Re: linux-next: build failure after merge of the arm64 tree

[Peter Collingbourne <pcc@google.com>]

On Wed, Aug 7, 2019 at 8:25 AM Will Deacon <will@kernel.org> wrote:
>
> Hello Masahiro,
>
> On Wed, Aug 07, 2019 at 11:43:02PM +0900, Masahiro Yamada wrote:
> > On Wed, Aug 7, 2019 at 8:46 PM Will Deacon <will@kernel.org> wrote:
> > > On Tue, Aug 06, 2019 at 07:34:36PM -0700, Peter Collingbourne wrote:
> > > > We're setting NM to something else based on a config option, which I
> > > > presume sets up some sort of circular dependency that confuses
> > > > Kconfig. Removing this fragment of the makefile (or appending
> > > > --synthetic unconditionally) also makes the problem go away.
> >
> > Exactly. This makes a circular dependency.
> > Kconfig determines the environment variable 'NM' has been changed,
> > and re-runs.
> >
> > > Yes, I think you're right. The lack of something like KBUILD_NMFLAGS means
> > > that architectures are forced to override NM entirely if they want to pass
> > > any specific options. Making that conditional on a Kconfig option appears
> > > to send the entire thing recursive.
> >
> > Adding KBUILD_NMFLAGS is probably the right thing to do.
> > (Is there somebody who wants to volunteer for this?)
>
> I don't think so ;) We don't do this for many other tools, and there's only
> this one case that really seems to require it.
>
> > But, for this particular case, I vote for
> > the entire removal of --synthetic.
> >
> > This dates back to 2004, and the commit message
> > did not clearly explain why it was needed.
> >
> > The PowerPC maintainers should re-evaluate
> > whether or not this flag is necessary.
> >
> > ppc32 is working without --synthetic,
> > so probably ppc64 would be ...
>
> Again, this is up to the PPC maintainers.
>
> > > diff --git a/init/Kconfig b/init/Kconfig
> > > index d96127ebc44e..a38027a06b79 100644
> > > --- a/init/Kconfig
> > > +++ b/init/Kconfig
> > > @@ -31,7 +31,7 @@ config CC_HAS_ASM_GOTO
> > >         def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
> > >
> > >  config TOOLS_SUPPORT_RELR
> > > -       def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(NM)" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh)
> > > +       def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(CROSS_COMPILE)nm" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh)
> > >
> > >  config CC_HAS_WARN_MAYBE_UNINITIALIZED
> > >         def_bool $(cc-option,-Wmaybe-uninitialized)
> >
> > Maybe,
> >
> > def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "OBJCOPY=$(OBJCOPY)"
> > $(srctree)/scripts/tools-support-relr.sh)
> >
> > will work.
>
> Oh yes, I prefer that. I've included the updated patch below, and I'll
> put it into -next shortly so that we resolve the build breakage. Hopefully
> we'll have a better fix once the ozlabs folks wake up.
>
> > Or more simply
> >
> > def_bool $(success,$(srctree)/scripts/tools-support-relr.sh)
> >
> > CC, LD, OBJCOPY, NM are environment variables,
> > so I think tools-support-relr.sh can directly use them.
>
> Maybe, but the other scripts invoked here tend to pass $(CC) through as
> an argument, and that helps with your observation below...:
>
> > However, this bypasses the detection of environment variable changes.
> > If a user passes NM= from the command line, Kconfig will no longer
> > notice the change of NM.
>
> ... but since my proposal also breaks this, then I think your idea of just
> dropping $NM for now is best.

Thanks Will, that sounds good to me as well. I verified that it fixes
the hang in ppc64 on my end, and that we can still produce RELR
relocations on arm64 by passing in llvm-nm as $NM.

> Will
>
> --->8
>
> From 71c67a31f09fa8fdd1495dffd96a5f0d4cef2ede Mon Sep 17 00:00:00 2001
> From: Will Deacon <will@kernel.org>
> Date: Wed, 7 Aug 2019 12:48:33 +0100
> Subject: [PATCH] init/Kconfig: Fix infinite Kconfig recursion on PPC
>
> Commit 5cf896fb6be3 ("arm64: Add support for relocating the kernel with
> RELR relocations") introduced CONFIG_TOOLS_SUPPORT_RELR, which checks
> for RELR support in the toolchain as part of the kernel configuration.
> During this procedure, "$(NM)" is invoked to see if it supports the new
> relocation format, however PowerPC conditionally overrides this variable
> in the architecture Makefile in order to pass '--synthetic' when
> targetting PPC64.
>
> This conditional override causes Kconfig to recurse forever, since
> CONFIG_TOOLS_SUPPORT_RELR cannot be determined without $(NM) being
> defined, but that in turn depends on CONFIG_PPC64:
>
>   $ make ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu-
>   scripts/kconfig/conf  --syncconfig Kconfig
>   scripts/kconfig/conf  --syncconfig Kconfig
>   scripts/kconfig/conf  --syncconfig Kconfig
>   [...]
>
> In this particular case, it looks like PowerPC may be able to pass
> '--synthetic' unconditionally to nm or even drop it altogether. While
> that is being resolved, let's just bodge the RELR check by picking up
> $(NM) directly from the environment in whatever state it happens to be
> in.
>
> Cc: Peter Collingbourne <pcc@google.com>
> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> Signed-off-by: Will Deacon <will@kernel.org>

Tested-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Peter Collingbourne <pcc@google.com>

Peter