Re: [PATCH] arm64: do not descend to vdso directories twice

From: Masahiro Yamada
Date: Mon Dec 21 2020 - 11:05:10 EST

On Mon, Dec 21, 2020 at 11:36 PM Vincenzo Frascino
<vincenzo.frascino@xxxxxxx> wrote:
>
> Hi Masahiro,
>
> On 12/18/20 2:45 AM, Masahiro Yamada wrote:
> > arm64 descends into each vdso directory twice; first in vdso_prepare,
> > second during the ordinary build process.
> >
> > PPC mimicked it and uncovered a problem [1]. In the first descend,
> > Kbuild directly visits the vdso directories, therefore it does not
> > inherit subdir-ccflags-y from upper directories.
> >
> > This means the command line parameters may differ between the two.
> > If it happens, the offset values in the generated headers might be
> > different from real offsets of vdso.so in the kernel.
> >
> > This potential danger should be avoided. The vdso directories are
> > built in the vdso_prepare stage, so the second descend is unneeded.
> >
> > [1]: https://lore.kernel.org/linux-kbuild/CAK7LNARAkJ3_-4gX0VA2UkapbOftuzfSTVMBbgbw=HD8n7N+7w@xxxxxxxxxxxxxx/T/#ma10dcb961fda13f36d42d58fa6cb2da988b7e73a
> >
>
> I could not reproduce the problem you are reporting on arm64. Could you please
> provide some steps?


As far as I see in arm64 Makefiles, you cannot reproduce it.
So, this is a _potential_ problem.

When somebody adds subdir-ccflags-y to arch/arm64/Kbuild or
arch/arm64/kernel/Makefile,
the real issue will suddenly come up.


arm64 is the first arch that implemented vdso-offsets.h

arch maintainers tend to copy Makefiles from existing ones.

Now PPC is the second with almost the same Makefile implementation.

This time, powerpc hit the problem because arch/powerpc/Kbuild
has subdir-ccflags-y.

I do not want to be bothered every time arch maintainers hit this issue.
So, I want to fix arm64 first,
then powerpc will eventually copy the correct Makefiles.




> In my case the vDSO library is not rebuilt as a result of the procedure reported
> in the email you linked at [1].
>
> > Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> > ---
> >
> > arch/arm64/Makefile | 10 ++++++----
> > arch/arm64/kernel/Makefile | 5 +++--
> > arch/arm64/kernel/{vdso/vdso.S => vdso-wrap.S} | 0
> > arch/arm64/kernel/vdso/Makefile | 1 -
> > arch/arm64/kernel/{vdso32/vdso.S => vdso32-wrap.S} | 0
> > arch/arm64/kernel/vdso32/Makefile | 1 -
> > 6 files changed, 9 insertions(+), 8 deletions(-)
> > rename arch/arm64/kernel/{vdso/vdso.S => vdso-wrap.S} (100%)
> > rename arch/arm64/kernel/{vdso32/vdso.S => vdso32-wrap.S} (100%)
> >
> > diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> > index 6a87d592bd00..f18d20a68170 100644
> > --- a/arch/arm64/Makefile
> > +++ b/arch/arm64/Makefile
> > @@ -179,10 +179,12 @@ ifeq ($(KBUILD_EXTMOD),)
> > # this hack.
> > prepare: vdso_prepare
> > vdso_prepare: prepare0
> > - $(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso include/generated/vdso-offsets.h
> > - $(if $(CONFIG_COMPAT_VDSO),$(Q)$(MAKE) \
> > - $(build)=arch/arm64/kernel/vdso32 \
> > - include/generated/vdso32-offsets.h)
> > + $(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso \
> > + include/generated/vdso-offsets.h arch/arm64/kernel/vdso/vdso.so
> > +ifdef CONFIG_COMPAT_VDSO
> > + $(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso32 \
> > + include/generated/vdso32-offsets.h arch/arm64/kernel/vdso32/vdso.so
> > +endif
> > endif
>
> The reason why it is currently done in two phases (a bit hacky as per comment)
> is because vdso-offsets.h is required to be generated before compiling kernel/.
> Please refer to the comment in arch/arm64/Makefile.

Yes, I know.

>
> Could you explain how your change satisfies the dependency?

vdso_prepare completes before Kbuild starts ordinary descending.
So *.so files and vdso-offsets.h exist
when Kbuild builds arch/arm64/kernel/.






> >
> > define archhelp
> > diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> > index 86364ab6f13f..42f6ad2c7eac 100644
> > --- a/arch/arm64/kernel/Makefile
> > +++ b/arch/arm64/kernel/Makefile
> > @@ -59,9 +59,10 @@ obj-$(CONFIG_CRASH_CORE) += crash_core.o
> > obj-$(CONFIG_ARM_SDE_INTERFACE) += sdei.o
> > obj-$(CONFIG_ARM64_PTR_AUTH) += pointer_auth.o
> > obj-$(CONFIG_ARM64_MTE) += mte.o
> > +obj-y += vdso-wrap.o
> > +obj-$(CONFIG_COMPAT_VDSO) += vdso32-wrap.o
> >
> > -obj-y += vdso/ probes/
> > -obj-$(CONFIG_COMPAT_VDSO) += vdso32/
> > +obj-y += probes/
> > head-y := head.o
> > extra-y += $(head-y) vmlinux.lds
> >
> > diff --git a/arch/arm64/kernel/vdso/vdso.S b/arch/arm64/kernel/vdso-wrap.S
> > similarity index 100%
> > rename from arch/arm64/kernel/vdso/vdso.S
> > rename to arch/arm64/kernel/vdso-wrap.S
> > diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
> > index a8f8e409e2bf..85222f64f394 100644
> > --- a/arch/arm64/kernel/vdso/Makefile
> > +++ b/arch/arm64/kernel/vdso/Makefile
> > @@ -45,7 +45,6 @@ endif
> > # Disable gcov profiling for VDSO code
> > GCOV_PROFILE := n
> >
> > -obj-y += vdso.o
> > targets += vdso.lds
> > CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
> >
> > diff --git a/arch/arm64/kernel/vdso32/vdso.S b/arch/arm64/kernel/vdso32-wrap.S
> > similarity index 100%
> > rename from arch/arm64/kernel/vdso32/vdso.S
> > rename to arch/arm64/kernel/vdso32-wrap.S
> > diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> > index a1e0f91e6cea..789ad420f16b 100644
> > --- a/arch/arm64/kernel/vdso32/Makefile
> > +++ b/arch/arm64/kernel/vdso32/Makefile
> > @@ -155,7 +155,6 @@ c-obj-vdso-gettimeofday := $(addprefix $(obj)/, $(c-obj-vdso-gettimeofday))
> > asm-obj-vdso := $(addprefix $(obj)/, $(asm-obj-vdso))
> > obj-vdso := $(c-obj-vdso) $(c-obj-vdso-gettimeofday) $(asm-obj-vdso)
> >
> > -obj-y += vdso.o
> > targets += vdso.lds
> > CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
> >
> >
>
> --
> Regards,
> Vincenzo



--
Best Regards
Masahiro Yamada