linux-kernel - Re: [PATCH 1/1] Kbuild, clang: add option for choosing a ThinLTO cache directory

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Date:   Tue, 13 Jul 2021 19:45:21 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Tor Vic <torvic9@...lbox.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 1/1] Kbuild, clang: add option for choosing a ThinLTO
 cache directory

On Mon, Jul 12, 2021 at 8:10 PM Tor Vic <torvic9@...lbox.org> wrote:
>
> On some distros and configurations, it might be useful to allow for
> specifying a directory where Clang stores its ThinLTO cache.
>
> More specifically, when building the VirtualBox extramodules on Arch with
> its proper 'makepkg' build system and DKMS, against an already installed
> ThinLTO kernel, the build fails because it tries to create the ThinLTO
> cache in a directory that is not user-writable.
>
> A similar problem has been reported with openSUSE's OBS build system.
>
> Add a Kconfig option that allows users to choose a directory in which
> Clang's ThinLTO can store its cache.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1104
> Signed-off-by: Tor Vic <torvic9@...lbox.org>


I do not understand.

Currently, .thinlto-cache is created in the same directory
as the other objects.  (right under $(KBUILD_EXTMOD))

If you build in a read-only directory, you cannot put
any build artifact (not limited to the thinlto cache) there.

Why did changing the location of .thinlto-cache
solve your problem?



> ---
>  Makefile                  |  5 +++--
>  arch/Kconfig              | 10 ++++++++++
>  scripts/Makefile.lib      |  4 ++++
>  scripts/Makefile.modfinal |  4 ++++
>  4 files changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index c3f9bd191b89..472bc8bfff03 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -932,7 +932,8 @@ endif
>  ifdef CONFIG_LTO_CLANG
>  ifdef CONFIG_LTO_CLANG_THIN
>  CC_FLAGS_LTO   := -flto=thin -fsplit-lto-unit
> -KBUILD_LDFLAGS += --thinlto-cache-dir=$(extmod_prefix).thinlto-cache
> +export thinlto-dir = $(if
> $(CONFIG_LTO_CLANG_THIN_CACHEDIR),$(CONFIG_LTO_CLANG_THIN_CACHEDIR)/)
> +KBUILD_LDFLAGS +=
> --thinlto-cache-dir=$(thinlto-dir)$(extmod_prefix).thinlto-cache
>  else
>  CC_FLAGS_LTO   := -flto
>  endif
> @@ -1728,7 +1729,7 @@ PHONY += compile_commands.json
>
>  clean-dirs := $(KBUILD_EXTMOD)
>  clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers
> $(KBUILD_EXTMOD)/modules.nsdeps \
> -       $(KBUILD_EXTMOD)/compile_commands.json $(KBUILD_EXTMOD)/.thinlto-cache
> +       $(KBUILD_EXTMOD)/compile_commands.json
> $(thinlto-dir)$(KBUILD_EXTMOD)/.thinlto-cache
>
>  PHONY += help
>  help:
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 129df498a8e1..19e4d140e12a 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -696,6 +696,16 @@ config LTO_CLANG_THIN
>             https://clang.llvm.org/docs/ThinLTO.html
>
>           If unsure, say Y.
> +
> +config LTO_CLANG_THIN_CACHEDIR
> +       string "Clang ThinLTO cache directory"
> +       depends on LTO_CLANG_THIN
> +       default ""
> +       help
> +         This option allows users to choose a directory that stores
> +         Clang's ThinLTO cache.
> +         Leave empty for default.
> +
>  endchoice
>
>  config ARCH_SUPPORTS_CFI_CLANG
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 10950559b223..bca87a6aa35b 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -197,6 +197,10 @@ endif
>  part-of-module = $(if $(filter $(basename $@).o, $(real-obj-m)),y)
>  quiet_modtag = $(if $(part-of-module),[M],   )
>
> +ifdef CONFIG_LTO_CLANG_THIN
> +KBUILD_LDFLAGS +=
> --thinlto-cache-dir=$(thinlto-dir)$(extmod-prefix).thinlto-cache
> +endif
> +
>  modkern_cflags =                                          \
>         $(if $(part-of-module),                           \
>                 $(KBUILD_CFLAGS_MODULE) $(CFLAGS_MODULE), \
> diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
> index 5e9b8057fb24..ab0d72e21318 100644
> --- a/scripts/Makefile.modfinal
> +++ b/scripts/Makefile.modfinal
> @@ -35,6 +35,10 @@ ifdef CONFIG_LTO_CLANG
>  # avoid a second slow LTO link
>  prelink-ext := .lto
>
> +ifdef CONFIG_LTO_CLANG_THIN
> +KBUILD_LDFLAGS +=
> --thinlto-cache-dir=$(thinlto-dir)$(extmod-prefix).thinlto-cache
> +endif # CONFIG_LTO_CLANG_THIN
> +
>  # ELF processing was skipped earlier because we didn't have native code,
>  # so let's now process the prelinked binary before we link the module.
>
> --
> 2.32.0



-- 
Best Regards
Masahiro Yamada