linux-kernel - Re: [PATCH RFC] kbuild: Prevent compiler mismatch with external modules

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Date:   Sat, 6 Mar 2021 01:28:22 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Justin Forbes <jforbes@...hat.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Miroslav Benes <mbenes@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Jessica Yu <jeyu@...nel.org>
Subject: Re: [PATCH RFC] kbuild: Prevent compiler mismatch with external modules

On Tue, Feb 2, 2021 at 6:13 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Fri, Jan 29, 2021 at 08:17:51AM +0900, Masahiro Yamada wrote:
> > [3]
> > Peterz already pointed out asm-goto as an example of ABI mismatch.
> >
> > I remember a trouble reported in the past due
> > to the mismatch of -mstack-protector-guard-offset.
> >
> > https://bugzilla.kernel.org/show_bug.cgi?id=201891
> >
> > This has already been fixed,
> > and it will no longer happen though.
>
> This is kind of concerning though.  It would be nice to somehow store
> KCLAGS in the config and warn if it changes unexpectedly.
>
> This can be a problem not only for OOT modules, but for regular kernel
> builds which have a .config copied from somewhere.
>
> Because of the toolchain-dependent kconfig options, features can
> silently disappear if the toolchain doesn't support them, due to a
> different compiler version, or even a missing library.
>
> > [2]
> >
> > As for this patch, it is wrong to do this check in the Makefile
> > parse stage.
> >
> > "make M=...  clean"
> > "make M=...  help"
> >
> > etc. will fail.
> > Such targets do not require the compiler in the first place.
> >
> > This check must be done before starting building something,
> >
> > Also, this patch is not applicable.
> > gcc-version.sh and clang-version.sh do not exist.
> > See linux-next.
>
> Something like so?

No, I do not think so.


>
> diff --git a/Makefile b/Makefile
> index 95ab9856f357..10ca621369fb 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1721,12 +1721,25 @@ KBUILD_MODULES := 1
>
>  build-dirs := $(KBUILD_EXTMOD)
>  PHONY += modules
> -modules: $(MODORDER)
> +modules: ext_compiler_check $(MODORDER)


For the single thread build, yes
GNU Make will run the targets from left to right,
so ext_compiler_check is run before compiling
any build artifact.

If -j <N> option is given, there is no guarantee
that ext_compiler_check finishes first.



>         $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
>
>  $(MODORDER): descend
>         @:
>
> +orig_name   := $(if $(CONFIG_CC_IS_GCC),GCC,CLANG)
> +orig_minor  := $(shell expr $(if $(CONFIG_CC_IS_GCC),$(CONFIG_GCC_VERSION),$(CONFIG_CLANG_VERSION)) / 100)
> +cur_namever := $(shell $(srctree)/scripts/cc-version.sh $(CC))
> +cur_name    := $(word 1,$(cur_namever))
> +cur_minor   := $(shell expr $(word 2,$(cur_namever)) / 100)

These are still calculated by 'make M=... clean' or 'make M=... help'.
Using '=' assignment solves it, but the code is still ugly.


I attached my alternative implementation.


> +PHONY += ext_compiler_check
> +ext_compiler_check:
> +       @if [ $(orig_name) != $(cur_name) ] || [ $(orig_minor) != $(cur_minor) ]; then \
> +               echo >&2 "warning: The compiler differs from the version which was used to build the kernel."; \
> +               echo >&2 "warning: Some kernel features are compiler-dependent."; \
> +               echo >&2 "warning: It's recommended that you change your compiler to match the version in the .config file."; \
> +       fi
> +
>  PHONY += modules_install
>  modules_install: _emodinst_ _emodinst_post
>
>

--
Best Regards
Masahiro Yamada

Download attachment "0001-kbuild-show-notice-if-a-different-compiler-is-used-f.patch" of type "application/x-patch" (1799 bytes)