lib/: set O_CLOEXEC on all fds

As found by
  git grep -E '(open|setmntent|pipe2?)\(' |
    grep -vE '((zfs|zpool)_|fd|dl|lzc_re|pidfile_|g_)open\('

FreeBSD's pidfile_open() says nothing about the flags of the files it
opens, but we can't do anything about it anyway; the implementation does
open all files with O_CLOEXEC

Consider this output with zpool.d/media appended with
"pid=$$; (ls -l /proc/$pid/fd > /dev/tty)":
  $ /sbin/zpool iostat -vc media
  lrwx------ 0 -> /dev/pts/0
  l-wx------ 1 -> 'pipe:[3278500]'
  l-wx------ 2 -> /dev/null
  lrwx------ 3 -> /dev/zfs
  lr-x------ 4 -> /proc/31895/mounts
  lrwx------ 5 -> /dev/zfs
  lr-x------ 10 -> /usr/lib/zfs-linux/zpool.d/media
vs
  $ ./zpool iostat -vc vendor,upath,iostat,media
  lrwx------ 0 -> /dev/pts/0
  l-wx------ 1 -> 'pipe:[3279887]'
  l-wx------ 2 -> /dev/null
  lr-x------ 10 -> /usr/lib/zfs-linux/zpool.d/media

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11866

lib/libzfs/os/linux/libzfs_pool_os.c

@@ -62,7 +62,7 @@ zpool_relabel_disk(libzfs_handle_t *hdl, const char *path, const char *msg)
 {
 	int fd, error;
 
-	if ((fd = open(path, O_RDWR|O_DIRECT)) < 0) {
+	if ((fd = open(path, O_RDWR|O_DIRECT|O_CLOEXEC)) < 0) {
 		zfs_error_aux(hdl, dgettext(TEXT_DOMAIN, "cannot "
 		    "relabel '%s': unable to open device: %d"), path, errno);
 		return (zfs_error(hdl, EZFS_OPENFAILED, msg));
@@ -107,7 +107,7 @@ read_efi_label(nvlist_t *config, diskaddr_t *sb)
 
 	(void) snprintf(diskname, sizeof (diskname), "%s%s", DISK_ROOT,
 	    strrchr(path, '/'));
-	if ((fd = open(diskname, O_RDONLY|O_DIRECT)) >= 0) {
+	if ((fd = open(diskname, O_RDONLY|O_DIRECT|O_CLOEXEC)) >= 0) {
 		struct dk_gpt *vtoc;
 
 		if ((err = efi_alloc_and_read(fd, &vtoc)) >= 0) {
@@ -159,7 +159,7 @@ zpool_label_disk_check(char *path)
 	struct dk_gpt *vtoc;
 	int fd, err;
 
-	if ((fd = open(path, O_RDONLY|O_DIRECT)) < 0)
+	if ((fd = open(path, O_RDONLY|O_DIRECT|O_CLOEXEC)) < 0)
 		return (errno);
 
 	if ((err = efi_alloc_and_read(fd, &vtoc)) != 0) {
@@ -190,7 +190,7 @@ zpool_label_name(char *label_name, int label_size)
 	uint64_t id = 0;
 	int fd;
 
-	fd = open("/dev/urandom", O_RDONLY);
+	fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC);
 	if (fd >= 0) {
 		if (read(fd, &id, sizeof (id)) != sizeof (id))
 			id = 0;
@@ -241,7 +241,7 @@ zpool_label_disk(libzfs_handle_t *hdl, zpool_handle_t *zhp, const char *name)
 
 	(void) snprintf(path, sizeof (path), "%s/%s", DISK_ROOT, name);
 
-	if ((fd = open(path, O_RDWR|O_DIRECT|O_EXCL)) < 0) {
+	if ((fd = open(path, O_RDWR|O_DIRECT|O_EXCL|O_CLOEXEC)) < 0) {
 		/*
 		 * This shouldn't happen.  We've long since verified that this
 		 * is a valid device.

lib/libzfs/os/linux/libzfs_sendrecv_os.c

@@ -35,7 +35,7 @@
 void
 libzfs_set_pipe_max(int infd)
 {
-	FILE *procf = fopen("/proc/sys/fs/pipe-max-size", "r");
+	FILE *procf = fopen("/proc/sys/fs/pipe-max-size", "re");
 
 	if (procf != NULL) {
 		unsigned long max_psize;

lib/libzfs/os/linux/libzfs_util_os.c

@@ -143,7 +143,7 @@ libzfs_load_module_impl(const char *module)
 
 	start = gethrtime();
 	do {
-		fd = open(ZFS_DEV, O_RDWR);
+		fd = open(ZFS_DEV, O_RDWR | O_CLOEXEC);
 		if (fd >= 0) {
 			(void) close(fd);
 			return (0);
@@ -195,7 +195,7 @@ zfs_version_kernel(char *version, int len)
 	int fd;
 	int rlen;
 
-	if ((fd = open(ZFS_SYSFS_DIR "/version", O_RDONLY)) == -1)
+	if ((fd = open(ZFS_SYSFS_DIR "/version", O_RDONLY | O_CLOEXEC)) == -1)
 		return (-1);
 
 	if ((rlen = read(fd, version, len)) == -1) {