tree-wide: rework mount api support checks

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/cgroups/cgfsng.c

@@ -1799,7 +1799,7 @@ static int __cg_mount_direct(int type, struct hierarchy *h,
 		fstype = "cgroup";
 	}
 
-	if (new_mount_api()) {
+	if (can_use_mount_api()) {
 		fd_fs = fs_prepare(fstype, -EBADF, "", 0, 0);
 		if (fd_fs < 0)
 			return log_error_errno(-errno, errno, "Failed to prepare filesystem context for %s", fstype);
@@ -1945,7 +1945,7 @@ __cgfsng_ops static bool cgfsng_mount(struct cgroup_ops *ops,
 	 * relying on RESOLVE_BENEATH so we need to skip the leading "/" in the
 	 * DEFAULT_CGROUP_MOUNTPOINT define.
 	 */
-	if (new_mount_api()) {
+	if (can_use_mount_api()) {
 		fd_fs = fs_prepare("tmpfs", -EBADF, "", 0, 0);
 		if (fd_fs < 0)
 			return log_error_errno(-errno, errno, "Failed to create new filesystem context for tmpfs");

src/lxc/conf.c

@@ -844,7 +844,7 @@ static int lxc_setup_ttys(struct lxc_conf *conf)
 						       "Failed to unlink %d(%s)",
 						       rootfs->dfd_dev, tty_name);
 
-			if (new_mount_api()) {
+			if (can_use_mount_api()) {
 				ret = fd_bind_mount(tty->pty, "",
 						    PROTECT_OPATH_FILE,
 						    PROTECT_LOOKUP_BENEATH_XDEV,
@@ -881,7 +881,7 @@ static int lxc_setup_ttys(struct lxc_conf *conf)
 						       "Failed to create tty mount target %d(%s)",
 						       rootfs->dfd_dev, rootfs->buf);
 
-			if (new_mount_api()) {
+			if (can_use_mount_api()) {
 				ret = fd_bind_mount(tty->pty, "",
 						    PROTECT_OPATH_FILE,
 						    PROTECT_LOOKUP_BENEATH_XDEV,
@@ -1074,7 +1074,7 @@ static int mount_autodev(const char *name, const struct lxc_rootfs *rootfs,
 		goto reset_umask;
 	}
 
-	if (new_mount_api()) {
+	if (can_use_mount_api()) {
 		fd_fs = fs_prepare("tmpfs", -EBADF, "", 0, 0);
 		if (fd_fs < 0)
 			return log_error_errno(-errno, errno, "Failed to prepare filesystem context for tmpfs");
@@ -1216,7 +1216,7 @@ static int lxc_fill_autodev(struct lxc_rootfs *rootfs)
 		if (ret < 0)
 			return ret_errno(EIO);
 
-		if (new_mount_api()) {
+		if (can_use_mount_api()) {
 			ret = fd_bind_mount(rootfs->dfd_host, rootfs->buf,
 					    PROTECT_OPATH_FILE,
 					    PROTECT_LOOKUP_BENEATH_XDEV,
@@ -1716,7 +1716,7 @@ static int lxc_setup_dev_console(struct lxc_rootfs *rootfs,
 	if (ret < 0)
 		return log_error_errno(-errno, errno, "Failed to set mode \"0%o\" to \"%s\"", S_IXUSR | S_IXGRP, console->name);
 
-	if (new_mount_api()) {
+	if (can_use_mount_api()) {
 		ret = lxc_bind_mount_console(console, rootfs->dfd_dev, "console");
 	} else {
 		ret = strnprintf(rootfs->buf, sizeof(rootfs->buf), "%s/dev/console", rootfs_path);
@@ -1779,7 +1779,7 @@ static int lxc_setup_ttydir_console(struct lxc_rootfs *rootfs,
 		return log_error_errno(-errno, errno, "Failed to set mode \"0%o\" to \"%s\"", S_IXUSR | S_IXGRP, console->name);
 
 	/* bind mount console->name to '/dev/<ttydir>/console' */
-	if (new_mount_api()) {
+	if (can_use_mount_api()) {
 		ret = strnprintf(rootfs->buf, sizeof(rootfs->buf), "%s/console", ttydir);
 		if (ret < 0)
 			return ret;
@@ -1793,7 +1793,7 @@ static int lxc_setup_ttydir_console(struct lxc_rootfs *rootfs,
 	DEBUG("Mounted \"%s\" onto \"%s\"", console->name, lxcpath);
 
 	/* bind mount '/dev/<ttydir>/console'  to '/dev/console'  */
-	if (new_mount_api()) {
+	if (can_use_mount_api()) {
 		ret = fd_bind_mount(rootfs->dfd_dev, rootfs->buf,
 				    PROTECT_OPATH_FILE, PROTECT_LOOKUP_BENEATH_XDEV,
 				    rootfs->dfd_dev, "console",

src/lxc/mount_utils.c

@@ -438,3 +438,53 @@ unsigned long add_required_remount_flags(const char *s, const char *d,
        return flags;
 #endif
 }
+
+bool can_use_mount_api(void)
+{
+	static int supported = -1;
+
+	if (supported == -1) {
+		__do_close int fd = -EBADF;
+
+		fd = openat2(-EBADF, "", NULL, 0);
+		if (fd > 0 || errno == ENOSYS) {
+			supported = 0;
+			return false;
+		}
+
+		fd = fsmount(-EBADF, 0, 0);
+		if (fd > 0 || errno == ENOSYS) {
+			supported = 0;
+			return false;
+		}
+
+		fd = fsconfig(-EBADF, -EINVAL, NULL, NULL, 0);
+		if (fd > 0 || errno == ENOSYS) {
+			supported = 0;
+			return false;
+		}
+
+		fd = fsopen(NULL, 0);
+		if (fd > 0 || errno == ENOSYS) {
+			supported = 0;
+			return false;
+		}
+
+		fd = move_mount(-EBADF, NULL, -EBADF, NULL, 0);
+		if (fd > 0 || errno == ENOSYS) {
+			supported = 0;
+			return false;
+		}
+
+		fd = open_tree(-EBADF, NULL, 0);
+		if (fd > 0 || errno == ENOSYS) {
+			supported = 0;
+			return false;
+		}
+
+		supported = 1;
+		TRACE("Kernel supports mount api");
+	}
+
+	return supported == 1;
+}

src/lxc/mount_utils.h

@@ -185,28 +185,6 @@ __hidden extern int fd_bind_mount(int dfd_from, const char *path_from,
 				  __u64 o_flags_to, __u64 resolve_flags_to,
 				  unsigned int attr_flags, bool recursive);
 
-/*
- * We use openat2() as indicator whether or not the new mount api is supported.
- * First, because openat2() has been introduced after all syscalls from the new
- * mount api we currently use and second because our hardened mount logic
- * relies on openat2() to safely resolve paths.
- */
-static inline bool new_mount_api(void)
-{
-	__do_close int fd = -EBADF;
-	static int supported = -1;
-
-	if (supported == -1) {
-		fd = openat2(-EBADF, "", NULL, 0);
-		if (fd < 0 && errno != ENOSYS)
-			supported = 1;
-		else
-			supported = 0;
-	}
-
-	return supported == 1;
-}
-
 __hidden extern int calc_remount_flags_new(int dfd_from, const char *path_from,
 					   __u64 o_flags_from,
 					   __u64 resolve_flags_from,
@@ -223,4 +201,6 @@ __hidden extern unsigned long add_required_remount_flags(const char *s,
 							 const char *d,
 							 unsigned long flags);
 
+__hidden extern bool can_use_mount_api(void);
+
 #endif /* __LXC_MOUNT_UTILS_H */