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

[Kees Cook <keescook@chromium.org>]

On Mon, Jul 12, 2021 at 11:10:04AM +0000, Tor Vic 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.

Ah-ha, good idea. Thanks! Question below...

> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1104
> Signed-off-by: Tor Vic <torvic9@mailbox.org>
> ---
>  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.

Why are these changes needed in Makefile.lib and Makefile.modfinal?
Isn't KBUILD_LDFLAGS already populated from the top-level Makefile?

-- 
Kees Cook