kunit: Introduce UAPI testing framework

Documentation/dev-tools/kunit/api/index.rst

@@ -9,6 +9,7 @@ API Reference
 	test
 	resource
 	functionredirection
+	uapi
 	clk
 	of
 	platformdevice
@@ -32,6 +33,10 @@ Documentation/dev-tools/kunit/api/functionredirection.rst
 
  - Documents the KUnit Function Redirection API
 
+Documentation/dev-tools/kunit/api/uapi.rst
+
+ - Documents the KUnit Userspace testing API
+
 Driver KUnit API
 ================
 

Documentation/dev-tools/kunit/api/uapi.rst

@@ -0,0 +1,14 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================
+Userspace Test API
+==================
+
+This file documents all of the userspace testing API.
+Userspace tests are built as :ref:`kbuild userprogs <kbuild_userprogs>`,
+linked statically and without any external dependencies.
+
+For the widest platform compatibility they should use nolibc, as provided by `init/Makefile.nolibc`.
+
+.. kernel-doc:: include/kunit/uapi.h
+   :internal:

Documentation/kbuild/makefiles.rst

@@ -891,6 +891,8 @@ This is possible in two ways:
     This will tell kbuild to build lxdialog even if not referenced in
     any rule.
 
+.. _kbuild_userprogs:
+
 Userspace Program support
 =========================
 

MAINTAINERS

@@ -14321,6 +14321,15 @@ S:	Maintained
 F:	Documentation/devicetree/bindings/leds/backlight/kinetic,ktz8866.yaml
 F:	drivers/video/backlight/ktz8866.c
 
+KUNIT UAPI TESTING FRAMEWORK (in addition to KERNEL UNIT TESTING FRAMEWORK)
+M:	Thomas Weißschuh <thomas.weissschuh@linutronix.de>
+S:	Maintained
+F:	include/kunit/uapi.h
+F:	lib/kunit/kunit-example-uapi.c
+F:	lib/kunit/kunit-test-uapi.c
+F:	lib/kunit/kunit-uapi.c
+F:	lib/kunit/uapi-preinit.c
+
 KVM PARAVIRT (KVM/paravirt)
 M:	Paolo Bonzini <pbonzini@redhat.com>
 R:	Vitaly Kuznetsov <vkuznets@redhat.com>
@@ -18811,6 +18820,8 @@ 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/
 

fs/coredump.c

@@ -1012,7 +1012,7 @@ static bool coredump_pipe(struct core_name *cn, struct coredump_params *cprm,
 		helper_argv[argi] = cn->corename + argv[argi];
 	helper_argv[argi] = NULL;
 
-	sub_info = call_usermodehelper_setup(helper_argv[0], helper_argv, NULL,
+	sub_info = call_usermodehelper_setup(AT_FDCWD, helper_argv[0], helper_argv, NULL,
 					     GFP_KERNEL, umh_coredump_setup,
 					     NULL, cprm);
 	if (!sub_info)

fs/exec.c

@@ -1846,7 +1846,7 @@ static int do_execveat_common(int fd, struct filename *filename,
 	return bprm_execve(bprm);
 }
 
-int kernel_execve(const char *kernel_filename,
+int kernel_execve(int dirfd, const char *kernel_filename,
 		  const char *const *argv, const char *const *envp)
 {
 	int retval;
@@ -1856,7 +1856,7 @@ int kernel_execve(const char *kernel_filename,
 		return -EINVAL;
 
 	CLASS(filename_kernel, filename)(kernel_filename);
-	CLASS(bprm, bprm)(AT_FDCWD, filename, 0);
+	CLASS(bprm, bprm)(dirfd, filename, 0);
 	if (IS_ERR(bprm))
 		return PTR_ERR(bprm);
 

include/kunit/uapi.h

@@ -0,0 +1,77 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * KUnit Userspace testing API.
+ *
+ * Copyright (C) 2026, Linutronix GmbH.
+ * Author: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
+ */
+
+#ifndef _KUNIT_UAPI_H
+#define _KUNIT_UAPI_H
+
+#include <linux/types.h>
+
+struct kunit;
+
+/**
+ * struct kunit_uapi_blob - Blob embedded build artifact
+ * @path: Path of the embedded artifact.
+ * @data: Start of the embedded data in memory.
+ * @end: End of the embedded data in memory.
+ */
+struct kunit_uapi_blob {
+	const char *const path;
+	const u8 *data;
+	const u8 *end;
+};
+
+#if IS_ENABLED(CONFIG_KUNIT_UAPI)
+
+/**
+ * KUNIT_UAPI_EMBED_BLOB() - Embed another build artifact into the kernel
+ * @_name: The name of symbol under which the artifact is embedded.
+ * @_path: Path to the artifact on disk.
+ *
+ * Embeds a build artifact like a userspace executable into the kernel or current module.
+ * The build artifact is read from disk and needs to be already built.
+ */
+#define KUNIT_UAPI_EMBED_BLOB(_name, _path)					\
+	asm (									\
+	"	.pushsection .rodata, \"a\"				\n"	\
+	"	.global " __stringify(CONCATENATE(_name, _data)) "	\n"	\
+	__stringify(CONCATENATE(_name, _data)) ":			\n"	\
+	"	.incbin " __stringify(_path) "				\n"	\
+	"	.size " __stringify(CONCATENATE(_name, _data)) ", "		\
+			". - " __stringify(CONCATENATE(_name, _data)) "	\n"	\
+	"	.global " __stringify(CONCATENATE(_name, _end)) "	\n"	\
+	__stringify(CONCATENATE(_name, _end)) ":			\n"	\
+	"	.popsection						\n"	\
+	);									\
+										\
+	extern const char CONCATENATE(_name, _data)[];				\
+	extern const char CONCATENATE(_name, _end)[];				\
+										\
+	static const struct kunit_uapi_blob _name = {				\
+		.path	= _path,						\
+		.data	= CONCATENATE(_name, _data),				\
+		.end	= CONCATENATE(_name, _end),				\
+	}									\
+
+#else /* !CONFIG_KUNIT_UAPI */
+
+/* Unresolved external reference, to be optimized away */
+#define KUNIT_UAPI_EMBED_BLOB(_name, _path)					\
+	extern const struct kunit_uapi_blob _name
+
+#endif /* CONFIG_KUNIT_UAPI */
+
+/**
+ * kunit_uapi_run_kselftest() - Run a userspace kselftest as part of kunit
+ * @test: The test context object.
+ * @executable: kselftest executable to run
+ *
+ * Runs the kselftest and forwards its TAP output and exit status to kunit.
+ */
+void kunit_uapi_run_kselftest(struct kunit *test, const struct kunit_uapi_blob *executable);
+
+#endif /* _KUNIT_UAPI_H */

include/linux/binfmts.h

@@ -144,7 +144,7 @@ int copy_string_kernel(const char *arg, struct linux_binprm *bprm);
 extern void set_binfmt(struct linux_binfmt *new);
 extern ssize_t read_code(struct file *, unsigned long, loff_t, size_t);
 
-int kernel_execve(const char *filename,
+int kernel_execve(int dirfd, const char *filename,
 		  const char *const *argv, const char *const *envp);
 
 #endif /* _LINUX_BINFMTS_H */

include/linux/mount.h

@@ -10,6 +10,7 @@
 #ifndef _LINUX_MOUNT_H
 #define _LINUX_MOUNT_H
 
+#include <linux/cleanup.h>
 #include <linux/types.h>
 #include <asm/barrier.h>
 
@@ -99,6 +100,7 @@ extern bool our_mnt(struct vfsmount *mnt);
 
 extern struct vfsmount *kern_mount(struct file_system_type *);
 extern void kern_unmount(struct vfsmount *mnt);
+DEFINE_FREE(kern_unmount, struct vfsmount *, if (_T) kern_unmount(_T));
 extern int may_umount_tree(struct vfsmount *);
 extern int may_umount(struct vfsmount *);
 int do_mount(const char *, const char __user *,

include/linux/umh.h

@@ -20,6 +20,7 @@ struct file;
 struct subprocess_info {
 	struct work_struct work;
 	struct completion *complete;
+	int dirfd;
 	const char *path;
 	char **argv;
 	char **envp;
@@ -34,7 +35,7 @@ extern int
 call_usermodehelper(const char *path, char **argv, char **envp, int wait);
 
 extern struct subprocess_info *
-call_usermodehelper_setup(const char *path, char **argv, char **envp,
+call_usermodehelper_setup(int dirfd, const char *path, char **argv, char **envp,
 			  gfp_t gfp_mask,
 			  int (*init)(struct subprocess_info *info, struct cred *new),
 			  void (*cleanup)(struct subprocess_info *), void *data);

init/Kconfig

@@ -91,6 +91,8 @@ config CC_CAN_LINK
 	default $(cc_can_link_user,$(m64-flag)) if 64BIT
 	default $(cc_can_link_user,$(m32-flag))
 
+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

init/Kconfig.nolibc

@@ -0,0 +1,16 @@
+# 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 SUPERH
+	default y if UML_X86
+	default y if X86

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 \
+		     -I$(objtree)/usr/include -I$(srctree)/tools/include/nolibc/
+
+NOLIBC_USERLDFLAGS := -nostdlib -nostdinc -static
+
+endif # CONFIG_ARCH_HAS_NOLIBC && CONFIG_HEADERS_INSTALL

init/main.c

@@ -1500,7 +1500,7 @@ static int run_init_process(const char *init_filename)
 	pr_debug("  with environment:\n");
 	for (p = envp_init; *p; p++)
 		pr_debug("    %s\n", *p);
-	return kernel_execve(init_filename, argv_init, envp_init);
+	return kernel_execve(AT_FDCWD, init_filename, argv_init, envp_init);
 }
 
 static int try_to_run_init_process(const char *init_filename)

kernel/module/kmod.c

@@ -95,7 +95,7 @@ static int call_modprobe(char *orig_module_name, int wait)
 	argv[3] = module_name;	/* check free_modprobe_argv() */
 	argv[4] = NULL;
 
-	info = call_usermodehelper_setup(modprobe_path, argv, envp, GFP_KERNEL,
+	info = call_usermodehelper_setup(AT_FDCWD, modprobe_path, argv, envp, GFP_KERNEL,
 					 NULL, free_modprobe_argv, NULL);
 	if (!info)
 		goto free_module_name;

kernel/umh.c

@@ -106,7 +106,7 @@ static int call_usermodehelper_exec_async(void *data)
 	commit_creds(new);
 
 	wait_for_initramfs();
-	retval = kernel_execve(sub_info->path,
+	retval = kernel_execve(sub_info->dirfd, sub_info->path,
 			       (const char *const *)sub_info->argv,
 			       (const char *const *)sub_info->envp);
 out:
@@ -331,6 +331,7 @@ static void helper_unlock(void)
 
 /**
  * call_usermodehelper_setup - prepare to call a usermode helper
+ * @dirfd: directory to resolve path against
  * @path: path to usermode executable
  * @argv: arg vector for process
  * @envp: environment for process
@@ -352,7 +353,7 @@ static void helper_unlock(void)
  * Function must be runnable in either a process context or the
  * context in which call_usermodehelper_exec is called.
  */
-struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv,
+struct subprocess_info *call_usermodehelper_setup(int dirfd, const char *path, char **argv,
 		char **envp, gfp_t gfp_mask,
 		int (*init)(struct subprocess_info *info, struct cred *new),
 		void (*cleanup)(struct subprocess_info *info),
@@ -366,8 +367,10 @@ struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv,
 	INIT_WORK(&sub_info->work, call_usermodehelper_exec_work);
 
 #ifdef CONFIG_STATIC_USERMODEHELPER
+	sub_info->dirfd = AT_FDCWD;
 	sub_info->path = CONFIG_STATIC_USERMODEHELPER_PATH;
 #else
+	sub_info->dirfd = dirfd;
 	sub_info->path = path;
 #endif
 	sub_info->argv = argv;
@@ -484,7 +487,7 @@ int call_usermodehelper(const char *path, char **argv, char **envp, int wait)
 	struct subprocess_info *info;
 	gfp_t gfp_mask = (wait == UMH_NO_WAIT) ? GFP_ATOMIC : GFP_KERNEL;
 
-	info = call_usermodehelper_setup(path, argv, envp, gfp_mask,
+	info = call_usermodehelper_setup(AT_FDCWD, path, argv, envp, gfp_mask,
 					 NULL, NULL, NULL);
 	if (info == NULL)
 		return -ENOMEM;

lib/kobject_uevent.c

@@ -625,7 +625,7 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
 			goto exit;
 
 		retval = -ENOMEM;
-		info = call_usermodehelper_setup(env->argv[0], env->argv,
+		info = call_usermodehelper_setup(AT_FDCWD, env->argv[0], env->argv,
 						 env->envp, GFP_KERNEL,
 						 NULL, cleanup_uevent_env, env);
 		if (info) {

lib/kunit/Kconfig

@@ -141,4 +141,20 @@ config KUNIT_UML_PCI
 
 	  If unsure, say N.
 
+config KUNIT_UAPI
+	tristate "KUnit UAPI testing framework"
+	depends on KUNIT
+	depends on ARCH_HAS_NOLIBC
+	depends on !STATIC_USERMODEHELPER
+	depends on !LTO_CLANG # https://github.com/llvm/llvm-project/issues/112920
+	select HEADERS_INSTALL
+	select DEVTMPFS
+	default KUNIT
+	help
+	  Enables support for building and running userspace selftests as part of kunit.
+	  These tests should be statically linked and use kselftest.h or kselftest_harness.h
+	  for status reporting.
+
+	  In most cases this should be left as its default.
+
 endif # KUNIT

lib/kunit/Makefile

@@ -1,3 +1,5 @@
+include $(srctree)/init/Makefile.nolibc
+
 obj-$(CONFIG_KUNIT) +=			kunit.o
 
 kunit-objs +=				test.o \
@@ -12,6 +14,13 @@ kunit-objs +=				test.o \
 					device.o \
 					platform.o
 
+userprogs +=				uapi-preinit
+uapi-preinit-userccflags +=		-static $(NOLIBC_USERCFLAGS)
+obj-$(CONFIG_KUNIT_UAPI) +=		kunit-uapi.o
+
+CFLAGS_kunit-uapi.o :=			-Wa,-I$(obj)
+$(obj)/kunit-uapi.o: $(obj)/uapi-preinit
+
 ifeq ($(CONFIG_KUNIT_DEBUGFS),y)
 kunit-objs +=				debugfs.o
 endif
@@ -20,6 +29,15 @@ endif
 obj-$(if $(CONFIG_KUNIT),y) +=		hooks.o
 
 obj-$(CONFIG_KUNIT_TEST) +=		kunit-test.o
+
+userprogs +=				kunit-test-uapi
+kunit-test-uapi-userccflags :=		-static $(NOLIBC_USERCFLAGS)
+
+ifdef CONFIG_KUNIT_UAPI
+CFLAGS_kunit-test.o :=			-Wa,-I$(obj)
+$(obj)/kunit-test.o: $(obj)/kunit-test-uapi
+endif
+
 obj-$(CONFIG_KUNIT_TEST) +=		platform-test.o
 
 # string-stream-test compiles built-in only.
@@ -29,3 +47,11 @@ obj-$(CONFIG_KUNIT_TEST) +=		assert_test.o
 endif
 
 obj-$(CONFIG_KUNIT_EXAMPLE_TEST) +=	kunit-example-test.o
+
+userprogs +=				kunit-example-uapi
+kunit-example-uapi-userccflags :=	-static $(NOLIBC_USERCFLAGS)
+
+ifdef CONFIG_KUNIT_UAPI
+CFLAGS_kunit-example-test.o :=		-Wa,-I$(obj)
+$(obj)/kunit-example-test.o: $(obj)/kunit-example-uapi
+endif