Patch "x86/paravirt: Make native_save_fl() extern inline" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

x86/paravirt: Make native_save_fl() extern inline

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-paravirt-make-native_save_fl-extern-inline.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From d0a8d9378d16eb3c69bd8e6d23779fbdbee3a8c7 Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <[email protected]>
Date: Thu, 21 Jun 2018 09:23:24 -0700
Subject: x86/paravirt: Make native_save_fl() extern inline

From: Nick Desaulniers <[email protected]>

commit d0a8d9378d16eb3c69bd8e6d23779fbdbee3a8c7 upstream.

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.

[Backport for 4.4. 4.4 is missing commit 784d5699eddc "x86: move exports to actual definitions" which doesn't apply cleanly, and not really worth backporting IMO. It's simpler to change this patch from upstream:
+ #include <asm-generic/export.h>
rather than
+ #include <asm/export.h>]

---
 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(-)

@@ -8,7 +8,7 @@
  * Interrupt control:
  */
 
-static inline unsigned long native_save_fl(void)
+extern inline unsigned long native_save_fl(void)
 {
 	unsigned long flags;
 

@@ -41,6 +41,7 @@ obj-y			+= alternative.o i8253.o pci-nom
 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/

@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <asm/asm.h>
+#include <asm-generic/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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/x86-paravirt-make-native_save_fl-extern-inline.patch