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

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Date:   Thu, 28 Jan 2021 13:01:19 +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 <linuxppc-dev@...ts.ozlabs.org>
Cc:     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 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] powerpc/vdso: fix unnecessary rebuilds of vgettimeofday.o

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.



>
>  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
>


-- 
Best Regards
Masahiro Yamada