[tip: x86/entry] x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y

[tip-bot2 for Andrea Righi]

The following commit has been merged into the x86/entry branch of tip:

Commit-ID:     e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
Gitweb:        https://git.kernel.org/tip/e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
Author:        Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
AuthorDate:    Thu, 14 Jan 2021 12:48:35 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 21 Jan 2021 08:11:52 +01:00

x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y

With CONFIG_PREEMPTION disabled, arch/x86/entry/thunk_64.o is just an
empty object file.

With the newer binutils (tested with 2.35.90.20210113-1ubuntu1) the GNU
assembler doesn't generate a symbol table for empty object files and
objtool fails with the following error when a valid symbol table cannot
be found:

  arch/x86/entry/thunk_64.o: warning: objtool: missing symbol table

To prevent this from happening, build thunk_$(BITS).o only if
CONFIG_PREEMPTION is enabled.

  BugLink: https://bugs.launchpad.net/bugs/1911359

Fixes: 320100a5ffe5 ("x86/entry: Remove the TRACE_IRQS cruft")
Signed-off-by: Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Link: https://lore.kernel.org/r/YAAvk0UQelq0Ae7+@xps-13-7390
---
 arch/x86/entry/Makefile   | 3 ++-
 arch/x86/entry/thunk_32.S | 2 --
 arch/x86/entry/thunk_64.S | 4 ----
 3 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
index 08bf95d..83c98da 100644
--- a/arch/x86/entry/Makefile
+++ b/arch/x86/entry/Makefile
@@ -21,12 +21,13 @@ CFLAGS_syscall_64.o		+= $(call cc-option,-Wno-override-init,)
 CFLAGS_syscall_32.o		+= $(call cc-option,-Wno-override-init,)
 CFLAGS_syscall_x32.o		+= $(call cc-option,-Wno-override-init,)
 
-obj-y				:= entry_$(BITS).o thunk_$(BITS).o syscall_$(BITS).o
+obj-y				:= entry_$(BITS).o syscall_$(BITS).o
 obj-y				+= common.o
 
 obj-y				+= vdso/
 obj-y				+= vsyscall/
 
+obj-$(CONFIG_PREEMPTION)	+= thunk_$(BITS).o
 obj-$(CONFIG_IA32_EMULATION)	+= entry_64_compat.o syscall_32.o
 obj-$(CONFIG_X86_X32_ABI)	+= syscall_x32.o
 
diff --git a/arch/x86/entry/thunk_32.S b/arch/x86/entry/thunk_32.S
index f1f96d4..5997ec0 100644
--- a/arch/x86/entry/thunk_32.S
+++ b/arch/x86/entry/thunk_32.S
@@ -29,10 +29,8 @@ SYM_CODE_START_NOALIGN(\name)
 SYM_CODE_END(\name)
 	.endm
 
-#ifdef CONFIG_PREEMPTION
 	THUNK preempt_schedule_thunk, preempt_schedule
 	THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
 	EXPORT_SYMBOL(preempt_schedule_thunk)
 	EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
-#endif
 
diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S
index 496b11e..9d543c4 100644
--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -31,14 +31,11 @@ SYM_FUNC_END(\name)
 	_ASM_NOKPROBE(\name)
 	.endm
 
-#ifdef CONFIG_PREEMPTION
 	THUNK preempt_schedule_thunk, preempt_schedule
 	THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
 	EXPORT_SYMBOL(preempt_schedule_thunk)
 	EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
-#endif
 
-#ifdef CONFIG_PREEMPTION
 SYM_CODE_START_LOCAL_NOALIGN(__thunk_restore)
 	popq %r11
 	popq %r10
@@ -53,4 +50,3 @@ SYM_CODE_START_LOCAL_NOALIGN(__thunk_restore)
 	ret
 	_ASM_NOKPROBE(__thunk_restore)
 SYM_CODE_END(__thunk_restore)
-#endif