selftests: add memfd_create() + sealing tests

Some basic tests to verify sealing on memfds works as expected and
guarantees the advertised semantics.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Ryan Lortie <desrt@desrt.ca>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Daniel Mack <zonque@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 945 insertions, 0 deletions

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index d10f95ce2ea4..6fd2a4402069 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -2,6 +2,7 @@ TARGETS = breakpoints
 TARGETS += cpu-hotplug
 TARGETS += efivarfs
 TARGETS += kcmp
+TARGETS += memfd
 TARGETS += memory-hotplug
 TARGETS += mqueue
 TARGETS += net
diff --git a/tools/testing/selftests/memfd/.gitignore b/tools/testing/selftests/memfd/.gitignore
new file mode 100644
index 000000000000..bcc8ee20c710
--- /dev/null
+++ b/tools/testing/selftests/memfd/.gitignore
@@ -0,0 +1,2 @@
+memfd_test
+memfd-test-file
diff --git a/tools/testing/selftests/memfd/Makefile b/tools/testing/selftests/memfd/Makefile
new file mode 100644
index 000000000000..36653b903140
--- /dev/null
+++ b/tools/testing/selftests/memfd/Makefile
@@ -0,0 +1,29 @@
+uname_M := $(shell uname -m 2>/dev/null || echo not)
+ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/i386/)
+ifeq ($(ARCH),i386)
+	ARCH := X86
+endif
+ifeq ($(ARCH),x86_64)
+	ARCH := X86
+endif
+
+CFLAGS += -I../../../../arch/x86/include/generated/uapi/
+CFLAGS += -I../../../../arch/x86/include/uapi/
+CFLAGS += -I../../../../include/uapi/
+CFLAGS += -I../../../../include/
+
+all:
+ifeq ($(ARCH),X86)
+	gcc $(CFLAGS) memfd_test.c -o memfd_test
+else
+	echo "Not an x86 target, can't build memfd selftest"
+endif
+
+run_tests: all
+ifeq ($(ARCH),X86)
+	gcc $(CFLAGS) memfd_test.c -o memfd_test
+endif
+	@./memfd_test || echo "memfd_test: [FAIL]"
+
+clean:
+	$(RM) memfd_test
diff --git a/tools/testing/selftests/memfd/memfd_test.c b/tools/testing/selftests/memfd/memfd_test.c
new file mode 100644
index 000000000000..3634c909b1b0
--- /dev/null
+++ b/tools/testing/selftests/memfd/memfd_test.c
@@ -0,0 +1,913 @@
+#define _GNU_SOURCE
+#define __EXPORTED_HEADERS__
+
+#include <errno.h>
+#include <inttypes.h>
+#include <limits.h>
+#include <linux/falloc.h>
+#include <linux/fcntl.h>
+#include <linux/memfd.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#define MFD_DEF_SIZE 8192
+#define STACK_SIZE 65535
+
+static int sys_memfd_create(const char *name,
+			    unsigned int flags)
+{
+	return syscall(__NR_memfd_create, name, flags);
+}
+
+static int mfd_assert_new(const char *name, loff_t sz, unsigned int flags)
+{
+	int r, fd;
+
+	fd = sys_memfd_create(name, flags);
+	if (fd < 0) {
+		printf("memfd_create(\"%s\", %u) failed: %m\n",
+		       name, flags);
+		abort();
+	}
+
+	r = ftruncate(fd, sz);
+	if (r < 0) {
+		printf("ftruncate(%llu) failed: %m\n", (unsigned long long)sz);
+		abort();
+	}
+
+	return fd;
+}
+
+static void mfd_fail_new(const char *name, unsigned int flags)
+{
+	int r;
+
+	r = sys_memfd_create(name, flags);
+	if (r >= 0) {
+		printf("memfd_create(\"%s\", %u) succeeded, but failure expected\n",
+		       name, flags);
+		close(r);
+		abort();
+	}
+}
+
+static __u64 mfd_assert_get_seals(int fd)
+{
+	long r;
+
+	r = fcntl(fd, F_GET_SEALS);
+	if (r < 0) {
+		printf("GET_SEALS(%d) failed: %m\n", fd);
+		abort();
+	}
+
+	return r;
+}
+
+static void mfd_assert_has_seals(int fd, __u64 seals)
+{
+	__u64 s;
+
+	s = mfd_assert_get_seals(fd);
+	if (s != seals) {
+		printf("%llu != %llu = GET_SEALS(%d)\n",
+		       (unsigned long long)seals, (unsigned long long)s, fd);
+		abort();
+	}
+}
+
+static void mfd_assert_add_seals(int fd, __u64 seals)
+{
+	long r;
+	__u64 s;
+
+	s = mfd_assert_get_seals(fd);
+	r = fcntl(fd, F_ADD_SEALS, seals);
+	if (r < 0) {
+		printf("ADD_SEALS(%d, %llu -> %llu) failed: %m\n",
+		       fd, (unsigned long long)s, (unsigned long long)seals);
+		abort();
+	}
+}
+
+static void mfd_fail_add_seals(int fd, __u64 seals)
+{
+	long r;
+	__u64 s;
+
+	r = fcntl(fd, F_GET_SEALS);
+	if (r < 0)
+		s = 0;
+	else
+		s = r;
+
+	r = fcntl(fd, F_ADD_SEALS, seals);
+	if (r >= 0) {
+		printf("ADD_SEALS(%d, %llu -> %llu) didn't fail as expected\n",
+		       fd, (unsigned long long)s, (unsigned long long)seals);
+		abort();
+	}
+}
+
+static void mfd_assert_size(int fd, size_t size)
+{
+	struct stat st;
+	int r;
+
+	r = fstat(fd, &st);
+	if (r < 0) {
+		printf("fstat(%d) failed: %m\n", fd);
+		abort();
+	} else if (st.st_size != size) {
+		printf("wrong file size %lld, but expected %lld\n",
+		       (long long)st.st_size, (long long)size);
+		abort();
+	}
+}
+
+static int mfd_assert_dup(int fd)
+{
+	int r;
+
+	r = dup(fd);
+	if (r < 0) {
+		printf("dup(%d) failed: %m\n", fd);
+		abort();
+	}
+
+	return r;
+}
+
+static void *mfd_assert_mmap_shared(int fd)
+{
+	void *p;
+
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ | PROT_WRITE,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+
+	return p;
+}
+
+static void *mfd_assert_mmap_private(int fd)
+{
+	void *p;
+
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ,
+		 MAP_PRIVATE,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+
+	return p;
+}
+
+static int mfd_assert_open(int fd, int flags, mode_t mode)
+{
+	char buf[512];
+	int r;
+
+	sprintf(buf, "/proc/self/fd/%d", fd);
+	r = open(buf, flags, mode);
+	if (r < 0) {
+		printf("open(%s) failed: %m\n", buf);
+		abort();
+	}
+
+	return r;
+}
+
+static void mfd_fail_open(int fd, int flags, mode_t mode)
+{
+	char buf[512];
+	int r;
+
+	sprintf(buf, "/proc/self/fd/%d", fd);
+	r = open(buf, flags, mode);
+	if (r >= 0) {
+		printf("open(%s) didn't fail as expected\n");
+		abort();
+	}
+}
+
+static void mfd_assert_read(int fd)
+{
+	char buf[16];
+	void *p;
+	ssize_t l;
+
+	l = read(fd, buf, sizeof(buf));
+	if (l != sizeof(buf)) {
+		printf("read() failed: %m\n");
+		abort();
+	}
+
+	/* verify PROT_READ *is* allowed */
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ,
+		 MAP_PRIVATE,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+	munmap(p, MFD_DEF_SIZE);
+
+	/* verify MAP_PRIVATE is *always* allowed (even writable) */
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ | PROT_WRITE,
+		 MAP_PRIVATE,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+	munmap(p, MFD_DEF_SIZE);
+}
+
+static void mfd_assert_write(int fd)
+{
+	ssize_t l;
+	void *p;
+	int r;
+
+	/* verify write() succeeds */
+	l = write(fd, "\0\0\0\0", 4);
+	if (l != 4) {
+		printf("write() failed: %m\n");
+		abort();
+	}
+
+	/* verify PROT_READ | PROT_WRITE is allowed */
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ | PROT_WRITE,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+	*(char *)p = 0;
+	munmap(p, MFD_DEF_SIZE);
+
+	/* verify PROT_WRITE is allowed */
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_WRITE,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+	*(char *)p = 0;
+	munmap(p, MFD_DEF_SIZE);
+
+	/* verify PROT_READ with MAP_SHARED is allowed and a following
+	 * mprotect(PROT_WRITE) allows writing */
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+
+	r = mprotect(p, MFD_DEF_SIZE, PROT_READ | PROT_WRITE);
+	if (r < 0) {
+		printf("mprotect() failed: %m\n");
+		abort();
+	}
+
+	*(char *)p = 0;
+	munmap(p, MFD_DEF_SIZE);
+
+	/* verify PUNCH_HOLE works */
+	r = fallocate(fd,
+		      FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+		      0,
+		      MFD_DEF_SIZE);
+	if (r < 0) {
+		printf("fallocate(PUNCH_HOLE) failed: %m\n");
+		abort();
+	}
+}
+
+static void mfd_fail_write(int fd)
+{
+	ssize_t l;
+	void *p;
+	int r;
+
+	/* verify write() fails */
+	l = write(fd, "data", 4);
+	if (l != -EPERM) {
+		printf("expected EPERM on write(), but got %d: %m\n", (int)l);
+		abort();
+	}
+
+	/* verify PROT_READ | PROT_WRITE is not allowed */
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ | PROT_WRITE,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p != MAP_FAILED) {
+		printf("mmap() didn't fail as expected\n");
+		abort();
+	}
+
+	/* verify PROT_WRITE is not allowed */
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_WRITE,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p != MAP_FAILED) {
+		printf("mmap() didn't fail as expected\n");
+		abort();
+	}
+
+	/* Verify PROT_READ with MAP_SHARED with a following mprotect is not
+	 * allowed. Note that for r/w the kernel already prevents the mmap. */
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p != MAP_FAILED) {
+		r = mprotect(p, MFD_DEF_SIZE, PROT_READ | PROT_WRITE);
+		if (r >= 0) {
+			printf("mmap()+mprotect() didn't fail as expected\n");
+			abort();
+		}
+	}
+
+	/* verify PUNCH_HOLE fails */
+	r = fallocate(fd,
+		      FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+		      0,
+		      MFD_DEF_SIZE);
+	if (r >= 0) {
+		printf("fallocate(PUNCH_HOLE) didn't fail as expected\n");
+		abort();
+	}
+}
+
+static void mfd_assert_shrink(int fd)
+{
+	int r, fd2;
+
+	r = ftruncate(fd, MFD_DEF_SIZE / 2);
+	if (r < 0) {
+		printf("ftruncate(SHRINK) failed: %m\n");
+		abort();
+	}
+
+	mfd_assert_size(fd, MFD_DEF_SIZE / 2);
+
+	fd2 = mfd_assert_open(fd,
+			      O_RDWR | O_CREAT | O_TRUNC,
+			      S_IRUSR | S_IWUSR);
+	close(fd2);
+
+	mfd_assert_size(fd, 0);
+}
+
+static void mfd_fail_shrink(int fd)
+{
+	int r;
+
+	r = ftruncate(fd, MFD_DEF_SIZE / 2);
+	if (r >= 0) {
+		printf("ftruncate(SHRINK) didn't fail as expected\n");
+		abort();
+	}
+
+	mfd_fail_open(fd,
+		      O_RDWR | O_CREAT | O_TRUNC,
+		      S_IRUSR | S_IWUSR);
+}
+
+static void mfd_assert_grow(int fd)
+{
+	int r;
+
+	r = ftruncate(fd, MFD_DEF_SIZE * 2);
+	if (r < 0) {
+		printf("ftruncate(GROW) failed: %m\n");
+		abort();
+	}
+
+	mfd_assert_size(fd, MFD_DEF_SIZE * 2);
+
+	r = fallocate(fd,
+		      0,
+		      0,
+		      MFD_DEF_SIZE * 4);
+	if (r < 0) {
+		printf("fallocate(ALLOC) failed: %m\n");
+		abort();
+	}
+
+	mfd_assert_size(fd, MFD_DEF_SIZE * 4);
+}
+
+static void mfd_fail_grow(int fd)
+{
+	int r;
+
+	r = ftruncate(fd, MFD_DEF_SIZE * 2);
+	if (r >= 0) {
+		printf("ftruncate(GROW) didn't fail as expected\n");
+		abort();
+	}
+
+	r = fallocate(fd,
+		      0,
+		      0,
+		      MFD_DEF_SIZE * 4);
+	if (r >= 0) {
+		printf("fallocate(ALLOC) didn't fail as expected\n");
+		abort();
+	}
+}
+
+static void mfd_assert_grow_write(int fd)
+{
+	static char buf[MFD_DEF_SIZE * 8];
+	ssize_t l;
+
+	l = pwrite(fd, buf, sizeof(buf), 0);
+	if (l != sizeof(buf)) {
+		printf("pwrite() failed: %m\n");
+		abort();
+	}
+
+	mfd_assert_size(fd, MFD_DEF_SIZE * 8);
+}
+
+static void mfd_fail_grow_write(int fd)
+{
+	static char buf[MFD_DEF_SIZE * 8];
+	ssize_t l;
+
+	l = pwrite(fd, buf, sizeof(buf), 0);
+	if (l == sizeof(buf)) {
+		printf("pwrite() didn't fail as expected\n");
+		abort();
+	}
+}
+
+static int idle_thread_fn(void *arg)
+{
+	sigset_t set;
+	int sig;
+
+	/* dummy waiter; SIGTERM terminates us anyway */
+	sigemptyset(&set);
+	sigaddset(&set, SIGTERM);
+	sigwait(&set, &sig);
+
+	return 0;
+}
+
+static pid_t spawn_idle_thread(unsigned int flags)
+{
+	uint8_t *stack;
+	pid_t pid;
+
+	stack = malloc(STACK_SIZE);
+	if (!stack) {
+		printf("malloc(STACK_SIZE) failed: %m\n");
+		abort();
+	}
+
+	pid = clone(idle_thread_fn,
+		    stack + STACK_SIZE,
+		    SIGCHLD | flags,
+		    NULL);
+	if (pid < 0) {
+		printf("clone() failed: %m\n");
+		abort();
+	}
+
+	return pid;
+}
+
+static void join_idle_thread(pid_t pid)
+{
+	kill(pid, SIGTERM);
+	waitpid(pid, NULL, 0);
+}
+
+/*
+ * Test memfd_create() syscall
+ * Verify syscall-argument validation, including name checks, flag validation
+ * and more.
+ */
+static void test_create(void)
+{
+	char buf[2048];
+	int fd;
+
+	/* test NULL name */
+	mfd_fail_new(NULL, 0);
+
+	/* test over-long name (not zero-terminated) */
+	memset(buf, 0xff, sizeof(buf));
+	mfd_fail_new(buf, 0);
+
+	/* test over-long zero-terminated name */
+	memset(buf, 0xff, sizeof(buf));
+	buf[sizeof(buf) - 1] = 0;
+	mfd_fail_new(buf, 0);
+
+	/* verify "" is a valid name */
+	fd = mfd_assert_new("", 0, 0);
+	close(fd);
+
+	/* verify invalid O_* open flags */
+	mfd_fail_new("", 0x0100);
+	mfd_fail_new("", ~MFD_CLOEXEC);
+	mfd_fail_new("", ~MFD_ALLOW_SEALING);
+	mfd_fail_new("", ~0);
+	mfd_fail_new("", 0x80000000U);
+
+	/* verify MFD_CLOEXEC is allowed */
+	fd = mfd_assert_new("", 0, MFD_CLOEXEC);
+	close(fd);
+
+	/* verify MFD_ALLOW_SEALING is allowed */
+	fd = mfd_assert_new("", 0, MFD_ALLOW_SEALING);
+	close(fd);
+
+	/* verify MFD_ALLOW_SEALING | MFD_CLOEXEC is allowed */
+	fd = mfd_assert_new("", 0, MFD_ALLOW_SEALING | MFD_CLOEXEC);
+	close(fd);
+}
+
+/*
+ * Test basic sealing
+ * A very basic sealing test to see whether setting/retrieving seals works.
+ */
+static void test_basic(void)
+{
+	int fd;
+
+	fd = mfd_assert_new("kern_memfd_basic",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+
+	/* add basic seals */
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_SHRINK |
+				 F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_SHRINK |
+				 F_SEAL_WRITE);
+
+	/* add them again */
+	mfd_assert_add_seals(fd, F_SEAL_SHRINK |
+				 F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_SHRINK |
+				 F_SEAL_WRITE);
+
+	/* add more seals and seal against sealing */
+	mfd_assert_add_seals(fd, F_SEAL_GROW | F_SEAL_SEAL);
+	mfd_assert_has_seals(fd, F_SEAL_SHRINK |
+				 F_SEAL_GROW |
+				 F_SEAL_WRITE |
+				 F_SEAL_SEAL);
+
+	/* verify that sealing no longer works */
+	mfd_fail_add_seals(fd, F_SEAL_GROW);
+	mfd_fail_add_seals(fd, 0);
+
+	close(fd);
+
+	/* verify sealing does not work without MFD_ALLOW_SEALING */
+	fd = mfd_assert_new("kern_memfd_basic",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC);
+	mfd_assert_has_seals(fd, F_SEAL_SEAL);
+	mfd_fail_add_seals(fd, F_SEAL_SHRINK |
+			       F_SEAL_GROW |
+			       F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_SEAL);
+	close(fd);
+}
+
+/*
+ * Test SEAL_WRITE
+ * Test whether SEAL_WRITE actually prevents modifications.
+ */
+static void test_seal_write(void)
+{
+	int fd;
+
+	fd = mfd_assert_new("kern_memfd_seal_write",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE);
+
+	mfd_assert_read(fd);
+	mfd_fail_write(fd);
+	mfd_assert_shrink(fd);
+	mfd_assert_grow(fd);
+	mfd_fail_grow_write(fd);
+
+	close(fd);
+}
+
+/*
+ * Test SEAL_SHRINK
+ * Test whether SEAL_SHRINK actually prevents shrinking
+ */
+static void test_seal_shrink(void)
+{
+	int fd;
+
+	fd = mfd_assert_new("kern_memfd_seal_shrink",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_SHRINK);
+	mfd_assert_has_seals(fd, F_SEAL_SHRINK);
+
+	mfd_assert_read(fd);
+	mfd_assert_write(fd);
+	mfd_fail_shrink(fd);
+	mfd_assert_grow(fd);
+	mfd_assert_grow_write(fd);
+
+	close(fd);
+}
+
+/*
+ * Test SEAL_GROW
+ * Test whether SEAL_GROW actually prevents growing
+ */
+static void test_seal_grow(void)
+{
+	int fd;
+
+	fd = mfd_assert_new("kern_memfd_seal_grow",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_GROW);
+	mfd_assert_has_seals(fd, F_SEAL_GROW);
+
+	mfd_assert_read(fd);
+	mfd_assert_write(fd);
+	mfd_assert_shrink(fd);
+	mfd_fail_grow(fd);
+	mfd_fail_grow_write(fd);
+
+	close(fd);
+}
+
+/*
+ * Test SEAL_SHRINK | SEAL_GROW
+ * Test whether SEAL_SHRINK | SEAL_GROW actually prevents resizing
+ */
+static void test_seal_resize(void)
+{
+	int fd;
+
+	fd = mfd_assert_new("kern_memfd_seal_resize",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_SHRINK | F_SEAL_GROW);
+	mfd_assert_has_seals(fd, F_SEAL_SHRINK | F_SEAL_GROW);
+
+	mfd_assert_read(fd);
+	mfd_assert_write(fd);
+	mfd_fail_shrink(fd);
+	mfd_fail_grow(fd);
+	mfd_fail_grow_write(fd);
+
+	close(fd);
+}
+
+/*
+ * Test sharing via dup()
+ * Test that seals are shared between dupped FDs and they're all equal.
+ */
+static void test_share_dup(void)
+{
+	int fd, fd2;
+
+	fd = mfd_assert_new("kern_memfd_share_dup",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+
+	fd2 = mfd_assert_dup(fd);
+	mfd_assert_has_seals(fd2, 0);
+
+	mfd_assert_add_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd2, F_SEAL_WRITE);
+
+	mfd_assert_add_seals(fd2, F_SEAL_SHRINK);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE | F_SEAL_SHRINK);
+	mfd_assert_has_seals(fd2, F_SEAL_WRITE | F_SEAL_SHRINK);
+
+	mfd_assert_add_seals(fd, F_SEAL_SEAL);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE | F_SEAL_SHRINK | F_SEAL_SEAL);
+	mfd_assert_has_seals(fd2, F_SEAL_WRITE | F_SEAL_SHRINK | F_SEAL_SEAL);
+
+	mfd_fail_add_seals(fd, F_SEAL_GROW);
+	mfd_fail_add_seals(fd2, F_SEAL_GROW);
+	mfd_fail_add_seals(fd, F_SEAL_SEAL);
+	mfd_fail_add_seals(fd2, F_SEAL_SEAL);
+
+	close(fd2);
+
+	mfd_fail_add_seals(fd, F_SEAL_GROW);
+	close(fd);
+}
+
+/*
+ * Test sealing with active mmap()s
+ * Modifying seals is only allowed if no other mmap() refs exist.
+ */
+static void test_share_mmap(void)
+{
+	int fd;
+	void *p;
+
+	fd = mfd_assert_new("kern_memfd_share_mmap",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+
+	/* shared/writable ref prevents sealing WRITE, but allows others */
+	p = mfd_assert_mmap_shared(fd);
+	mfd_fail_add_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_SHRINK);
+	mfd_assert_has_seals(fd, F_SEAL_SHRINK);
+	munmap(p, MFD_DEF_SIZE);
+
+	/* readable ref allows sealing */
+	p = mfd_assert_mmap_private(fd);
+	mfd_assert_add_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE | F_SEAL_SHRINK);
+	munmap(p, MFD_DEF_SIZE);
+
+	close(fd);
+}
+
+/*
+ * Test sealing with open(/proc/self/fd/%d)
+ * Via /proc we can get access to a separate file-context for the same memfd.
+ * This is *not* like dup(), but like a real separate open(). Make sure the
+ * semantics are as expected and we correctly check for RDONLY / WRONLY / RDWR.
+ */
+static void test_share_open(void)
+{
+	int fd, fd2;
+
+	fd = mfd_assert_new("kern_memfd_share_open",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+
+	fd2 = mfd_assert_open(fd, O_RDWR, 0);
+	mfd_assert_add_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd2, F_SEAL_WRITE);
+
+	mfd_assert_add_seals(fd2, F_SEAL_SHRINK);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE | F_SEAL_SHRINK);
+	mfd_assert_has_seals(fd2, F_SEAL_WRITE | F_SEAL_SHRINK);
+
+	close(fd);
+	fd = mfd_assert_open(fd2, O_RDONLY, 0);
+
+	mfd_fail_add_seals(fd, F_SEAL_SEAL);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE | F_SEAL_SHRINK);
+	mfd_assert_has_seals(fd2, F_SEAL_WRITE | F_SEAL_SHRINK);
+
+	close(fd2);
+	fd2 = mfd_assert_open(fd, O_RDWR, 0);
+
+	mfd_assert_add_seals(fd2, F_SEAL_SEAL);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE | F_SEAL_SHRINK | F_SEAL_SEAL);
+	mfd_assert_has_seals(fd2, F_SEAL_WRITE | F_SEAL_SHRINK | F_SEAL_SEAL);
+
+	close(fd2);
+	close(fd);
+}
+
+/*
+ * Test sharing via fork()
+ * Test whether seal-modifications work as expected with forked childs.
+ */
+static void test_share_fork(void)
+{
+	int fd;
+	pid_t pid;
+
+	fd = mfd_assert_new("kern_memfd_share_fork",
+			    MFD_DEF_SIZE,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+
+	pid = spawn_idle_thread(0);
+	mfd_assert_add_seals(fd, F_SEAL_SEAL);
+	mfd_assert_has_seals(fd, F_SEAL_SEAL);
+
+	mfd_fail_add_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_SEAL);
+
+	join_idle_thread(pid);
+
+	mfd_fail_add_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_SEAL);
+
+	close(fd);
+}
+
+int main(int argc, char **argv)
+{
+	pid_t pid;
+
+	printf("memfd: CREATE\n");
+	test_create();
+	printf("memfd: BASIC\n");
+	test_basic();
+
+	printf("memfd: SEAL-WRITE\n");
+	test_seal_write();
+	printf("memfd: SEAL-SHRINK\n");
+	test_seal_shrink();
+	printf("memfd: SEAL-GROW\n");
+	test_seal_grow();
+	printf("memfd: SEAL-RESIZE\n");
+	test_seal_resize();
+
+	printf("memfd: SHARE-DUP\n");
+	test_share_dup();
+	printf("memfd: SHARE-MMAP\n");
+	test_share_mmap();
+	printf("memfd: SHARE-OPEN\n");
+	test_share_open();
+	printf("memfd: SHARE-FORK\n");
+	test_share_fork();
+
+	/* Run test-suite in a multi-threaded environment with a shared
+	 * file-table. */
+	pid = spawn_idle_thread(CLONE_FILES | CLONE_FS | CLONE_VM);
+	printf("memfd: SHARE-DUP (shared file-table)\n");
+	test_share_dup();
+	printf("memfd: SHARE-MMAP (shared file-table)\n");
+	test_share_mmap();
+	printf("memfd: SHARE-OPEN (shared file-table)\n");
+	test_share_open();
+	printf("memfd: SHARE-FORK (shared file-table)\n");
+	test_share_fork();
+	join_idle_thread(pid);
+
+	printf("memfd: DONE\n");
+
+	return 0;
+}