[PATCH] fs: add an ioctl to get an owning userns for a superblock

[Andrei Vagin <avagin@xxxxxxxxxx>]

The introduced ioctl returns a file descriptor that refers to a owning
user namespace for a superblock which is associated with a target file
descriptor.

EPERM is returned if the current process doesn't have CAP_SYS_ADMIN in
the returned user namespace.

This information is required to dump and restore mount namespaces. We
need to know to which user namespace a superblock is belonged to.

We already have the SIOCGSKNS ioctl for sockets to get a network
namespace, so it looks reasonable to use the same interface for
superblocks too.

This functionality can be useful for users in order to understand
a running system.

Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>
---
 fs/ioctl.c              | 23 +++++++++++++++++++++++
 include/uapi/linux/fs.h |  2 ++
 2 files changed, 25 insertions(+)

diff --git a/fs/ioctl.c b/fs/ioctl.c
index 569db68..22bbf37 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -16,6 +16,8 @@
 #include <linux/buffer_head.h>
 #include <linux/falloc.h>
 #include <linux/sched/signal.h>
+#include <linux/proc_fs.h>
+#include <linux/user_namespace.h>
 
 #include "internal.h"
 
@@ -614,6 +616,25 @@ static int ioctl_file_dedupe_range(struct file *file, void __user *arg)
 	return ret;
 }
 
+static struct ns_common *get_sb_userns(struct ns_common *ns_common)
+{
+	struct user_namespace *ns;
+
+	ns = container_of(ns_common, struct user_namespace, ns);
+
+	return &get_user_ns(ns)->ns;
+}
+
+static int ioctl_fs_sb_userns(struct file *filp)
+{
+	struct super_block *sb = file_inode(filp)->i_sb;
+
+	if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
+		return -EPERM;
+
+	return open_related_ns(&sb->s_user_ns->ns, get_sb_userns);
+}
+
 /*
  * When you add any new common ioctls to the switches above and below
  * please update compat_sys_ioctl() too.
@@ -677,6 +698,8 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
 
 	case FIDEDUPERANGE:
 		return ioctl_file_dedupe_range(filp, argp);
+	case FS_IOC_SB_USERNS:
+		return ioctl_fs_sb_userns(filp);
 
 	default:
 		if (S_ISREG(inode->i_mode))
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 33423aa..26ef2d5 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -246,6 +246,8 @@ struct fsxattr {
 #define FICLONE		_IOW(0x94, 9, int)
 #define FICLONERANGE	_IOW(0x94, 13, struct file_clone_range)
 #define FIDEDUPERANGE	_IOWR(0x94, 54, struct file_dedupe_range)
+/* Get a file descriptor to an owning userns for a superblock */
+#define FS_IOC_SB_USERNS		_IOR('X', 55, int)
 
 #define	FS_IOC_GETFLAGS			_IOR('f', 1, long)
 #define	FS_IOC_SETFLAGS			_IOW('f', 2, long)
-- 
2.9.3