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Date: Thu, 7 Jun 2018 11:39:46 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>, hpa@...or.com,
mingo@...hat.com, Thomas Gleixner <tglx@...utronix.de>
Cc: linux-efi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
x86@...nel.org, virtualization@...ts.linux-foundation.org,
Alistair Strachan <astrachan@...gle.com>,
Manoj Gupta <manojgupta@...gle.com>,
Greg Hackmann <ghackmann@...gle.com>, sedat.dilek@...il.com,
tstellar@...hat.com, Kees Cook <keescook@...gle.com>,
Masahiro Yamada <yamada.masahiro@...ionext.com>,
Michal Marek <michal.lkml@...kovi.net>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
geert@...ux-m68k.org, Will Deacon <will.deacon@....com>,
mawilcox@...rosoft.com, Arnd Bergmann <arnd@...db.de>,
David Rientjes <rientjes@...gle.com>, acme@...hat.com,
Philippe Ombredanne <pombredanne@...b.com>,
Andrey Ryabinin <aryabinin@...tuozzo.com>,
Kate Stewart <kstewart@...uxfoundation.org>,
boris.ostrovsky@...cle.com, "J. Kiszka" <jan.kiszka@...mens.com>,
rostedt@...dmis.org, kirill.shutemov@...ux.intel.com,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
akataria@...are.com, brijesh.singh@....com,
Cao jin <caoj.fnst@...fujitsu.com>,
Greg KH <gregkh@...uxfoundation.org>,
jarkko.sakkinen@...ux.intel.com, jgross@...e.com,
Josh Poimboeuf <jpoimboe@...hat.com>,
Matthias Kaehlcke <mka@...omium.org>, thomas.lendacky@....com,
Thiebaud Weksteen <tweek@...gle.com>, mjg59@...gle.com,
joe@...ches.com
Subject: Re: [PATCH v3 3/3] x86: paravirt: make native_save_fl extern inline
On Thu, Jun 7, 2018 at 11:32 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> native_save_fl() is marked static inline, but by using it as
> a function pointer in arch/x86/kernel/paravirt.c, it MUST be outlined.
>
> paravirt's use of native_save_fl() also requires that no GPRs other than
> %rax are clobbered.
>
> Compilers have different heuristics which they use to emit stack guard
> code, the emittance of which can break paravirt's callee saved assumption
> by clobbering %rcx.
>
> Marking a function definition extern inline means that if this version
> cannot be inlined, then the out-of-line version will be preferred. By
> having the out-of-line version be implemented in assembly, it cannot be
> instrumented with a stack protector, which might violate custom calling
> conventions that code like paravirt rely on.
>
> The semantics of extern inline has changed since gnu89. This means that
> folks using GCC versions >= 5.1 may see symbol redefinition errors at
> link time for subdirs that override KBUILD_CFLAGS (making the C standard
> used implicit) regardless of this patch. This has been cleaned up
> earlier in the patch set, but is left as a note in the commit message
> for future travelers.
>
> Reports:
> https://lkml.org/lkml/2018/5/7/534
> https://github.com/ClangBuiltLinux/linux/issues/16
>
> Discussion:
> https://bugs.llvm.org/show_bug.cgi?id=37512
> https://lkml.org/lkml/2018/5/24/1371
>
> Thanks to the many folks that participated in the discussion.
>
> Debugged-by: Alistair Strachan <astrachan@...gle.com>
> Debugged-by: Matthias Kaehlcke <mka@...omium.org>
> Reported-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Suggested-by: H. Peter Anvin <hpa@...or.com>
> Suggested-by: Tom Stellar <tstellar@...hat.com>
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> ---
> arch/x86/include/asm/irqflags.h | 2 +-
> arch/x86/kernel/Makefile | 1 +
> arch/x86/kernel/irqflags.S | 26 ++++++++++++++++++++++++++
> 3 files changed, 28 insertions(+), 1 deletion(-)
> create mode 100644 arch/x86/kernel/irqflags.S
>
> diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
> index 89f08955fff7..c4fc17220df9 100644
> --- a/arch/x86/include/asm/irqflags.h
> +++ b/arch/x86/include/asm/irqflags.h
> @@ -13,7 +13,7 @@
> * Interrupt control:
> */
>
> -static inline unsigned long native_save_fl(void)
> +extern inline unsigned long native_save_fl(void)
> {
> unsigned long flags;
>
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 02d6f5cf4e70..8824d01c0c35 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -61,6 +61,7 @@ obj-y += alternative.o i8253.o hw_breakpoint.o
> obj-y += tsc.o tsc_msr.o io_delay.o rtc.o
> obj-y += pci-iommu_table.o
> obj-y += resource.o
> +obj-y += irqflags.o
>
> obj-y += process.o
> obj-y += fpu/
> diff --git a/arch/x86/kernel/irqflags.S b/arch/x86/kernel/irqflags.S
> new file mode 100644
> index 000000000000..ddeeaac8adda
> --- /dev/null
> +++ b/arch/x86/kernel/irqflags.S
> @@ -0,0 +1,26 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#include <asm/asm.h>
> +#include <asm/export.h>
> +#include <linux/linkage.h>
> +
> +/*
> + * unsigned long native_save_fl(void)
> + */
> +ENTRY(native_save_fl)
> + pushf
> + pop %_ASM_AX
> + ret
> +ENDPROC(native_save_fl)
> +EXPORT_SYMBOL(native_save_fl)
> +
> +/*
> + * void native_restore_fl(unsigned long flags)
> + * %eax/%rdi: flags
> + */
> +ENTRY(native_restore_fl)
> + push %_ASM_ARG1
> + popf
> + ret
> +ENDPROC(native_restore_fl)
> +EXPORT_SYMBOL(native_restore_fl)
> --
> 2.17.1.1185.g55be947832-goog
>
Probably should have mentioned in the notes/cover letter that this was
rebased on hpa's patch (2/3) in this series. As a change from v2.
--
Thanks,
~Nick Desaulniers
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