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

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Message-ID: <07ee85bb-e6d6-d66e-1282-591716037625@csgroup.eu>
Date:   Wed, 31 Mar 2021 11:03:09 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>, 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 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] powerpc/vdso: fix unnecessary rebuilds of
 vgettimeofday.o



Le 28/01/2021 à 05:01, Masahiro Yamada a écrit :
> On Thu, Dec 24, 2020 at 2:12 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>>
>> 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>
>> ---
> 
> 
> Michael, please take a  look at this.
> 
> The unneeded rebuild problem is still remaining.

Seems like with this patch, vdso32_wrapper.o is not rebuilt when vdso32.so.dbg is rebuilt.

Leading to ... disasters.

I'll send a patch


> 
> 
>>
>>   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
>>
> 
>