x86/vdso/32: Assemble sigreturn.S separately

sigreturn.S was historically included by the various
__kernel_vsyscall implementations due to assumptions about all the
32-bit vDSO images having the sigreturn symbols at the same address.

Those assumptions were removed in v3.16, and as of v4.4, there is only
a single 32-bit vDSO left.

Simplify the build process by assembling sigreturn.S into a normal
object file.

Signed-off-by: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/entry/vdso/Makefile

···
 override obj-dirs = $(dir $(obj)) $(obj)/vdso32/
 
 targets += vdso32/vdso32.lds
-targets += vdso32/note.o vdso32/vclock_gettime.o vdso32/system_call.o
+targets += vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o
 targets += vdso32/vclock_gettime.o
 
 KBUILD_AFLAGS_32 := $(filter-out -m64,$(KBUILD_AFLAGS)) -DBUILD_VDSO
···
 		      $(obj)/vdso32/vdso32.lds \
 		      $(obj)/vdso32/vclock_gettime.o \
 		      $(obj)/vdso32/note.o \
-		      $(obj)/vdso32/system_call.o
+		      $(obj)/vdso32/system_call.o \
+		      $(obj)/vdso32/sigreturn.o
 	$(call if_changed,vdso)
 
 #

arch/x86/entry/vdso/vdso32/sigreturn.S

···
-/*
- * Common code for the sigreturn entry points in vDSO images.
- * So far this code is the same for both int80 and sysenter versions.
- * This file is #include'd by int80.S et al to define them first thing.
- * The kernel assumes that the addresses of these routines are constant
- * for all vDSO implementations.
- */
-
 #include <linux/linkage.h>
 #include <asm/unistd_32.h>
 #include <asm/asm-offsets.h>

arch/x86/entry/vdso/vdso32/system_call.S

···
  * AT_SYSINFO entry point
 */
 
+#include <linux/linkage.h>
 #include <asm/dwarf2.h>
 #include <asm/cpufeatures.h>
 #include <asm/alternative-asm.h>
-
-/*
- * First get the common code for the sigreturn entry points.
- * This must come first.
- */
-#include "sigreturn.S"
 
 	.text
 	.globl __kernel_vsyscall