linux-kernel - [tip:x86/urgent] x86/paravirt: Make native_save

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Date:   Tue, 3 Jul 2018 09:00:18 -0700
From:   tip-bot for Nick Desaulniers <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     peterz@...radead.org, tstellar@...hat.com, ndesaulniers@...gle.com,
        jgross@...e.com, torvalds@...ux-foundation.org, mka@...omium.org,
        arnd@...db.de, mingo@...nel.org, linux-kernel@...r.kernel.org,
        astrachan@...gle.com, sedat.dilek@...il.com, tglx@...utronix.de,
        hpa@...or.com
Subject: [tip:x86/urgent] x86/paravirt: Make native_save_fl() extern inline

Commit-ID:  d0a8d9378d16eb3c69bd8e6d23779fbdbee3a8c7
Gitweb:     https://git.kernel.org/tip/d0a8d9378d16eb3c69bd8e6d23779fbdbee3a8c7
Author:     Nick Desaulniers <ndesaulniers@...gle.com>
AuthorDate: Thu, 21 Jun 2018 09:23:24 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 3 Jul 2018 10:56:27 +0200

x86/paravirt: Make native_save_fl() extern inline

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>
Suggested-by: Arnd Bergmann <arnd@...db.de>
Suggested-by: H. Peter Anvin <hpa@...or.com>
Suggested-by: Tom Stellar <tstellar@...hat.com>
Reported-by: Sedat Dilek <sedat.dilek@...il.com>
Tested-by: Sedat Dilek <sedat.dilek@...il.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
Acked-by: Juergen Gross <jgross@...e.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: acme@...hat.com
Cc: akataria@...are.com
Cc: akpm@...ux-foundation.org
Cc: andrea.parri@...rulasolutions.com
Cc: ard.biesheuvel@...aro.org
Cc: aryabinin@...tuozzo.com
Cc: astrachan@...gle.com
Cc: boris.ostrovsky@...cle.com
Cc: brijesh.singh@....com
Cc: caoj.fnst@...fujitsu.com
Cc: geert@...ux-m68k.org
Cc: ghackmann@...gle.com
Cc: gregkh@...uxfoundation.org
Cc: jan.kiszka@...mens.com
Cc: jarkko.sakkinen@...ux.intel.com
Cc: joe@...ches.com
Cc: jpoimboe@...hat.com
Cc: keescook@...gle.com
Cc: kirill.shutemov@...ux.intel.com
Cc: kstewart@...uxfoundation.org
Cc: linux-efi@...r.kernel.org
Cc: linux-kbuild@...r.kernel.org
Cc: manojgupta@...gle.com
Cc: mawilcox@...rosoft.com
Cc: michal.lkml@...kovi.net
Cc: mjg59@...gle.com
Cc: mka@...omium.org
Cc: pombredanne@...b.com
Cc: rientjes@...gle.com
Cc: rostedt@...dmis.org
Cc: thomas.lendacky@....com
Cc: tweek@...gle.com
Cc: virtualization@...ts.linux-foundation.org
Cc: will.deacon@....com
Cc: yamada.masahiro@...ionext.com
Link: http://lkml.kernel.org/r/20180621162324.36656-4-ndesaulniers@google.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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(-)

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)