kbuild: Add environment variables for userprogs flags

Bugzilla: https://bugzilla.redhat.com/2216526

Conflicts: we already have 1e24078113ae ("Kbuild: use -std=gnu11 for
		KBUILD_USERCFLAGS")

commit f67695c9962e5f444549b3437fb8d840ec6222c8
Author: Elliot Berman <quic_eberman@quicinc.com>
Date:   Tue Feb 1 13:35:42 2022 -0800

    kbuild: Add environment variables for userprogs flags

    Allow additional arguments be passed to userprogs compilation.
    Reproducible clang builds need to provide a sysroot and gcc path to
    ensure the 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.

    Clang might 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=/path/to/sysroot

    This can also be used to specify different sysroots such as musl or
    bionic which may be installed on the host in paths that the compiler
    may not search by default.

    Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
    Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
    Reviewed-by: Fangrui Song <maskray@google.com>
    Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Signed-off-by: Wander Lairson Costa <wander@redhat.com>

Documentation/kbuild/kbuild.rst

@@ -77,6 +77,17 @@ HOSTLDLIBS
 ----------
 Additional libraries to link against when building host programs.
 
+.. _userkbuildflags:
+
+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

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, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will also be used.
+
 5.4 When userspace programs are actually built
 ----------------------------------------------
 

Makefile

@@ -447,12 +447,13 @@ HOSTCC	= gcc
 HOSTCXX	= g++
 endif
 
-export KBUILD_USERCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
-			      -O2 -fomit-frame-pointer -std=gnu11 \
-			      -Wdeclaration-after-statement
-export KBUILD_USERLDFLAGS :=
+KBUILD_USERHOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
+			 -O2 -fomit-frame-pointer -std=gnu11 \
+			 -Wdeclaration-after-statement
+KBUILD_USERCFLAGS  := $(KBUILD_USERHOSTCFLAGS) $(USERCFLAGS)
+KBUILD_USERLDFLAGS := $(USERLDFLAGS)
 
-KBUILD_HOSTCFLAGS   := $(KBUILD_USERCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
+KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
 KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
 KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
 KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
@@ -547,6 +548,7 @@ 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

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))

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.