[PATCHv4 1/4] x86/boot/compressed/64: Fix build with GCC < 5

[Kirill A. Shutemov]

0-day reported this build error:

   arch/x86/boot/compressed/pgtable_64.o: In function `l5_paging_required':
   pgtable_64.c:(.text+0x22): undefined reference to `__force_order'

The issue is only with GCC < 5 and when KASLR is disabled. Newer GCC
works fine.

__force_order is used by special_insns.h asm code to force instruction
serialization.

It doesn't actually referenced from the code, but GCC < 5 with -fPIE
would still generate undefined symbol.

I didn't noticed this before and failed to move __force_order definition
from pagetable.c (which compiles only with KASLR enabled) to
pgtable_64.c.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Fixes: 10c9a5346f72 ("x86/boot/compressed/64: Detect and handle 5-level paging at boot-time")
Cc: stable@xxxxxxxxxxxxxxx
---
 arch/x86/boot/compressed/pagetable.c  |  3 ---
 arch/x86/boot/compressed/pgtable_64.c | 11 +++++++++++
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/boot/compressed/pagetable.c b/arch/x86/boot/compressed/pagetable.c
index 6bd51de4475c..250826ac216e 100644
--- a/arch/x86/boot/compressed/pagetable.c
+++ b/arch/x86/boot/compressed/pagetable.c
@@ -38,9 +38,6 @@
 #define __PAGE_OFFSET __PAGE_OFFSET_BASE
 #include "../../mm/ident_map.c"
 
-/* Used by pgtable.h asm code to force instruction serialization. */
-unsigned long __force_order;
-
 /* Used to track our page table allocation area. */
 struct alloc_pgt_data {
 	unsigned char *pgt_buf;
diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index 7bcf03b376da..491fa2d08bca 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -1,5 +1,16 @@
 #include <asm/processor.h>
 
+/*
+ * __force_order is used by special_insns.h asm code to force instruction
+ * serialization.
+ *
+ * It doesn't actually referenced from the code, but GCC < 5 with -fPIE
+ * would still generate undefined symbol.
+ *
+ * Let's workaround this by defining the variable.
+ */
+unsigned long __force_order;
+
 int l5_paging_required(void)
 {
 	/* Check if leaf 7 is supported. */
-- 
2.15.0