linux-kernel - Re: [PATCH 06/11] x86-64: Fix %cs value in convert_from

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Message-ID: <AANLkTi=G+=Hc3e8Z+HH6yJPGi4TM3TyYZ6UvdUpHyBsC@mail.gmail.com>
Date:	Sun, 29 Aug 2010 21:41:06 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/11] x86-64: Fix %cs value in convert_from_fxsr()

On Sat, Aug 28, 2010 at 7:04 PM, Brian Gerst <brgerst@...il.com> wrote:
> While %ds still contains the userspace selector, %cs is KERNEL_CS
> at this point.  Always get %cs from pt_regs.
>
> It actually is possible to get the correct segments for compat tasks,
> but that involves using the [f]xsave instruction without a REX.W prefix.
>
> Signed-off-by: Brian Gerst <brgerst@...il.com>

It might be just me but the above description doesn't explain
anything. What's the problem here? What is this fixing?

> ---
>  arch/x86/kernel/i387.c |   18 ++++++++----------
>  1 files changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
> index c795675..b1a732d 100644
> --- a/arch/x86/kernel/i387.c
> +++ b/arch/x86/kernel/i387.c
> @@ -383,19 +383,17 @@ convert_from_fxsr(struct user_i387_ia32_struct *env, struct task_struct *tsk)
>  #ifdef CONFIG_X86_64
>        env->fip = fxsave->rip;
>        env->foo = fxsave->rdp;
> +       /*
> +        * should be actually ds/cs at fpu exception time, but
> +        * that information is not available in 64bit mode.
> +        */
> +       env->fcs = task_pt_regs(tsk)->cs;
>        if (tsk == current) {
> -               /*
> -                * should be actually ds/cs at fpu exception time, but
> -                * that information is not available in 64bit mode.
> -                */
> -               asm("mov %%ds, %[fos]" : [fos] "=r" (env->fos));
> -               asm("mov %%cs, %[fcs]" : [fcs] "=r" (env->fcs));
> +               savesegment(ds, env->fos);
>        } else {
> -               struct pt_regs *regs = task_pt_regs(tsk);
> -
> -               env->fos = 0xffff0000 | tsk->thread.ds;
> -               env->fcs = regs->cs;
> +               env->fos = tsk->thread.ds;
>        }
> +       env->fos |= 0xffff0000;
>  #else
>        env->fip = fxsave->fip;
>        env->fcs = (u16) fxsave->fcs | ((u32) fxsave->fop << 16);
> --
> 1.7.2.2
>
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