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Subject[PATCH] fs: add an ioctl to get an owning userns for a superblock
Date
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@zeniv.linux.org.uk>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
---
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
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