powerpc/vdso: fix unnecessary rebuilds of vgettimeofday.o

vgettimeofday.o is unnecessarily rebuilt. Adding it to 'targets' is not
enough to fix the issue. Kbuild is correctly rebuilding it because the
command line is changed.

PowerPC builds each vdso directory twice; first in vdso_prepare to
generate vdso{32,64}-offsets.h, second as part of the ordinary build
process to embed vdso{32,64}.so.dbg into the kernel.

The problem shows up when CONFIG_PPC_WERROR=y due to the following line
in arch/powerpc/Kbuild:

  subdir-ccflags-$(CONFIG_PPC_WERROR) := -Werror

In the preparation stage, Kbuild directly visits the vdso directories,
hence it does not inherit subdir-ccflags-y. In the second descend,
Kbuild adds -Werror, which results in the command line flipping
with/without -Werror.

It implies a potential danger; if a more critical flag that would impact
the resulted vdso, the offsets recorded in the headers might be different
from real offsets in the embedded vdso images.

Removing the unneeded second descend solves the problem.

Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/linuxppc-dev/87tuslxhry.fsf@mpe.ellerman.id.au/
Link: https://lore.kernel.org/r/20201223171142.707053-1-masahiroy@kernel.org

5 files changed, 4 insertions, 11 deletions

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index fe2ef598e2ea..79ee7750937d 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -51,7 +51,7 @@ obj-y				+= ptrace/
 obj-$(CONFIG_PPC64)		+= setup_64.o \
 				   paca.o nvram_64.o note.o syscall_64.o
 obj-$(CONFIG_COMPAT)		+= sys_ppc32.o signal_32.o
-obj-$(CONFIG_VDSO32)		+= vdso32/
+obj-$(CONFIG_VDSO32)		+= vdso32_wrapper.o
 obj-$(CONFIG_PPC_WATCHDOG)	+= watchdog.o
 obj-$(CONFIG_HAVE_HW_BREAKPOINT)	+= hw_breakpoint.o
 obj-$(CONFIG_PPC_DAWR)		+= dawr.o
@@ -60,7 +60,7 @@ obj-$(CONFIG_PPC_BOOK3S_64)	+= cpu_setup_power.o
 obj-$(CONFIG_PPC_BOOK3S_64)	+= mce.o mce_power.o
 obj-$(CONFIG_PPC_BOOK3E_64)	+= exceptions-64e.o idle_book3e.o
 obj-$(CONFIG_PPC_BARRIER_NOSPEC) += security.o
-obj-$(CONFIG_PPC64)		+= vdso64/
+obj-$(CONFIG_PPC64)		+= vdso64_wrapper.o
 obj-$(CONFIG_ALTIVEC)		+= vecemu.o
 obj-$(CONFIG_PPC_BOOK3S_IDLE)	+= idle_book3s.o
 procfs-y			:= proc_powerpc.o
diff --git a/arch/powerpc/kernel/vdso32/Makefile b/arch/powerpc/kernel/vdso32/Makefile
index 9cb6f524854b..7d9a6fee0e3d 100644
--- a/arch/powerpc/kernel/vdso32/Makefile
+++ b/arch/powerpc/kernel/vdso32/Makefile
@@ -30,7 +30,7 @@ CC32FLAGS += -m32
 KBUILD_CFLAGS := $(filter-out -mcmodel=medium -mabi=elfv1 -mabi=elfv2 -mcall-aixdesc,$(KBUILD_CFLAGS))
 endif
 
-targets := $(obj-vdso32) vdso32.so.dbg
+targets := $(obj-vdso32) vdso32.so.dbg vgettimeofday.o
 obj-vdso32 := $(addprefix $(obj)/, $(obj-vdso32))
 
 GCOV_PROFILE := n
@@ -46,9 +46,6 @@ obj-y += vdso32_wrapper.o
 targets += vdso32.lds
 CPPFLAGS_vdso32.lds += -P -C -Upowerpc
 
-# Force dependency (incbin is bad)
-$(obj)/vdso32_wrapper.o : $(obj)/vdso32.so.dbg
-
 # link rule for the .so file, .lds has to be first
 $(obj)/vdso32.so.dbg: $(src)/vdso32.lds $(obj-vdso32) $(obj)/vgettimeofday.o FORCE
 	$(call if_changed,vdso32ld_and_check)
diff --git a/arch/powerpc/kernel/vdso32/vdso32_wrapper.S b/arch/powerpc/kernel/vdso32_wrapper.S
index 3f5ef035b0a9..3f5ef035b0a9 100644
--- a/arch/powerpc/kernel/vdso32/vdso32_wrapper.S
+++ b/arch/powerpc/kernel/vdso32_wrapper.S
diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
index bf363ff37152..a06fb9b461fc 100644
--- a/arch/powerpc/kernel/vdso64/Makefile
+++ b/arch/powerpc/kernel/vdso64/Makefile
@@ -17,7 +17,7 @@ endif
 
 # Build rules
 
-targets := $(obj-vdso64) vdso64.so.dbg
+targets := $(obj-vdso64) vdso64.so.dbg vgettimeofday.o
 obj-vdso64 := $(addprefix $(obj)/, $(obj-vdso64))
 
 GCOV_PROFILE := n
@@ -29,15 +29,11 @@ ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
 	-Wl,-soname=linux-vdso64.so.1 -Wl,--hash-style=both
 asflags-y := -D__VDSO64__ -s
 
-obj-y += vdso64_wrapper.o
 targets += vdso64.lds
 CPPFLAGS_vdso64.lds += -P -C -U$(ARCH)
 
 $(obj)/vgettimeofday.o: %.o: %.c FORCE
 
-# Force dependency (incbin is bad)
-$(obj)/vdso64_wrapper.o : $(obj)/vdso64.so.dbg
-
 # link rule for the .so file, .lds has to be first
 $(obj)/vdso64.so.dbg: $(src)/vdso64.lds $(obj-vdso64) $(obj)/vgettimeofday.o FORCE
 	$(call if_changed,vdso64ld_and_check)
diff --git a/arch/powerpc/kernel/vdso64/vdso64_wrapper.S b/arch/powerpc/kernel/vdso64_wrapper.S
index 1d56d81fe3b3..1d56d81fe3b3 100644
--- a/arch/powerpc/kernel/vdso64/vdso64_wrapper.S
+++ b/arch/powerpc/kernel/vdso64_wrapper.S