[PATCH v2 07/11] um: add infrastructure to build files using nolibc

Benjamin Berg
2025-09-19T15:36:10Z

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

---
v2:
- Do not include nolibc.h via CFLAGS
- Make syscall guard more explicit
- Remove __UM_NOLIBC__ define, it is not needed
- Fix out-of-tree building
---
arch/um/Makefile | 20 +++++++++++++++++++-
arch/um/include/shared/os.h | 2 ++
arch/um/include/shared/user.h | 1 -
arch/um/scripts/Makefile.rules | 8 +++++++-
4 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index f7c509262568..c3a81df50911 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -78,6 +78,24 @@ 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 $(srctree)/tools/include \
+ -D__EXPORTED_HEADERS__ \
+ -D__UM_HOST__ \
+ -DNOLIBC_NO_RUNTIME \
+ -DNOLIBC_IGNORE_ERRNO \
+ -nostdlib -nostdinc -static \
+ -I$(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 $(objtree)/include/generated/autoconf.h \
+ -include $(srctree)/tools/include/linux/kconfig.h \
+ -include $(srctree)/include/linux/kern_levels.h \
+ -include $(srctree)/$(ARCH_DIR)/include/shared/user.h
+
#This will adjust *FLAGS accordingly to the platform.
include $(srctree)/$(ARCH_DIR)/Makefile-os-Linux

@@ -160,4 +178,4 @@ 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..a131ae9e00db 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 NOLIBC
long syscall(long number, ...);
+#endif

/* irqflags tracing */
extern void block_signals_trace(void);
diff --git a/arch/um/include/shared/user.h b/arch/um/include/shared/user.h
index c9b853e1282f..e32bdd032029 100644
--- a/arch/um/include/shared/user.h
+++ b/arch/um/include/shared/user.h
@@ -16,7 +16,6 @@
*/
#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))

-#include <linux/types.h>
#include <stddef.h>

extern void panic(const char *fmt, ...)
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