proc: Convert proc_mount to use mount_ns.

Move the call of get_pid_ns, the call of proc_parse_options, and
the setting of s_iflags into proc_fill_super so that mount_ns
can be used.

Convert proc_mount to call mount_ns and remove the now unnecessary
code.

Acked-by: Seth Forshee <seth.forshee@canonical.com>
Reviewed-by: Djalal Harouni <tixxdz@gmail.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/proc/inode.c

@@ -457,12 +457,17 @@ struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
 	return inode;
 }
 
-int proc_fill_super(struct super_block *s)
+int proc_fill_super(struct super_block *s, void *data, int silent)
 {
+	struct pid_namespace *ns = get_pid_ns(s->s_fs_info);
 	struct inode *root_inode;
 	int ret;
 
-	s->s_iflags |= SB_I_USERNS_VISIBLE;
+	if (!proc_parse_options(data, ns))
+		return -EINVAL;
+
+	/* User space would break if executables appear on proc */
+	s->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC;
 	s->s_flags |= MS_NODIRATIME | MS_NOSUID | MS_NOEXEC;
 	s->s_blocksize = 1024;
 	s->s_blocksize_bits = 10;

fs/proc/internal.h

@@ -212,7 +212,7 @@ extern const struct inode_operations proc_pid_link_inode_operations;
 
 extern void proc_init_inodecache(void);
 extern struct inode *proc_get_inode(struct super_block *, struct proc_dir_entry *);
-extern int proc_fill_super(struct super_block *);
+extern int proc_fill_super(struct super_block *, void *data, int flags);
 extern void proc_entry_rundown(struct proc_dir_entry *);
 
 /*
@@ -268,6 +268,7 @@ static inline void proc_tty_init(void) {}
  * root.c
  */
 extern struct proc_dir_entry proc_root;
+extern int proc_parse_options(char *options, struct pid_namespace *pid);
 
 extern void proc_self_init(void);
 extern int proc_remount(struct super_block *, int *, char *);

fs/proc/root.c

@@ -23,21 +23,6 @@
 
 #include "internal.h"
 
-static int proc_test_super(struct super_block *sb, void *data)
-{
-	return sb->s_fs_info == data;
-}
-
-static int proc_set_super(struct super_block *sb, void *data)
-{
-	int err = set_anon_super(sb, NULL);
-	if (!err) {
-		struct pid_namespace *ns = (struct pid_namespace *)data;
-		sb->s_fs_info = get_pid_ns(ns);
-	}
-	return err;
-}
-
 enum {
 	Opt_gid, Opt_hidepid, Opt_err,
 };
@@ -48,7 +33,7 @@ static const match_table_t tokens = {
 	{Opt_err, NULL},
 };
 
-static int proc_parse_options(char *options, struct pid_namespace *pid)
+int proc_parse_options(char *options, struct pid_namespace *pid)
 {
 	char *p;
 	substring_t args[MAX_OPT_ARGS];
@@ -100,45 +85,16 @@ int proc_remount(struct super_block *sb, int *flags, char *data)
 static struct dentry *proc_mount(struct file_system_type *fs_type,
 	int flags, const char *dev_name, void *data)
 {
-	int err;
-	struct super_block *sb;
 	struct pid_namespace *ns;
-	char *options;
 
 	if (flags & MS_KERNMOUNT) {
-		ns = (struct pid_namespace *)data;
-		options = NULL;
+		ns = data;
+		data = NULL;
 	} else {
 		ns = task_active_pid_ns(current);
-		options = data;
-
-		/* Does the mounter have privilege over the pid namespace? */
-		if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
-			return ERR_PTR(-EPERM);
-	}
-
-	sb = sget(fs_type, proc_test_super, proc_set_super, flags, ns);
-	if (IS_ERR(sb))
-		return ERR_CAST(sb);
-
-	if (!proc_parse_options(options, ns)) {
-		deactivate_locked_super(sb);
-		return ERR_PTR(-EINVAL);
-	}
-
-	if (!sb->s_root) {
-		err = proc_fill_super(sb);
-		if (err) {
-			deactivate_locked_super(sb);
-			return ERR_PTR(err);
-		}
-
-		sb->s_flags |= MS_ACTIVE;
-		/* User space would break if executables appear on proc */
-		sb->s_iflags |= SB_I_NOEXEC;
 	}
 
-	return dget(sb->s_root);
+	return mount_ns(fs_type, flags, data, ns, ns->user_ns, proc_fill_super);
 }
 
 static void proc_kill_sb(struct super_block *sb)