Linux Virtualization - Jul 2018 - [PATCH 4.4 018/107] x86/paravirt: Make native_save_fl() extern inline

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nick Desaulniers <ndesaulniers at google.com>

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>]

Debugged-by: Alistair Strachan <astrachan at google.com>
Debugged-by: Matthias Kaehlcke <mka at chromium.org>
Suggested-by: Arnd Bergmann <arnd at arndb.de>
Suggested-by: H. Peter Anvin <hpa at zytor.com>
Suggested-by: Tom Stellar <tstellar at redhat.com>
Reported-by: Sedat Dilek <sedat.dilek at gmail.com>
Tested-by: Sedat Dilek <sedat.dilek at gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers at google.com>
Acked-by: Juergen Gross <jgross at suse.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: acme at redhat.com
Cc: akataria at vmware.com
Cc: akpm at linux-foundation.org
Cc: andrea.parri at amarulasolutions.com
Cc: ard.biesheuvel at linaro.org
Cc: aryabinin at virtuozzo.com
Cc: astrachan at google.com
Cc: boris.ostrovsky at oracle.com
Cc: brijesh.singh at amd.com
Cc: caoj.fnst at cn.fujitsu.com
Cc: geert at linux-m68k.org
Cc: ghackmann at google.com
Cc: gregkh at linuxfoundation.org
Cc: jan.kiszka at siemens.com
Cc: jarkko.sakkinen at linux.intel.com
Cc: joe at perches.com
Cc: jpoimboe at redhat.com
Cc: keescook at google.com
Cc: kirill.shutemov at linux.intel.com
Cc: kstewart at linuxfoundation.org
Cc: linux-efi at vger.kernel.org
Cc: linux-kbuild at vger.kernel.org
Cc: manojgupta at google.com
Cc: mawilcox at microsoft.com
Cc: michal.lkml at markovi.net
Cc: mjg59 at google.com
Cc: mka at chromium.org
Cc: pombredanne at nexb.com
Cc: rientjes at google.com
Cc: rostedt at goodmis.org
Cc: thomas.lendacky at amd.com
Cc: tweek at google.com
Cc: virtualization at lists.linux-foundation.org
Cc: will.deacon at arm.com
Cc: yamada.masahiro at socionext.com
Link: http://lkml.kernel.org/r/20180621162324.36656-4-ndesaulniers at google.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.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(-)

--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -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;
 
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -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/
--- /dev/null
+++ b/arch/x86/kernel/irqflags.S
@@ -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)