Message-ID: <202505081651.EAF0C6B@keescook>
Date: Thu, 8 May 2025 16:59:48 -0700
From: Kees Cook <kees@...nel.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Petr Pavlu <petr.pavlu@...e.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Justin Stitt <justinstitt@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Richard Weinberger <richard@....at>,
	Anton Ivanov <anton.ivanov@...bridgegreys.com>,
	Johannes Berg <johannes@...solutions.net>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	linux-kbuild@...r.kernel.org, kasan-dev@...glegroups.com,
	linux-um@...ts.infradead.org
Subject: Re: [PATCH v2 0/3] Detect changed compiler dependencies for full
 rebuild

On Fri, May 09, 2025 at 08:13:18AM +0900, Masahiro Yamada wrote:
> On Fri, May 9, 2025 at 1:56 AM Kees Cook <kees@...nel.org> wrote:
> >
> > On Fri, May 09, 2025 at 01:44:09AM +0900, Masahiro Yamada wrote:
> > > On Sun, May 4, 2025 at 2:37 AM Kees Cook <kees@...nel.org> wrote:
> > > >
> > > > On Sat, May 03, 2025 at 06:39:28PM +0900, Masahiro Yamada wrote:
> > > > > On Sat, May 3, 2025 at 7:54 AM Kees Cook <kees@...nel.org> wrote:
> > > > > >
> > > > > >  v2:
> > > > > >   - switch from -include to -I with a -D gated include compiler-version.h
> > > > > >  v1: https://lore.kernel.org/lkml/20250501193839.work.525-kees@kernel.org/
> > > > >
> > > > >
> > > > > What do you think of my patch as a prerequisite?
> > > > > https://lore.kernel.org/linux-kbuild/20250503084145.1994176-1-masahiroy@kernel.org/T/#u
> > > > > Perhaps, can you implement this series more simply?
> > > > >
> > > > > My idea is to touch a single include/generated/global-rebuild.h
> > > > > rather than multiple files such as gcc-plugins-deps.h, integer-wrap.h, etc.
> > > > >
> > > > > When the file is touched, the entire kernel source tree will be rebuilt.
> > > > > This may rebuild more than needed (e.g. vdso) but I do not think
> > > > > it is a big deal.
> > > >
> > > > This is roughly where I started when trying to implement this, but I
> > > > didn't like the ergonomics of needing to scatter "touch" calls all over,
> > > > which was especially difficult for targets that shared a build rule but
> > > > may not all need to trigger a global rebuild. But what ultimately pushed
> > > > me away from it was when I needed to notice if a non-built source file
> > > > changed (the Clang .scl file), and I saw that I need to be dependency
> > > > driven rather than target driven. (Though perhaps there is a way to
> > > > address this with your global-rebuild.h?)
> > > >
> > > > As far as doing a full rebuild, if it had been available last week, I
> > > > probably would have used it, but now given the work that Nicolas, you,
> > > > and I have put into this, we have a viable way (I think) to make this
> > > > more specific. It does end up being a waste of time/resources to rebuild
> > > > stuff that doesn't need to be (efi-stub, vdso, boot code, etc), and that
> > > > does add up when I'm iterating on something that keeps triggering a full
> > > > rebuild. We already have to do the argument filtering for targets that
> > > > don't want randstruct, etc, so why not capitalize on that and make the
> > > > rebuild avoid those files too?
> > >
> > >
> > > efi-stub, vdso are very small.
> > >
> > > Unless this turns out to be painful, I prefer
> > > a simpler implementation.
> > >
> > > You will see how .scl file is handled.
> > >
> > > See the below code:
> > >
> > >
> > > diff --git a/Kbuild b/Kbuild
> > > index f327ca86990c..85747239314c 100644
> > > --- a/Kbuild
> > > +++ b/Kbuild
> > > @@ -67,10 +67,20 @@ targets += $(atomic-checks)
> > >  $(atomic-checks): $(obj)/.checked-%: include/linux/atomic/%  FORCE
> > >         $(call if_changed,check_sha1)
> > >
> > > +rebuild-$(CONFIG_GCC_PLUGINS)          += $(addprefix
> > > scripts/gcc-plugins/, $(GCC_PLUGIN))
> > > +rebuild-$(CONFIG_RANDSTRUCT)           += include/generated/randstruct_hash.h
> >
> > These are in $(objtree)
> 
> Yes.
> 
> > > +rebuild-$(CONFIG_UBSAN_INTEGER_WRAP)   += scripts/integer-wrap-ignore.scl
> >
> > This is in $(srctree)
> 
> Yes.
> 
> > > +
> > > +quiet_cmd_touch = TOUCH   $@
> > > +      cmd_touch = touch $@
> > > +
> > > +include/generated/global-rebuild.h: $(rebuild-y)
> > > +       $(call cmd,touch)
> >
> > Is this rule going to find the right versions of the dependencies?
> 
> I think so, but please test it.

The patch was white-space damaged and wrapped, but I rebuilt it manually
and it mostly works. There still seems to be some ordering issues, as
some stuff gets rebuilt on a record build:

# Clean the tree and pick an "everything" build
$ make O=gcc-test clean allmodconfig -s

# Make a target normally
$ make O=gcc-test kernel/seccomp.o -s

# Touch a gcc plugin that was in .config
$ touch scripts/gcc-plugins/stackleak_plugin.c

# Build and a full rebuild is triggered (good)
$ make O=gcc-test kernel/seccomp.o
make[1]: Entering directory '/srv/code/gcc-test'
  GEN     Makefile
  DESCEND objtool
  HOSTCXX scripts/gcc-plugins/stackleak_plugin.so
  INSTALL libsubcmd_headers
  TOUCH   include/generated/global-rebuild.h
  CC      kernel/bounds.s
  CC      arch/x86/kernel/asm-offsets.s
  CALL    ../scripts/checksyscalls.sh
  CC      kernel/seccomp.o
make[1]: Leaving directory '/srv/code/gcc-test'

# Build again, but more stuff gets built
$ make O=gcc-test kernel/seccomp.o
make[1]: Entering directory '/srv/code/gcc-test'
  GEN     Makefile
  DESCEND objtool
  CC      scripts/mod/empty.o
  CC      scripts/mod/devicetable-offsets.s
  INSTALL libsubcmd_headers
  MKELF   scripts/mod/elfconfig.h
  HOSTCC  scripts/mod/modpost.o
  HOSTCC  scripts/mod/sumversion.o
  HOSTCC  scripts/mod/symsearch.o
  HOSTCC  scripts/mod/file2alias.o
  HOSTLD  scripts/mod/modpost
  CALL    ../scripts/checksyscalls.sh
make[1]: Leaving directory '/srv/code/gcc-test'

# Third time finally everything is stable
$ hmake O=gcc-test kernel/seccomp.o
make[1]: Entering directory '/srv/code/gcc-test'
  GEN     Makefile
  DESCEND objtool
  CALL    ../scripts/checksyscalls.sh
  INSTALL libsubcmd_headers
make[1]: Leaving directory '/srv/code/gcc-test'


Note that scripts/mod/* gets rebuilt on the second rebuild.


-- 
Kees Cook

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