This is xnu-4903.221.2. See this file in:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <assert.h>
#include <signal.h>
#include <spawn.h>
#include <spawn_private.h>
#include <stdint.h>
#include <sys/sysctl.h>
#include <sys/spawn_internal.h>
#include <sys/kern_memorystatus.h>
#include <mach-o/dyld.h>

#include <darwintest.h>
#include <darwintest_utils.h>

T_GLOBAL_META(
	T_META_NAMESPACE("xnu.vm"),
	T_META_CHECK_LEAKS(false)
);

extern char **environ;

/*
 * This test file contains two sub-tests which attempt to verify
 * the allowing or not allowing of a corpse for crashreporter when
 * a task exceeds its memory allocation limit. vm_map_fork() is the
 * kernel routine used to generate a corpse task.
 *
 * A corpse is allowed to be taken if a task's memory resource limit that
 * is exceeded is less than 1/4 of the system wide task limit.
 * If the amount exceeds 1/4 the sytem wide limit, then the corpse is disallowed.
 *
 * If the device under test is already under pressure, the test
 * could fail due to jetsam cutting in and killing the parent, child or
 * other necessary testing processes.
 */

/* Test variants */
#define TEST_ALLOWED	 0x1
#define TEST_NOT_ALLOWED 0x2

/*
 * Values which the kernel OR's into the PID when a corpse
 * is either allowed or disallowed for the
 * kern.memorystatus_vm_map_fork_pidwatch sysctl.
 */
#define MEMORYSTATUS_VM_MAP_FORK_ALLOWED	0x100000000ul
#define MEMORYSTATUS_VM_MAP_FORK_NOT_ALLOWED 0x200000000ul

/*
 * The memory allocation happens in a child process, this
 * is stuff to deal with creating and managing the child.
 * The child will only execute the T_HELPER_DECL.
 */
static char testpath[PATH_MAX];
static uint32_t testpath_size = sizeof(testpath);
#define LIMIT_DELTA_MB 5 /* an arbitrary limit delta */
#define MEGABYTE	(1024 * 1024)

/*
 * The child process communicates back to parent via an exit() code.
 */
enum child_exits {
	NORMAL_EXIT = 0,
	NO_MEMSIZE_ARG,
	INVALID_MEMSIZE,
	MALLOC_FAILED,
	NUM_CHILD_EXIT
};
static char *child_exit_why[] = {
	"normal exit",
	"no memsize argument to child",
	"invalid memsize argument to child",
	"malloc() failed",
};

/*
 * Corpse collection only happens in development kernels.
 * So we need this to detect if the test is relevant.
 */
static boolean_t
is_development_kernel(void)
{
	int ret;
	int dev = 0;
	size_t dev_size = sizeof(dev);

	ret = sysctlbyname("kern.development", &dev, &dev_size, NULL, 0);
	if (ret != 0) {
		return FALSE;
	}

	return (dev != 0);
}

/*
 * Set/Get the sysctl used to determine if corpse collection occurs.
 * This is done by the kernel checking for a specific PID.
 */
static void
set_memorystatus_vm_map_fork_pidwatch(pid_t pid)
{
	uint64_t new_value = (uint64_t)pid;
	size_t new_len = sizeof(new_value);
	int err;

	err = sysctlbyname("kern.memorystatus_vm_map_fork_pidwatch", NULL, NULL, &new_value, new_len);
	T_QUIET;
	T_ASSERT_POSIX_SUCCESS(err, "set sysctlbyname(kern.memorystatus_vm_map_fork_pidwatch...) failed");
	return;
}

static uint64_t
get_memorystatus_vm_map_fork_pidwatch()
{
	uint64_t value = 0;
	size_t val_len = sizeof(value);
	int err;

	err = sysctlbyname("kern.memorystatus_vm_map_fork_pidwatch", &value, &val_len, NULL, 0);
	T_QUIET;
	T_ASSERT_POSIX_SUCCESS(err, "get sysctlbyname(kern.memorystatus_vm_map_fork_pidwatch...) failed");

	return value;
}

/*
 * We want to avoid jetsam giving us bad results, if possible. So check if there's
 * enough memory for the test to run, waiting briefly for some to free up.
 */
static void
wait_for_free_mem(int need_mb)
{
	int64_t		memsize;
	int		memorystatus_level;
	size_t		size;
	int64_t		avail;
	int		err;
	int		try;

	/*
	 * get amount of memory in the machine
	 */
	size = sizeof(memsize);
	err = sysctlbyname("hw.memsize", &memsize, &size, NULL, 0);
	T_QUIET; T_ASSERT_POSIX_SUCCESS(err, "sysctlbyname(hw.memsize...) failed");

	/*
	 * Use a loop to briefly sleep and recheck if short on memory.
	 */
	try = 1;
	for (;;) {

		/*
		 * memorystatus_level is a percentage of memory available. For example 20 means 1/5 of memory.
		 * It currently doesn't exist on macOS but neither does jetsam, so pass the test there.
		 */
		size = sizeof(memorystatus_level);
		if (sysctlbyname("kern.memorystatus_level", &memorystatus_level, &size, NULL, 0) != 0)
			return;
		T_QUIET; T_ASSERT_LE(memorystatus_level, 100, "memorystatus_level too high");
		T_QUIET; T_ASSERT_GT(memorystatus_level, 0, "memorystatus_level negative");

		/*
		 * jetsam kicks in at memory status level of 15%, so subtract that much out of what's available.
		 */
		avail = MAX(0, (memsize * (memorystatus_level - 15)) / 100);

		/*
		 * We're good to go if there's more than enough available.
		 */
		if ((int64_t)need_mb * MEGABYTE < avail)
			return;

		/*
		 * issue a message to log and sleep briefly to see if we can get more memory
		 */
		if (try-- == 0)
			break;
		T_LOG("Need %d MB, only %d MB available. sleeping 5 seconds for more to free. memorystatus_level %d",
		    need_mb, (int)(avail / MEGABYTE), memorystatus_level);
		sleep(5);
	}
	T_SKIP("Needed %d MB, but only %d MB available. Skipping test to avoid jetsam issues.",
	    need_mb, (int)(avail / MEGABYTE));
}


/*
 * The main test calls this to spawn child process which will run and
 * exceed some memory limit. The child is initially suspended so that
 * we can do the sysctl calls before it runs.
 * Since this is a libdarwintest, the "-n" names the T_HELPER_DECL() that
 * we want to run. The arguments specific to the test follow a "--".
 */
static pid_t
spawn_child_process(
	char * const executable,
	char * const memlimit,
	short flags,
	int priority,
	int active_limit_mb,
	int inactive_limit_mb)
{
	posix_spawnattr_t spawn_attrs;
	int err;
	pid_t child_pid;
	char * const argv_child[] = { executable, "-n", "child_process", "--", memlimit, NULL };

	err = posix_spawnattr_init(&spawn_attrs);
	T_QUIET; T_ASSERT_POSIX_SUCCESS(err, "  posix_spawnattr_init() failed");

	err = posix_spawnattr_setflags(&spawn_attrs, POSIX_SPAWN_START_SUSPENDED);
	T_QUIET; T_ASSERT_POSIX_SUCCESS(err, "  posix_spawnattr_setflags() failed");

	err = posix_spawnattr_setjetsam_ext(&spawn_attrs, flags, priority, active_limit_mb, inactive_limit_mb);
	T_QUIET; T_ASSERT_POSIX_SUCCESS(err, "  posix_spawnattr_setjetsam_ext() failed");

	err = posix_spawn(&child_pid, executable, NULL, &spawn_attrs, argv_child, environ);
	T_QUIET; T_ASSERT_POSIX_SUCCESS(err, "  posix_spawn() failed");

	return child_pid;
}


/*
 * The parent calls this to continue the suspended child, then wait for its result.
 * We collect its resource usage to vefiry the expected amount allocated.
 */
static void
test_child_process(pid_t child_pid, int *status, struct rusage *ru)
{
	int err = 0;
	pid_t got_pid;

	T_LOG("  continuing child[%d]\n", child_pid);

	err = kill(child_pid, SIGCONT);
	T_QUIET; T_ASSERT_POSIX_SUCCESS(err, "  kill(%d, SIGCONT) failed", child_pid);

	T_LOG("  waiting for child[%d] to exit", child_pid);

	got_pid = wait4(child_pid, status, 0, ru);
	T_QUIET; T_ASSERT_EQ(child_pid, got_pid, "  wait4(%d, ...) returned %d", child_pid, got_pid);
}

/*
 * The child process executes this code. The easiest way, with given darwintest infrastructure,
 * it has to return information is via exit status.
 */
T_HELPER_DECL(child_process, "child allocates memory to failure")
{
#define BYTESPERALLOC	MEGABYTE
#define BYTESINEXCESS	(2 * MEGABYTE) /* 2 MB - arbitrary */
	char *limit;
	long limit_mb = 0;
	long max_bytes_to_munch, bytes_remaining, bytes_this_munch;
	void *mem = NULL;

	/*
	 * This helper is run in a child process. The helper sees one argument
	 * as a string which is the amount of memory in megabytes to allocate.
	 */
	if (argc != 1)
		exit(NO_MEMSIZE_ARG);

	limit = argv[0];
	errno = 0;
	limit_mb = strtol(limit, NULL, 10);
	if (errno != 0 || limit_mb <= 0)
		exit(INVALID_MEMSIZE);

	/* Compute in excess of assigned limit */
	max_bytes_to_munch = limit_mb * MEGABYTE;
	max_bytes_to_munch += BYTESINEXCESS;

	for (bytes_remaining = max_bytes_to_munch; bytes_remaining > 0; bytes_remaining -= bytes_this_munch) {
		bytes_this_munch = MIN(bytes_remaining, BYTESPERALLOC);

		mem = malloc((size_t)bytes_this_munch);
		if (mem == NULL)
			exit(MALLOC_FAILED);
		arc4random_buf(mem, (size_t)bytes_this_munch);
	}

	/* We chewed up all the memory we were asked to. */
	exit(NORMAL_EXIT);
}


/*
 * Actual test body.
 */
static void
memorystatus_vm_map_fork_parent(int test_variant)
{
	int		max_task_pmem = 0; /* MB */
	size_t		size = 0;
	int		active_limit_mb = 0;
	int		inactive_limit_mb = 0;
	short		flags = 0;
	char		memlimit_str[16];
	pid_t		child_pid;
	int		child_status;
	uint64_t	kernel_pidwatch_val;
	uint64_t 	expected_pidwatch_val;
	int		ret;
	struct rusage	ru;
	enum child_exits exit_val;

	/*
	 * The code to set/get the pidwatch sysctl is only in
	 * development kernels. Skip the test if not on one.
	 */
	if (!is_development_kernel()) {
		T_SKIP("Can't test on release kernel");
	}

	/*
	 * Determine a memory limit based on system having one or not.
	 */
	size = sizeof(max_task_pmem);
	(void)sysctlbyname("kern.max_task_pmem", &max_task_pmem, &size, NULL, 0);
	if (max_task_pmem <= 0)
		max_task_pmem = 0;

	if (test_variant == TEST_ALLOWED) {
		
		/*
		 * Tell the child to allocate less than 1/4 the system wide limit.
		 */
		if (max_task_pmem / 4 - LIMIT_DELTA_MB <= 0) {
			active_limit_mb = LIMIT_DELTA_MB;
		} else {
			active_limit_mb = max_task_pmem / 4 - LIMIT_DELTA_MB;
		}
		expected_pidwatch_val = MEMORYSTATUS_VM_MAP_FORK_ALLOWED;

	} else { /* TEST_NOT_ALLOWED */

		/*
		 * Tell the child to allocate more than 1/4 the system wide limit.
		 */
		active_limit_mb = (max_task_pmem / 4) + LIMIT_DELTA_MB;
		if (max_task_pmem == 0) {
			expected_pidwatch_val = MEMORYSTATUS_VM_MAP_FORK_ALLOWED;
		} else {
			expected_pidwatch_val = MEMORYSTATUS_VM_MAP_FORK_NOT_ALLOWED;
		}

	}
	inactive_limit_mb = active_limit_mb;
	T_LOG("using limit of %d Meg", active_limit_mb);

	/*
	 * When run as part of a larger suite, a previous test
	 * may have left the system temporarily with too little
	 * memory to run this test. We try to detect if there is
	 * enough free memory to proceed, waiting a little bit
	 * for memory to free up.
	 */
	wait_for_free_mem(active_limit_mb);

#if defined(__x86_64__)
	/*
	 * vm_map_fork() is always allowed on desktop.
	 */
	expected_pidwatch_val = MEMORYSTATUS_VM_MAP_FORK_ALLOWED;
#endif

	/*
	 * Prepare the arguments needed to spawn the child process.
	 */
	memset (memlimit_str, 0, sizeof(memlimit_str));
	(void)sprintf(memlimit_str, "%d", active_limit_mb);

	ret = _NSGetExecutablePath(testpath, &testpath_size);
	T_QUIET; T_ASSERT_POSIX_SUCCESS(ret, "_NSGetExecutablePath(%s, ...)", testpath);

	/*
	 * We put the child process in FOREGROUND to try and keep jetsam's hands off it.
	 */
	child_pid = spawn_child_process(testpath, memlimit_str, flags,
	    JETSAM_PRIORITY_FOREGROUND, active_limit_mb, inactive_limit_mb);

	expected_pidwatch_val |= (uint64_t)child_pid;

	/*
	 * We only reach here if parent successfully spawned child process.
	 */
	T_LOG("  spawned child_pid[%d] with memlimit %s (%d)MB\n",
	    child_pid, memlimit_str, active_limit_mb);

	/*
	 * Set the kernel's pidwatch to look for the child.
	 */
	(void)set_memorystatus_vm_map_fork_pidwatch((pid_t)0);
	(void)set_memorystatus_vm_map_fork_pidwatch(child_pid);

	/*
	 * Let the child run and wait for it to finish.
	 */
	test_child_process(child_pid, &child_status, &ru);
	T_LOG("Child exited with max_rss of %ld", ru.ru_maxrss);

	/*
	 * Retrieve the kernel's pidwatch value. This should now indicate
	 * if the corpse was allowed or not.
	 */
	kernel_pidwatch_val = get_memorystatus_vm_map_fork_pidwatch();
	(void)set_memorystatus_vm_map_fork_pidwatch((pid_t)0);

	/*
	 * If the child died abnormally, the test is invalid.
	 */
	if (!WIFEXITED(child_status)) {
		if (WIFSIGNALED(child_status)) {
			/* jetsam kills a process with SIGKILL */
			if (WTERMSIG(child_status) == SIGKILL)
				T_LOG("Child appears to have been a jetsam victim");
			T_SKIP("Child terminated by signal %d test result invalid", WTERMSIG(child_status));
		}
		T_SKIP("child did not exit normally (status=%d) test result invalid", child_status);
	}

	/*
	 * We don't expect the child to exit for any other reason than success
	 */
	exit_val = (enum child_exits)WEXITSTATUS(child_status);
	T_QUIET; T_ASSERT_EQ(exit_val, NORMAL_EXIT, "child exit due to: %s", 
	    (0 < exit_val && exit_val < NUM_CHILD_EXIT) ? child_exit_why[exit_val] : "unknown");

	/*
	 * If the kernel aborted generating a corpse for other reasons, the test is invalid.
	 */
	if (kernel_pidwatch_val == -1ull) {
		T_SKIP("corpse generation was aborted by kernel");
	}

	/*
	 * We should always have made it through the vm_map_fork() checks in the kernel for this test.
	 */
	T_QUIET; T_ASSERT_NE_ULLONG(kernel_pidwatch_val, (uint64_t)child_pid, "child didn't trigger corpse generation");

	T_EXPECT_EQ(kernel_pidwatch_val, expected_pidwatch_val, "kernel value 0x%llx - expected 0x%llx",
	    kernel_pidwatch_val, expected_pidwatch_val);
}

/*
 * The order of these 2 test functions is important. They will be executed by the test framwork in order.
 *
 * We test "not allowed first", then "allowed". If it were the other way around, the corpse from the "allowed"
 * test would likely cause memory pressure and jetsam would likely kill the "not allowed" test.
 */
T_DECL(memorystatus_vm_map_fork_test_not_allowed, "test that corpse generation was not allowed")
{
	memorystatus_vm_map_fork_parent(TEST_NOT_ALLOWED);
}

T_DECL(memorystatus_vm_map_fork_test_allowed, "test corpse generation allowed")
{

	memorystatus_vm_map_fork_parent(TEST_ALLOWED);
}