linux-kernel - [PATCH 7/9] um: add infrastructure to build files using nolibc

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Message-ID: <20250915071115.1429196-8-benjamin@sipsolutions.net>
Date: Mon, 15 Sep 2025 09:11:13 +0200
From: Benjamin Berg <benjamin@...solutions.net>
To: linux-um@...ts.infradead.org,
	Willy Tarreau <w@....eu>,
	Thomas Weißschuh <linux@...ssschuh.net>,
	linux-kselftest@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: linux-kernel@...r.kernel.org,
	Tiwei Bie <tiwei.btw@...group.com>,
	Benjamin Berg <benjamin.berg@...el.com>
Subject: [PATCH 7/9] um: add infrastructure to build files using nolibc

From: Benjamin Berg <benjamin.berg@...el.com>

Add NOLIBC_CFLAGS and NOLIBC_OBJS to build files against nolibc rather
than libc. With this it is possible to move to nolibc in smaller steps.

Set NOLIBC_IGNORE_ERRNO, as the nolibc errno implementation is overly
simple and cannot handle threading. nolibc provides sys_* functions that
do not emulate the libc errno behaviour and can be used instead.

Guard the syscall definition as it is a macro in nolibc.

Signed-off-by: Benjamin Berg <benjamin.berg@...el.com>
---
 arch/um/Makefile               | 21 ++++++++++++++++++++-
 arch/um/include/shared/os.h    |  2 ++
 arch/um/scripts/Makefile.rules |  8 +++++++-
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index a3f27d791cc0..54aa26fcc32b 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -78,6 +78,25 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
 		-include $(srctree)/include/linux/kern_levels.h \
 		-include $(srctree)/$(ARCH_DIR)/include/shared/user.h
 
+NOLIBC_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
+		$(ARCH_INCLUDE) $(MODE_INCLUDE) $(filter -I%,$(CFLAGS)) \
+		-I $(srctree)/tools/include \
+		-D__EXPORTED_HEADERS__ \
+		-D__UM_HOST__ -D__UM_NOLIBC__ \
+		-DNOLIBC_NO_STARTCODE \
+		-DNOLIBC_IGNORE_ERRNO \
+		-nostdlib -nostdinc -static \
+		-I$(srctree)/include/uapi \
+		-I$(srctree)/$(HOST_DIR)/include/uapi \
+		-I$(objtree)/$(HOST_DIR)/include/generated/uapi \
+		-I $(srctree)/tools/include/nolibc \
+		-I $(srctree)/usr/include \
+		-include $(srctree)/include/generated/autoconf.h \
+		-include $(srctree)/tools/include/linux/kconfig.h \
+		-include $(srctree)/include/linux/kern_levels.h \
+		-include $(srctree)/tools/include/nolibc/nolibc.h \
+		-include $(srctree)/$(ARCH_DIR)/include/shared/user.h
+
 #This will adjust *FLAGS accordingly to the platform.
 include $(srctree)/$(ARCH_DIR)/Makefile-os-Linux
 
@@ -157,4 +176,4 @@ archclean:
 		-o -name '*.gcov' \) -type f -print | xargs rm -f
 	$(Q)$(MAKE) -f $(srctree)/Makefile ARCH=$(HEADER_ARCH) clean
 
-export HEADER_ARCH SUBARCH USER_CFLAGS CFLAGS_NO_HARDENING DEV_NULL_PATH
+export HEADER_ARCH SUBARCH USER_CFLAGS NOLIBC_CFLAGS CFLAGS_NO_HARDENING DEV_NULL_PATH
diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
index b35cc8ce333b..3e9f3ae61658 100644
--- a/arch/um/include/shared/os.h
+++ b/arch/um/include/shared/os.h
@@ -327,7 +327,9 @@ extern int __ignore_sigio_fd(int fd);
 /* tty.c */
 extern int get_pty(void);
 
+#ifndef syscall
 long syscall(long number, ...);
+#endif
 
 /* irqflags tracing */
 extern void block_signals_trace(void);
diff --git a/arch/um/scripts/Makefile.rules b/arch/um/scripts/Makefile.rules
index b4a2e0058503..5f07551935c3 100644
--- a/arch/um/scripts/Makefile.rules
+++ b/arch/um/scripts/Makefile.rules
@@ -11,6 +11,12 @@ USER_OBJS := $(foreach file,$(USER_OBJS),$(obj)/$(file))
 $(USER_OBJS:.o=.%): \
 	c_flags = -Wp,-MD,$(depfile) $(USER_CFLAGS) $(CFLAGS_$(basetarget).o)
 
+# Similar USER_OBJS but compiled against nolibc (may include kernel headers?)
+NOLIBC_OBJS := $(foreach file,$(NOLIBC_OBJS),$(obj)/$(file))
+
+$(NOLIBC_OBJS:.o=.%): \
+	c_flags = -Wp,-MD,$(depfile) $(NOLIBC_CFLAGS) $(CFLAGS_$(basetarget).o)
+
 # These are like USER_OBJS but filter USER_CFLAGS through unprofile instead of
 # using it directly.
 UNPROFILE_OBJS := $(foreach file,$(UNPROFILE_OBJS),$(obj)/$(file))
@@ -18,7 +24,7 @@ UNPROFILE_OBJS := $(foreach file,$(UNPROFILE_OBJS),$(obj)/$(file))
 $(UNPROFILE_OBJS:.o=.%): \
 	c_flags = -Wp,-MD,$(depfile) $(call unprofile,$(USER_CFLAGS)) $(CFLAGS_$(basetarget).o)
 
-$(USER_OBJS) $(UNPROFILE_OBJS): \
+$(USER_OBJS) $(NOLIBC_OBJS) $(UNPROFILE_OBJS): \
 	CHECKFLAGS := $(patsubst $(NOSTDINC_FLAGS),,$(CHECKFLAGS))
 
 # The stubs can't try to call mcount or update basic block data
-- 
2.51.0