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

From: Tor Vic
Date: Mon Jul 12 2021 - 07:10:21 EST


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@xxxxxxxxxxx>
---
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