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    Subject[PATCH 5.18 0953/1095] x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y
    Date
    From: Andrea Righi <andrea.righi@canonical.com>

    [ Upstream commit de979c83574abf6e78f3fa65b716515c91b2613d ]

    With CONFIG_PREEMPTION disabled, arch/x86/entry/thunk_$(BITS).o becomes
    an empty object file.

    With some old versions of binutils (i.e., 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>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Link: https://lore.kernel.org/r/Ys/Ke7EWjcX+ZlXO@arighi-desktop
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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(-)

    diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
    index eeadbd7d92cc..ca2fe186994b 100644
    --- a/arch/x86/entry/Makefile
    +++ b/arch/x86/entry/Makefile
    @@ -11,12 +11,13 @@ CFLAGS_REMOVE_common.o = $(CC_FLAGS_FTRACE)

    CFLAGS_common.o += -fno-stack-protector

    -obj-y := entry.o entry_$(BITS).o thunk_$(BITS).o syscall_$(BITS).o
    +obj-y := entry.o entry_$(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 7591bab060f7..ff6e7003da97 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 505b488fcc65..f38b07d2768b 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
    diff --git a/arch/x86/um/Makefile b/arch/x86/um/Makefile
    index ba5789c35809..a8cde4e8ab11 100644
    --- a/arch/x86/um/Makefile
    +++ b/arch/x86/um/Makefile
    @@ -28,7 +28,8 @@ 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

    --
    2.35.1


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