linux-kernel - Re: [x86] do_arch_prctl

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Date:	Tue, 18 Nov 2008 18:35:07 +0100
From:	Eric Lacombe <goretux@...il.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [x86] do_arch_prctl - bug?

Le mardi 18 novembre 2008 15:45:56, vous avez écrit :
> On Tue, 18 Nov 2008 15:33:32 +0100
>
> Eric Lacombe <goretux@...il.com> wrote:
> > Hello,
> >
> > I would like to know why the ARCH_SET_GS action of sys_arch_prctl,
> > write the MSR MSR_KERNEL_GS_BASE and not the MSR MSR_GS_BASE when the
> > variable "doit" equals 1? Is that a bug?
>
> I don't think it is.
> The trick is that we use "swapgs" on entering/leaving the kernel, and
> that will "swap" gs with the MSR, so when we return to userspace, GS
> gets loaded from the MSR_KERNEL_GS_BASE ...

Yeah when we enter the kernel swapgs is used, so the MSR_GS_BASE is switched 
with the MSR_KERNEL_GS_BASE.

In fact, what I certainly misunderstand is why load_gs_index use swapgs 
inside.
>From that function, I trust that only when gs is loaded, its hidden part is 
loaded with the MSR_GS_BASE.

ENTRY(native_load_gs_index)
        CFI_STARTPROC
        pushf
        CFI_ADJUST_CFA_OFFSET 8
        DISABLE_INTERRUPTS(CLBR_ANY | ~(CLBR_RDI))
        SWAPGS
gs_change:     
        movl %edi,%gs   
2:      mfence          /* workaround */
        SWAPGS
        popf
        CFI_ADJUST_CFA_OFFSET -8
        ret
        CFI_ENDPROC
ENDPROC(native_load_gs_index)

Regards,

	Eric

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