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Subject[PATCH v2] 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@canonical.com>
---
arch/x86/entry/Makefile | 3 ++-
arch/x86/entry/thunk_32.S | 2 --
arch/x86/entry/thunk_64.S | 4 ----
arch/x86/um/Makefile | 3 ++-
4 files changed, 4 insertions(+), 8 deletions(-)

ChangeLog (v1 -> v2):
- do not break UML build

diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
index 08bf95dbc911..83c98dae74a6 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 f1f96d4d8cd6..5997ec0b4b17 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 ccd32877a3c4..c7cf79be7231 100644
--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -36,14 +36,11 @@ 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(.L_restore)
popq %r11
popq %r10
@@ -58,4 +55,3 @@ SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
ret
_ASM_NOKPROBE(.L_restore)
SYM_CODE_END(.L_restore)
-#endif
diff --git a/arch/x86/um/Makefile b/arch/x86/um/Makefile
index 77f70b969d14..3113800da63a 100644
--- a/arch/x86/um/Makefile
+++ b/arch/x86/um/Makefile
@@ -27,7 +27,8 @@ else

obj-y += syscalls_64.o vdso/

-subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o ../entry/thunk_64.o
+subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o
+subarch-$(CONFIG_PREEMPTION) += ../entry/thunk_64.o

endif

--
2.29.2
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