Re: [PATCH v2] kbuild: Add environment variables for userprogs flags

[Fangrui Song <maskray@google.com>]

On 2022-01-28, Elliot Berman wrote:
>Allow additional arguments be passed to userprogs compilation.
>Reproducible clang builds need to provide a sysroot and gcc path to
>ensure same toolchain is used across hosts. KCFLAGS is not currently
>used for any user programs compilation, so add new USERCFLAGS and
>USERLDFLAGS which serves similar purpose as HOSTCFLAGS/HOSTLDFLAGS.
>
>Specifically, I'm trying to force CC_CAN_LINK to consistently fail in
>an environment where a user sysroot is not specifically available.
>Currently, Clang might automatically detect GCC installation on hosts
>which have it installed to a default location in /. With addition of
>these environment variables, you can specify flags such as:
>
>$ make USERCFLAGS=--sysroot=/dev/null USERLDFLAGS=-Wl,--sysroot=/dev/null
>
>to force sysroot detection to fail.

-Wl,--sysroot=/dev/null => --sysroot

As I mentioned in
https://lore.kernel.org/all/20220128031549.w5a4bilxbkppagfu@google.com/
-Wl,--sysroot=/dev/null does not suppress search paths like -L/lib .


>Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
>
>Changes since v1: Addressed minor nits from Nick
>---
> Documentation/kbuild/kbuild.rst    | 9 +++++++++
> Documentation/kbuild/makefiles.rst | 2 ++
> Makefile                           | 9 ++++++---
> init/Kconfig                       | 8 ++++----
> usr/include/Makefile               | 3 +++
> 5 files changed, 24 insertions(+), 7 deletions(-)
>
>diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
>index 2d1fc03d346e..3e7467b19c8f 100644
>--- a/Documentation/kbuild/kbuild.rst
>+++ b/Documentation/kbuild/kbuild.rst
>@@ -77,6 +77,15 @@ HOSTLDLIBS
> ----------
> Additional libraries to link against when building host programs.
>
>+USERCFLAGS
>+----------
>+Additional options used for $(CC) when compiling userprogs.
>+
>+USERLDFLAGS
>+----------
>+Additional options used for $(LD) when linking userprogs. userprogs are linked
>+with CC, so $(USERLDFLAGS) should include "-Wl," prefix as applicable.
>+
> KBUILD_KCONFIG
> --------------
> Set the top-level Kconfig file to the value of this environment
>diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
>index b008b90b92c9..39fb70f59429 100644
>--- a/Documentation/kbuild/makefiles.rst
>+++ b/Documentation/kbuild/makefiles.rst
>@@ -982,6 +982,8 @@ The syntax is quite similar. The difference is to use "userprogs" instead of
>
> 	When linking bpfilter_umh, it will be passed the extra option -static.
>
>+	From command line, USERCFLAGS and USERLDFLAGS will also be used (See kbuild.rst)
>+
> 5.4 When userspace programs are actually built
> ----------------------------------------------
>
>diff --git a/Makefile b/Makefile
>index 45278d508d81..8a2324e6bda8 100644
>--- a/Makefile
>+++ b/Makefile
>@@ -431,15 +431,17 @@ HOSTCC	= gcc
> HOSTCXX	= g++
> endif
>
>-export KBUILD_USERCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
>-			      -O2 -fomit-frame-pointer -std=gnu89
>-export KBUILD_USERLDFLAGS :=
>+KBUILD_USERCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
>+		     -O2 -fomit-frame-pointer -std=gnu89
>+KBUILD_USERLDFLAGS := $(USERLDFLAGS)
>
> KBUILD_HOSTCFLAGS   := $(KBUILD_USERCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
> KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
> KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
> KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
>
>+KBUILD_USERCFLAGS   += $(USERCFLAGS)
>+
> # Make variables (CC, etc...)
> CPP		= $(CC) -E
> ifneq ($(LLVM),)
>@@ -530,6 +532,7 @@ export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS LEX YACC AW
> export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
> export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD
> export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
>+export KBUILD_USERCFLAGS KBUILD_USERLDFLAGS
>
> export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
> export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
>diff --git a/init/Kconfig b/init/Kconfig
>index f2ae41e6717f..164706c38e8b 100644
>--- a/init/Kconfig
>+++ b/init/Kconfig
>@@ -62,13 +62,13 @@ config LLD_VERSION
>
> config CC_CAN_LINK
> 	bool
>-	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(m64-flag)) if 64BIT
>-	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(m32-flag))
>+	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m64-flag)) if 64BIT
>+	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m32-flag))
>
> config CC_CAN_LINK_STATIC
> 	bool
>-	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(m64-flag) -static) if 64BIT
>-	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(m32-flag) -static)
>+	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m64-flag) -static) if 64BIT
>+	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m32-flag) -static)
>
> config CC_HAS_ASM_GOTO
> 	def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
>diff --git a/usr/include/Makefile b/usr/include/Makefile
>index 1c2ae1368079..6a8c7dd9ccaf 100644
>--- a/usr/include/Makefile
>+++ b/usr/include/Makefile
>@@ -12,6 +12,9 @@ UAPI_CFLAGS := -std=c90 -Wall -Werror=implicit-function-declaration
> # It is here just because CONFIG_CC_CAN_LINK is tested with -m32 or -m64.
> UAPI_CFLAGS += $(filter -m32 -m64, $(KBUILD_CFLAGS))
>
>+# USERCFLAGS might contain sysroot location for CC
>+UAPI_CFLAGS += $(USERCFLAGS)
>+
> override c_flags = $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I$(objtree)/usr/include
>
> # The following are excluded for now because they fail to build.
>-- 
>2.25.1
>
>