -- 
> 1.5.5.1
> 

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