[v1] selftests/mm: add simple VM_PFNMAP tests based on mmap'ing /dev/mem

diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index ad4d6043a60f0..ae6f994d3add7 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile
@@ -84,6 +84,7 @@  TEST_GEN_FILES += mremap_test
 TEST_GEN_FILES += mseal_test
 TEST_GEN_FILES += on-fault-limit
 TEST_GEN_FILES += pagemap_ioctl
+TEST_GEN_FILES += pfnmap
 TEST_GEN_FILES += thuge-gen
 TEST_GEN_FILES += transhuge-stress
 TEST_GEN_FILES += uffd-stress
diff --git a/tools/testing/selftests/mm/pfnmap.c b/tools/testing/selftests/mm/pfnmap.c
new file mode 100644
index 0000000000000..59be2f3221124
--- /dev/null
+++ b/tools/testing/selftests/mm/pfnmap.c
@@ -0,0 +1,278 @@ 
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Basic VM_PFNMAP tests relying on mmap() of '/dev/mem'
+ *
+ * Copyright 2025, Red Hat, Inc.
+ *
+ * Author(s): David Hildenbrand <david@redhat.com>
+ */
+#define _GNU_SOURCE
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <setjmp.h>
+#include <linux/mman.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+
+#include "../kselftest.h"
+#include "vm_util.h"
+
+static size_t pagesize;
+static int pagemap_fd;
+static int dev_mem_fd;
+static sigjmp_buf env;
+
+static void signal_handler(int sig)
+{
+	if (sig == SIGSEGV)
+		siglongjmp(env, 1);
+	siglongjmp(env, 2);
+}
+
+static void sense_support(void)
+{
+	char *addr, tmp;
+	int ret;
+
+	dev_mem_fd = open("/dev/mem", O_RDONLY);
+	if (dev_mem_fd < 0)
+		ksft_exit_skip("Cannot open '/dev/mem': %s\n", strerror(errno));
+
+	/* We'll require the first two pages throughout our tests ... */
+	addr = mmap(0, pagesize * 2, PROT_READ, MAP_SHARED, dev_mem_fd, 0);
+	if (addr == MAP_FAILED)
+		ksft_exit_skip("Cannot mmap '/dev/mem'");
+
+	/* ... and want to be able to read from them. */
+	ret = sigsetjmp(env, 1);
+	if (!ret) {
+		tmp = *addr + *(addr + pagesize);
+		asm volatile("" : "+r" (tmp));
+	}
+	if (ret)
+		ksft_exit_skip("Cannot read-access mmap'ed '/dev/mem'");
+
+	munmap(addr, pagesize * 2);
+}
+
+static void test_madvise(void)
+{
+#define INIT_ADVICE(nr) { nr, #nr}
+	const struct {
+		int nr;
+		const char *name;
+	} advices[] = {
+		INIT_ADVICE(MADV_DONTNEED),
+		INIT_ADVICE(MADV_DONTNEED_LOCKED),
+		INIT_ADVICE(MADV_FREE),
+		INIT_ADVICE(MADV_WIPEONFORK),
+		INIT_ADVICE(MADV_COLD),
+		INIT_ADVICE(MADV_PAGEOUT),
+		INIT_ADVICE(MADV_POPULATE_READ),
+		INIT_ADVICE(MADV_POPULATE_WRITE),
+	};
+	char *addr;
+	int ret, i;
+
+	addr = mmap(0, pagesize, PROT_READ, MAP_SHARED, dev_mem_fd, 0);
+	if (addr == MAP_FAILED)
+		ksft_exit_fail_msg("mmap() failed: %s\n", strerror(errno));
+
+	/* All these advices must be rejected. */
+	for (i = 0; i < ARRAY_SIZE(advices); i++) {
+		ret = madvise(addr, pagesize, advices[i].nr);
+		ksft_test_result(ret && errno == EINVAL,
+				 "madvise(%s) should be disallowed\n",
+				 advices[i].name);
+	}
+
+	munmap(addr, pagesize);
+}
+
+static void test_munmap_splitting(void)
+{
+	char *addr1, *addr2;
+	int ret;
+
+	addr1 = mmap(0, pagesize * 2, PROT_READ, MAP_SHARED, dev_mem_fd, 0);
+	if (addr1 == MAP_FAILED)
+		ksft_exit_fail_msg("mmap() failed: %s\n", strerror(errno));
+
+	/* Unmap the first pages. */
+	ret = munmap(addr1, pagesize);
+	ksft_test_result(!ret, "munmap() splitting\n");
+
+	/* Remap the first page while the second page is still mapped. */
+	addr2 = mmap(0, pagesize, PROT_READ, MAP_SHARED, dev_mem_fd, 0);
+	ksft_test_result(addr2 != MAP_FAILED, "mmap() after splitting\n");
+
+	if (addr2 != MAP_FAILED)
+		munmap(addr2, pagesize);
+	if (!ret)
+		munmap(addr1 + pagesize, pagesize);
+	else
+		munmap(addr1, pagesize * 2);
+}
+
+static void test_mremap_fixed(void)
+{
+	char *addr, *new_addr, *ret;
+
+	addr = mmap(0, pagesize * 2, PROT_READ, MAP_SHARED, dev_mem_fd, 0);
+	if (addr == MAP_FAILED)
+		ksft_exit_fail_msg("mmap() failed: %s\n", strerror(errno));
+
+	/* Reserve a destination area. */
+	new_addr = mmap(0, pagesize * 2, PROT_READ, MAP_ANON | MAP_PRIVATE, -1, 0);
+	if (new_addr == MAP_FAILED)
+		ksft_exit_fail_msg("mmap() failed: %s\n", strerror(errno));
+
+	/* mremap() over our destination. */
+	ret = mremap(addr, pagesize * 2, pagesize * 2,
+		     MREMAP_FIXED | MREMAP_MAYMOVE, new_addr);
+	ksft_test_result(ret == new_addr, "mremap(MREMAP_FIXED)\n");
+	if (ret != new_addr)
+		munmap(new_addr, pagesize * 2);
+	munmap(addr, pagesize * 2);
+}
+
+static void test_mremap_shrinking(void)
+{
+	char *addr, *ret;
+
+	addr = mmap(0, pagesize * 2, PROT_READ, MAP_SHARED, dev_mem_fd, 0);
+	if (addr == MAP_FAILED)
+		ksft_exit_fail_msg("mmap() failed: %s\n", strerror(errno));
+
+	/* Shrinking is expected to work. */
+	ret = mremap(addr, pagesize * 2, pagesize, 0);
+	ksft_test_result(ret == addr, "mremap() shrinking\n");
+	if (ret != addr)
+		munmap(addr, pagesize * 2);
+	else
+		munmap(addr, pagesize);
+}
+
+static void test_mremap_growing(void)
+{
+	char *addr, *ret;
+
+	addr = mmap(0, pagesize, PROT_READ, MAP_SHARED, dev_mem_fd, 0);
+	if (addr == MAP_FAILED)
+		ksft_exit_fail_msg("mmap() failed: %s\n", strerror(errno));
+
+	/* Growing is not expected to work. */
+	ret = mremap(addr, pagesize, pagesize * 2, MREMAP_MAYMOVE);
+	ksft_test_result(ret == MAP_FAILED,
+			 "mremap() growing should be disallowed\n");
+	if (ret == MAP_FAILED)
+		munmap(addr, pagesize);
+	else
+		munmap(ret, pagesize * 2);
+}
+
+static void test_mprotect(void)
+{
+	char *addr, tmp;
+	int ret;
+
+	addr = mmap(0, pagesize, PROT_READ, MAP_SHARED, dev_mem_fd, 0);
+	if (addr == MAP_FAILED)
+		ksft_exit_fail_msg("mmap() failed: %s\n", strerror(errno));
+
+	/* With PROT_NONE, read access must result in SIGSEGV. */
+	ret = mprotect(addr, pagesize, PROT_NONE);
+	ksft_test_result(!ret, "mprotect(PROT_NONE)\n");
+
+	ret = sigsetjmp(env, 1);
+	if (!ret) {
+		tmp = *addr;
+		asm volatile("" : "+r" (tmp));
+	}
+	ksft_test_result(ret == 1, "SIGSEGV expected\n");
+
+	/* With PROT_READ, read access must again succeed. */
+	ret = mprotect(addr, pagesize, PROT_READ);
+	ksft_test_result(!ret, "mprotect(PROT_READ)\n");
+
+	ret = sigsetjmp(env, 1);
+	if (!ret) {
+		tmp = *addr;
+		asm volatile("" : "+r" (tmp));
+	}
+	ksft_test_result(!ret, "SIGSEGV not expected\n");
+
+	munmap(addr, pagesize);
+}
+
+static void test_fork(void)
+{
+	char *addr, tmp;
+	int ret;
+
+	addr = mmap(0, pagesize, PROT_READ, MAP_SHARED, dev_mem_fd, 0);
+	if (addr == MAP_FAILED)
+		ksft_exit_fail_msg("mmap() failed: %s\n", strerror(errno));
+
+	/* fork() a child and test if the child can access the page. */
+	ret = fork();
+	if (ret < 0) {
+		ksft_test_result_fail("fork()\n");
+		goto out;
+	} else if (!ret) {
+		ret = sigsetjmp(env, 1);
+		if (!ret) {
+			tmp = *addr;
+			asm volatile("" : "+r" (tmp));
+		}
+		/* Return the result to the parent. */
+		exit(ret);
+	}
+	ksft_test_result_pass("fork()\n");
+
+	/* Wait for our child and obtain the result. */
+	wait(&ret);
+	if (WIFEXITED(ret))
+		ret = WEXITSTATUS(ret);
+	else
+		ret = -EINVAL;
+
+	ksft_test_result(!ret, "SIGSEGV in child not expected\n");
+out:
+	munmap(addr, pagesize);
+}
+
+int main(int argc, char **argv)
+{
+	int err;
+
+	ksft_print_header();
+	ksft_set_plan(19);
+
+	pagesize = getpagesize();
+	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+	if (pagemap_fd < 0)
+		ksft_exit_fail_msg("opening pagemap failed\n");
+	if (signal(SIGSEGV, signal_handler) == SIG_ERR)
+		ksft_exit_fail_msg("signal() failed: %s\n", strerror(errno));
+
+	sense_support();
+	test_madvise();
+	test_munmap_splitting();
+	test_mremap_fixed();
+	test_mremap_shrinking();
+	test_mremap_growing();
+	test_mprotect();
+	test_fork();
+
+	err = ksft_get_fail_cnt();
+	if (err)
+		ksft_exit_fail_msg("%d out of %d tests failed\n",
+				   err, ksft_test_num());
+	ksft_exit_pass();
+}