[PATCH v4 05/15] init: add nolibc build support

["Thomas Weißschuh" <thomas.weissschuh@linutronix.de>]

Building userspace applications through the kbuild "userprogs" framework
requires a libc. Kernel toolchains often do not contain a libc.
In this case it is useful to use the nolibc library from the kernel tree.
Nolibc does not support all architectures and requires compiler flags.

Add a kconfig option, so users can know where it is available and provide a
variable for common options.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 MAINTAINERS          |  2 ++
 init/Kconfig         |  2 ++
 init/Kconfig.nolibc  | 15 +++++++++++++++
 init/Makefile.nolibc | 13 +++++++++++++
 4 files changed, 32 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index a92290fffa163f9fe8fe3f04bf66426f9a894409..e806158cc6798cf97a4aab58c038fb5351d469aa 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17702,6 +17702,8 @@ M:	Willy Tarreau <w@1wt.eu>
 M:	Thomas Weißschuh <linux@weissschuh.net>
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/nolibc/linux-nolibc.git
+F:	init/Kconfig.nolibc
+F:	init/Makefile.nolibc
 F:	tools/include/nolibc/
 F:	tools/testing/selftests/nolibc/
 
diff --git a/init/Kconfig b/init/Kconfig
index 26cafbad4f1560fb56b4bef31ae29baf54175661..0af62f135192e0470e16eb6bb2fbb45ac38f4b81 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -92,6 +92,8 @@ config CC_CAN_LINK_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)
 
+source "init/Kconfig.nolibc"
+
 # Fixed in GCC 14, 13.3, 12.4 and 11.5
 # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921
 config GCC_ASM_GOTO_OUTPUT_BROKEN
diff --git a/init/Kconfig.nolibc b/init/Kconfig.nolibc
new file mode 100644
index 0000000000000000000000000000000000000000..29cbc5437e70cbc5e256f00b74d0ab4801b40de7
--- /dev/null
+++ b/init/Kconfig.nolibc
@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config ARCH_HAS_NOLIBC
+	bool
+	default y if ARM
+	default y if ARM64
+	default y if LOONGARCH
+	default y if M68K
+	default y if MIPS
+	default y if PPC
+	default y if RISCV
+	default y if S390
+	default y if SPARC
+	default y if UML_X86
+	default y if X86
diff --git a/init/Makefile.nolibc b/init/Makefile.nolibc
new file mode 100644
index 0000000000000000000000000000000000000000..a8a193f78e34e0c4a6004e871bc206e06925f88a
--- /dev/null
+++ b/init/Makefile.nolibc
@@ -0,0 +1,13 @@
+# SPDX-License-Identifier: GPL-2.0
+# Compiler flags, which are necessary to build userspace applications with the
+# in-kernel libc "nolibc".
+
+ifeq ($(and $(CONFIG_ARCH_HAS_NOLIBC),$(CONFIG_HEADERS_INSTALL)),y)
+
+NOLIBC_USERCFLAGS := -nostdlib -nostdinc -static -ffreestanding \
+		     -fno-asynchronous-unwind-tables -fno-stack-protector \
+		     -isystem $(objtree)/usr/include -isystem $(srctree)/tools/include/nolibc/
+
+NOLIBC_USERLDFLAGS := -nostdlib -nostdinc -static
+
+endif # CONFIG_ARCH_HAS_NOLIBC && CONFIG_HEADERS_INSTALL

-- 
2.50.0