[4.4,018/107] x86/paravirt: Make native_save_fl() extern inline

Commit Message

Greg Kroah-Hartman July 23, 2018, 12:41 p.m. UTC

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

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

From: Nick Desaulniers <[e-mail]>

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 <[e-mail]>
Debugged-by: Matthias Kaehlcke <[e-mail]>
Suggested-by: Arnd Bergmann <[e-mail]>
Suggested-by: H. Peter Anvin <[e-mail]>
Suggested-by: Tom Stellar <[e-mail]>
Reported-by: Sedat Dilek <[e-mail]>
Tested-by: Sedat Dilek <[e-mail]>
Signed-off-by: Nick Desaulniers <[e-mail]>
Acked-by: Juergen Gross <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
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Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

---
 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(-)



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Comments

Ben Hutchings Aug. 24, 2018, 11:08 p.m. UTC | #1

On Mon, 2018-07-23 at 14:41 +0200, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Nick Desaulniers <[e-mail]>
> 
> 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.
[...]

Shouldn't native_restore_fl() be changed similarly?  You added an out-
of-line definition, but it doesn't seem like it will actually be used.

Ben.

Nick Desaulniers Aug. 27, 2018, 9:06 p.m. UTC | #2

On Fri, Aug 24, 2018 at 4:08 PM Ben Hutchings
<[e-mail]> wrote:
>
> On Mon, 2018-07-23 at 14:41 +0200, Greg Kroah-Hartman wrote:
> > 4.4-stable review patch.  If anyone has any objections, please let me know.
> >
> > ------------------
> >
> > From: Nick Desaulniers <[e-mail]>
> >
> > 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.
> [...]
>
> Shouldn't native_restore_fl() be changed similarly?  You added an out-
> of-line definition, but it doesn't seem like it will actually be used.
>

Good catch, will send a patch with your reported by. Thank you for the report.

Patch

--- 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)