take the targets of /proc/*/ns/* symlinks to separate fs

New pseudo-filesystem: nsfs.  Targets of /proc/*/ns/* live there now.
It's not mountable (not even registered, so it's not in /proc/filesystems,
etc.).  Files on it *are* bindable - we explicitly permit that in do_loopback().

This stuff lives in fs/nsfs.c now; proc_ns_fget() moved there as well.
get_proc_ns() is a macro now (it's simply returning ->i_private; would
have been an inline, if not for header ordering headache).
proc_ns_inode() is an ex-parrot.  The interface used in procfs is
ns_get_path(path, task, ops) and ns_get_name(buf, size, task, ops).

Dentries and inodes are never hashed; a non-counting reference to dentry
is stashed in ns_common (removed by ->d_prune()) and reused by ns_get_path()
if present.  See ns_get_path()/ns_prune_dentry/nsfs_evict() for details
of that mechanism.

As the result, proc_ns_follow_link() has stopped poking in nd->path.mnt;
it does nd_jump_link() on a consistent <vfsmount,dentry> pair it gets
from ns_get_path().

Signed-off-by: Al Viro <[email protected]>

fs/Makefile

@@ -11,7 +11,7 @@ obj-y :=	open.o read_write.o file_table.o super.o \
 		attr.o bad_inode.o file.o filesystems.o namespace.o \
 		seq_file.o xattr.o libfs.o fs-writeback.o \
 		pnode.o splice.o sync.o utimes.o \
-		stack.o fs_struct.o statfs.o fs_pin.o
+		stack.o fs_struct.o statfs.o fs_pin.o nsfs.o
 
 ifeq ($(CONFIG_BLOCK),y)
 obj-y +=	buffer.o block_dev.o direct-io.o mpage.o

fs/internal.h

@@ -147,3 +147,8 @@ extern const struct file_operations pipefifo_fops;
  */
 extern void sb_pin_kill(struct super_block *sb);
 extern void mnt_pin_kill(struct mount *m);
+
+/*
+ * fs/nsfs.c
+ */
+extern struct dentry_operations ns_dentry_operations;

fs/namespace.c

@@ -1569,8 +1569,8 @@ SYSCALL_DEFINE1(oldumount, char __user *, name)
 static bool is_mnt_ns_file(struct dentry *dentry)
 {
 	/* Is this a proxy for a mount namespace? */
-	struct inode *inode = dentry->d_inode;
-	return proc_ns_inode(inode) && dentry->d_fsdata == &mntns_operations;
+	return dentry->d_op == &ns_dentry_operations &&
+	       dentry->d_fsdata == &mntns_operations;
 }
 
 struct mnt_namespace *to_mnt_ns(struct ns_common *ns)
@@ -2016,7 +2016,10 @@ static int do_loopback(struct path *path, const char *old_name,
 	if (IS_MNT_UNBINDABLE(old))
 		goto out2;
 
-	if (!check_mnt(parent) || !check_mnt(old))
+	if (!check_mnt(parent))
+		goto out2;
+
+	if (!check_mnt(old) && old_path.dentry->d_op != &ns_dentry_operations)
 		goto out2;
 
 	if (!recurse && has_locked_children(old, old_path.dentry))

fs/nsfs.c

@@ -0,0 +1,161 @@
+#include <linux/mount.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/proc_ns.h>
+#include <linux/magic.h>
+#include <linux/ktime.h>
+
+static struct vfsmount *nsfs_mnt;
+
+static const struct file_operations ns_file_operations = {
+	.llseek		= no_llseek,
+};
+
+static char *ns_dname(struct dentry *dentry, char *buffer, int buflen)
+{
+	struct inode *inode = dentry->d_inode;
+	const struct proc_ns_operations *ns_ops = dentry->d_fsdata;
+
+	return dynamic_dname(dentry, buffer, buflen, "%s:[%lu]",
+		ns_ops->name, inode->i_ino);
+}
+
+static void ns_prune_dentry(struct dentry *dentry)
+{
+	struct inode *inode = dentry->d_inode;
+	if (inode) {
+		struct ns_common *ns = inode->i_private;
+		atomic_long_set(&ns->stashed, 0);
+	}
+}
+
+const struct dentry_operations ns_dentry_operations =
+{
+	.d_prune	= ns_prune_dentry,
+	.d_delete	= always_delete_dentry,
+	.d_dname	= ns_dname,
+};
+
+static void nsfs_evict(struct inode *inode)
+{
+	struct ns_common *ns = inode->i_private;
+	clear_inode(inode);
+	ns->ops->put(ns);
+}
+
+void *ns_get_path(struct path *path, struct task_struct *task,
+			const struct proc_ns_operations *ns_ops)
+{
+	struct vfsmount *mnt = mntget(nsfs_mnt);
+	struct qstr qname = { .name = "", };
+	struct dentry *dentry;
+	struct inode *inode;
+	struct ns_common *ns;
+	unsigned long d;
+
+again:
+	ns = ns_ops->get(task);
+	if (!ns) {
+		mntput(mnt);
+		return ERR_PTR(-ENOENT);
+	}
+	rcu_read_lock();
+	d = atomic_long_read(&ns->stashed);
+	if (!d)
+		goto slow;
+	dentry = (struct dentry *)d;
+	if (!lockref_get_not_dead(&dentry->d_lockref))
+		goto slow;
+	rcu_read_unlock();
+	ns_ops->put(ns);
+got_it:
+	path->mnt = mnt;
+	path->dentry = dentry;
+	return NULL;
+slow:
+	rcu_read_unlock();
+	inode = new_inode_pseudo(mnt->mnt_sb);
+	if (!inode) {
+		ns_ops->put(ns);
+		mntput(mnt);
+		return ERR_PTR(-ENOMEM);
+	}
+	inode->i_ino = ns->inum;
+	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
+	inode->i_flags |= S_IMMUTABLE;
+	inode->i_mode = S_IFREG | S_IRUGO;
+	inode->i_fop = &ns_file_operations;
+	inode->i_private = ns;
+
+	dentry = d_alloc_pseudo(mnt->mnt_sb, &qname);
+	if (!dentry) {
+		iput(inode);
+		mntput(mnt);
+		return ERR_PTR(-ENOMEM);
+	}
+	d_instantiate(dentry, inode);
+	dentry->d_fsdata = (void *)ns_ops;
+	d = atomic_long_cmpxchg(&ns->stashed, 0, (unsigned long)dentry);
+	if (d) {
+		d_delete(dentry);	/* make sure ->d_prune() does nothing */
+		dput(dentry);
+		cpu_relax();
+		goto again;
+	}
+	goto got_it;
+}
+
+int ns_get_name(char *buf, size_t size, struct task_struct *task,
+			const struct proc_ns_operations *ns_ops)
+{
+	struct ns_common *ns;
+	int res = -ENOENT;
+	ns = ns_ops->get(task);
+	if (ns) {
+		res = snprintf(buf, size, "%s:[%u]", ns_ops->name, ns->inum);
+		ns_ops->put(ns);
+	}
+	return res;
+}
+
+struct file *proc_ns_fget(int fd)
+{
+	struct file *file;
+
+	file = fget(fd);
+	if (!file)
+		return ERR_PTR(-EBADF);
+
+	if (file->f_op != &ns_file_operations)
+		goto out_invalid;
+
+	return file;
+
+out_invalid:
+	fput(file);
+	return ERR_PTR(-EINVAL);
+}
+
+static const struct super_operations nsfs_ops = {
+	.statfs = simple_statfs,
+	.evict_inode = nsfs_evict,
+};
+static struct dentry *nsfs_mount(struct file_system_type *fs_type,
+			int flags, const char *dev_name, void *data)
+{
+	return mount_pseudo(fs_type, "nsfs:", &nsfs_ops,
+			&ns_dentry_operations, NSFS_MAGIC);
+}
+static struct file_system_type nsfs = {
+	.name = "nsfs",
+	.mount = nsfs_mount,
+	.kill_sb = kill_anon_super,
+};
+
+void __init nsfs_init(void)
+{
+	nsfs_mnt = kern_mount(&nsfs);
+	if (IS_ERR(nsfs_mnt))
+		panic("can't set nsfs up\n");
+	nsfs_mnt->mnt_sb->s_flags &= ~MS_NOUSER;
+}

fs/proc/inode.c

@@ -32,7 +32,6 @@ static void proc_evict_inode(struct inode *inode)
 {
 	struct proc_dir_entry *de;
 	struct ctl_table_header *head;
-	struct ns_common *ns;
 
 	truncate_inode_pages_final(&inode->i_data);
 	clear_inode(inode);
@@ -49,10 +48,6 @@ static void proc_evict_inode(struct inode *inode)
 		RCU_INIT_POINTER(PROC_I(inode)->sysctl, NULL);
 		sysctl_head_put(head);
 	}
-	/* Release any associated namespace */
-	ns = PROC_I(inode)->ns.ns;
-	if (ns && ns->ops)
-		ns->ops->put(ns);
 }
 
 static struct kmem_cache * proc_inode_cachep;