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SubjectRe: [x86] do_arch_prctl - bug?
From
Date
Le mardi 18 novembre 2008 15:45:56, vous avez écrit :
> On Tue, 18 Nov 2008 15:33:32 +0100
>
> Eric Lacombe <goretux@gmail.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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