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

[Christophe Leroy]

Le 28/01/2021 à 05:01, Masahiro Yamada a écrit :
> On Thu, Dec 24, 2020 at 2:12 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx/
> > Reported-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> > ---
>
>
> 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