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Date: Wed, 22 May 2019 08:57:33 -0700
From: Thomas Garnier <thgarnie@...omium.org>
To: "H . Peter Anvin" <hpa@...or.com>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Kristen Carlson Accardi <kristen@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	"the arch/x86 maintainers" <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Nadav Amit <namit@...are.com>, 
	Jann Horn <jannh@...gle.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 03/12] x86: Add macro to get symbol address for PIE support

On Mon, May 20, 2019 at 8:13 PM <hpa@...or.com> wrote:
>
> On May 20, 2019 4:19:28 PM PDT, Thomas Garnier <thgarnie@...omium.org> wrote:
> >From: Thomas Garnier <thgarnie@...gle.com>
> >
> >Add a new _ASM_MOVABS macro to fetch a symbol address. It will be used
> >to replace "_ASM_MOV $<symbol>, %dst" code construct that are not
> >compatible with PIE.
> >
> >Signed-off-by: Thomas Garnier <thgarnie@...gle.com>
> >---
> > arch/x86/include/asm/asm.h | 1 +
> > 1 file changed, 1 insertion(+)
> >
> >diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
> >index 3ff577c0b102..3a686057e882 100644
> >--- a/arch/x86/include/asm/asm.h
> >+++ b/arch/x86/include/asm/asm.h
> >@@ -30,6 +30,7 @@
> > #define _ASM_ALIGN    __ASM_SEL(.balign 4, .balign 8)
> >
> > #define _ASM_MOV      __ASM_SIZE(mov)
> >+#define _ASM_MOVABS   __ASM_SEL(movl, movabsq)
> > #define _ASM_INC      __ASM_SIZE(inc)
> > #define _ASM_DEC      __ASM_SIZE(dec)
> > #define _ASM_ADD      __ASM_SIZE(add)
>
> This is just about *always* wrong on x86-86. We should be using leaq sym(%rip),%reg. If it isn't reachable by leaq, then it is a non-PIE symbol like percpu. You do have to keep those distinct!

Yes, I agree. This patch is just having a shortcut when it is a
non-PIE symbol. The other patches try to separate the use cases where
a leaq sym(%rip) would work versus the need for a movabsq. There are
multiple cases where relative references are not possible because the
memory layout is different (hibernation, early boot or others).

> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.

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