cgroups: fix cgroup mounting

src/lxc/cgroups/cgfsng.c

@@ -45,6 +45,7 @@
 #include "mainloop.h"
 #include "memory_utils.h"
 #include "storage/storage.h"
+#include "syscall_wrappers.h"
 #include "utils.h"
 
 #ifndef HAVE_STRLCPY
@@ -1805,11 +1806,12 @@ static inline int cg_mount_cgroup_full(int type, struct hierarchy *h,
 }
 
 __cgfsng_ops static bool cgfsng_mount(struct cgroup_ops *ops,
-				      struct lxc_handler *handler,
-				      const char *root, int type)
+				      struct lxc_conf *conf, int type)
 {
 	__do_free char *cgroup_root = NULL;
 	bool has_cgns = false, wants_force_mount = false;
+	struct lxc_rootfs *rootfs = &conf->rootfs;
+	const char *root = rootfs->path ? rootfs->mount : "";
 	int ret;
 
 	if (!ops)
@@ -1818,7 +1820,7 @@ __cgfsng_ops static bool cgfsng_mount(struct cgroup_ops *ops,
 	if (!ops->hierarchies)
 		return true;
 
-	if (!handler || !handler->conf)
+	if (!conf)
 		return ret_set_errno(false, EINVAL);
 
 	if ((type & LXC_AUTO_CGROUP_MASK) == 0)
@@ -1830,7 +1832,7 @@ __cgfsng_ops static bool cgfsng_mount(struct cgroup_ops *ops,
 	}
 
 	if (!wants_force_mount) {
-		wants_force_mount = !lxc_wants_cap(CAP_SYS_ADMIN, handler->conf);
+		wants_force_mount = !lxc_wants_cap(CAP_SYS_ADMIN, conf);
 
 		/*
 		 * Most recent distro versions currently have init system that
@@ -1874,16 +1876,15 @@ __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.
 	 */
-	ret = safe_mount_beneath(root, NULL,
-				 DEFAULT_CGROUP_MOUNTPOINT_RELATIVE,
-				 "tmpfs",
-				 MS_NOSUID | MS_NODEV | MS_NOEXEC | MS_RELATIME,
-				 "size=10240k,mode=755");
+	ret = mount_at(rootfs->mntpt_fd, NULL, DEFAULT_CGROUP_MOUNTPOINT_RELATIVE,
+		       PROTECT_OPATH_DIRECTORY, PROTECT_LOOKUP_BENEATH_XDEV,
+		       "tmpfs", MS_NOSUID | MS_NODEV | MS_NOEXEC | MS_RELATIME,
+		       "size=10240k,mode=755");
 	if (ret < 0) {
 		if (errno != ENOSYS)
 			return log_error_errno(false, errno,
 					       "Failed to mount tmpfs on %s",
-					       DEFAULT_CGROUP_MOUNTPOINT);
+					       DEFAULT_CGROUP_MOUNTPOINT_RELATIVE);
 
 		ret = safe_mount(NULL, cgroup_root, "tmpfs",
 				 MS_NOSUID | MS_NODEV | MS_NOEXEC | MS_RELATIME,

src/lxc/cgroups/cgroup.h

@@ -172,8 +172,7 @@ struct cgroup_ops {
 	bool (*chown)(struct cgroup_ops *ops, struct lxc_conf *conf);
 	bool (*attach)(struct cgroup_ops *ops, const struct lxc_conf *conf,
 		       const char *name, const char *lxcpath, pid_t pid);
-	bool (*mount)(struct cgroup_ops *ops, struct lxc_handler *handler,
-		      const char *root, int type);
+	bool (*mount)(struct cgroup_ops *ops, struct lxc_conf *conf, int type);
 	bool (*devices_activate)(struct cgroup_ops *ops,
 				 struct lxc_handler *handler);
 	bool (*monitor_delegate_controllers)(struct cgroup_ops *ops);

src/lxc/conf.c

@@ -734,10 +734,7 @@ static int lxc_mount_auto_mounts(struct lxc_conf *conf, int flags, struct lxc_ha
 		if (flags & LXC_AUTO_CGROUP_FORCE)
 			cg_flags |= LXC_AUTO_CGROUP_FORCE;
 
-		if (!handler->cgroup_ops->mount(handler->cgroup_ops,
-						handler,
-						rootfs->path ? rootfs->mount : "",
-						cg_flags))
+		if (!handler->cgroup_ops->mount(handler->cgroup_ops, conf, cg_flags))
 			return log_error_errno(-1, errno, "Failed to mount \"/sys/fs/cgroup\"");
 	}
 

src/lxc/utils.c

@@ -1208,6 +1208,58 @@ int safe_mount(const char *src, const char *dest, const char *fstype,
 	return 0;
 }
 
+int mount_at(int dfd,
+	     const char *src_under_dfd,
+	     const char *dst_under_dfd,
+	     __u64 o_flags,
+	     __u64 resolve_flags,
+	     const char *fstype,
+	     unsigned int mnt_flags,
+	     const void *data)
+{
+	__do_close int source_fd = -EBADF, target_fd = -EBADF;
+	struct lxc_open_how how = {
+		.flags		= o_flags,
+		.resolve	= resolve_flags,
+	};
+	int ret;
+	char src_buf[LXC_PROC_PID_FD_LEN], dst_buf[LXC_PROC_PID_FD_LEN];
+
+	if (dfd < 0)
+		return ret_errno(EINVAL);
+
+	if (!is_empty_string(src_buf) && *src_buf == '/')
+		return log_error_errno(-EINVAL, EINVAL, "Absolute path specified");
+
+	if (is_empty_string(dst_under_dfd))
+		return log_error_errno(-EINVAL, EINVAL, "No target path specified");
+
+	if (!is_empty_string(src_under_dfd)) {
+		source_fd = openat2(dfd, src_under_dfd, &how, sizeof(how));
+		if (source_fd < 0)
+			return -errno;
+
+		ret = snprintf(src_buf, sizeof(src_buf), "/proc/self/fd/%d", source_fd);
+		if (ret < 0 || ret >= sizeof(src_buf))
+			return -EIO;
+	}
+
+	target_fd = openat2(dfd, dst_under_dfd, &how, sizeof(how));
+	if (target_fd < 0)
+		return log_error_errno(-errno, errno, "Failed to open %d(%s)", dfd, dst_under_dfd);
+
+	ret = snprintf(dst_buf, sizeof(dst_buf), "/proc/self/fd/%d", target_fd);
+	if (ret < 0 || ret >= sizeof(dst_buf))
+		return -EIO;
+
+	if (!is_empty_string(src_buf))
+		ret = mount(src_buf, dst_buf, fstype, mnt_flags, data);
+	else
+		ret = mount(NULL, dst_buf, fstype, mnt_flags, data);
+
+	return ret;
+}
+
 int open_devnull(void)
 {
 	int fd = open("/dev/null", O_RDWR);

src/lxc/utils.h

@@ -243,5 +243,9 @@ __hidden extern int safe_mount_beneath(const char *beneath, const char *src, con
 				       const char *fstype, unsigned int flags, const void *data);
 __hidden extern int safe_mount_beneath_at(int beneat_fd, const char *src, const char *dst,
 					  const char *fstype, unsigned int flags, const void *data);
+__hidden extern int mount_at(int dfd, const char *src_under_dfd,
+			     const char *dst_under_dfd, __u64 o_flags,
+			     __u64 resolve_flags, const char *fstype,
+			     unsigned int mnt_flags, const void *data);
 
 #endif /* __LXC_UTILS_H */