The devpts filesystem has a notion of a system or primary instance of devpts. To retain the notion of a primary system instance of devpts the code needs a way to allow userspace to mount the internally mounted instance of devpts when it is not currently mounted by userspace. The new helper mount_super_once allows that.
The function mount_super_once ignores vfsmounts that are in use but are not mounted in userspace. The set of ignored mounts includes internal mounts, lazily unmounted instances of mounts, and loopback/bind mounts show root directory is not a filesystems root directory. Furthermore mount_super_once only allows exporting a mount to userspace if it can account for every reference in s_active on the superblock. Ensuring races do not allow two simultaneous mount requestions export the same filesystem to userspace. Signed-off-by: "Eric W. Biederman" <[email protected]> --- fs/super.c | 34 ++++++++++++++++++++++++++++++++++ include/linux/fs.h | 2 ++ 2 files changed, 36 insertions(+) diff --git a/fs/super.c b/fs/super.c index 74914b1bae70..4a6552395fad 100644 --- a/fs/super.c +++ b/fs/super.c @@ -33,6 +33,7 @@ #include <linux/cleancache.h> #include <linux/fsnotify.h> #include <linux/lockdep.h> +#include "mount.h" #include "internal.h" @@ -1101,6 +1102,39 @@ struct dentry *mount_single(struct file_system_type *fs_type, } EXPORT_SYMBOL(mount_single); +struct dentry *mount_super_once(struct super_block *sb, int flags, void *data) +{ + /* Allow mounting the specified superblock by userspace if there + * are not any existing userspace mounts of it. + */ + struct mount *mnt; + int count = 0; + + /* Walk through the existing mounts of this superblock. Fail + * if any of those mounts are exported to userspace. Otherwise + * increment s_active if the superblock s_active count matches + * the number of mounts in the list. This ensures that multiple + * simultaneous calls to mount_super_once do not race and result + * in the filesystem exported to userspace multiple times. + */ + lock_mount_hash(); + list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) { + if (is_mounted(&mnt->mnt) && (mnt->mnt.mnt_root == sb->s_root)) { + unlock_mount_hash(); + return ERR_PTR(-EBUSY); + } + count++; + } + if (atomic_cmpxchg(&sb->s_active, count, count + 1) != count) { + unlock_mount_hash(); + return ERR_PTR(-EBUSY); + } + unlock_mount_hash(); + down_write(&sb->s_umount); + do_remount_sb(sb, flags, data, 0); + return dget(sb->s_root); +} + struct dentry * mount_fs(struct file_system_type *type, int flags, const char *name, void *data) { diff --git a/include/linux/fs.h b/include/linux/fs.h index aade033bed49..465c1155bf72 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2002,6 +2002,8 @@ extern struct dentry *mount_single(struct file_system_type *fs_type, extern struct dentry *mount_nodev(struct file_system_type *fs_type, int flags, void *data, int (*fill_super)(struct super_block *, void *, int)); +extern struct dentry *mount_super_once(struct super_block *sb, + int flags, void *data); extern struct dentry *mount_subtree(struct vfsmount *mnt, const char *path); void generic_shutdown_super(struct super_block *sb); void kill_block_super(struct super_block *sb); -- 2.8.1
