linux-kernel - [PATCH 1/2] powerpc/vdso: fix unnecessary rebuilds of vgettimeofday.o

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Message-Id: <20201223171142.707053-1-masahiroy@kernel.org>
Date:   Thu, 24 Dec 2020 02:11:41 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Daniel Axtens <dja@...ens.net>,
        Greentime Hu <green.hu@...il.com>,
        Michal Suchanek <msuchanek@...e.de>,
        Nicholas Piggin <npiggin@...il.com>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] 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.

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

 arch/powerpc/kernel/Makefile                      | 4 ++--
 arch/powerpc/kernel/vdso32/Makefile               | 5 +----
 arch/powerpc/kernel/{vdso32 => }/vdso32_wrapper.S | 0
 arch/powerpc/kernel/vdso64/Makefile               | 6 +-----
 arch/powerpc/kernel/{vdso64 => }/vdso64_wrapper.S | 0
 5 files changed, 4 insertions(+), 11 deletions(-)
 rename arch/powerpc/kernel/{vdso32 => }/vdso32_wrapper.S (100%)
 rename arch/powerpc/kernel/{vdso64 => }/vdso64_wrapper.S (100%)

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 59aa2944ecae..42fc3de89b39 100644
--- a/arch/powerpc/kernel/vdso32/Makefile
+++ b/arch/powerpc/kernel/vdso32/Makefile
@@ -30,7 +30,7 @@ CC32FLAGS += -m32
 KBUILD_CFLAGS := $(filter-out -mcmodel=medium,$(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
similarity index 100%
rename from arch/powerpc/kernel/vdso32/vdso32_wrapper.S
rename to arch/powerpc/kernel/vdso32_wrapper.S
diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
index d365810a689a..b50b39fedf74 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
similarity index 100%
rename from arch/powerpc/kernel/vdso64/vdso64_wrapper.S
rename to arch/powerpc/kernel/vdso64_wrapper.S
-- 
2.27.0