[PATCH net-next v2 3/3] tun: Add ioctl() SIOCGSKNS cmd to allow obtaining net ns of tun device

[Kirill Tkhai <ktkhai@virtuozzo.com>]

This patch adds possibility to get tun device's net namespace fd
in the same way we allow to do that for sockets.

Socket ioctl numbers do not intersect with tun-specific, and there
is already SIOCSIFHWADDR used in tun code. So, SIOCGSKNS number
is choosen instead of custom-made for this functionality.

Note, that open_related_ns() uses plain get_net_ns() and it's safe
(net can't be already dead at this moment):

  tun socket is allocated via sk_alloc() with zero last arg (kern = 0).
  So, each alive socket increments net::count, and the socket is definitely
  alive during ioctl syscall.

Also, common variable net is introduced, so small cleanup in TUNSETIFF
is made.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 drivers/net/tun.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 81e6cc951e7f..8e9a0ac644d2 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -78,6 +78,7 @@
 #include <linux/mutex.h>
 
 #include <linux/uaccess.h>
+#include <linux/proc_fs.h>
 
 /* Uncomment to enable debugging */
 /* #define TUN_DEBUG 1 */
@@ -2793,6 +2794,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 	struct tun_struct *tun;
 	void __user* argp = (void __user*)arg;
 	struct ifreq ifr;
+	struct net *net;
 	kuid_t owner;
 	kgid_t group;
 	int sndbuf;
@@ -2801,7 +2803,8 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 	int le;
 	int ret;
 
-	if (cmd == TUNSETIFF || cmd == TUNSETQUEUE || _IOC_TYPE(cmd) == SOCK_IOC_TYPE) {
+	if (cmd == TUNSETIFF || cmd == TUNSETQUEUE ||
+	    (_IOC_TYPE(cmd) == SOCK_IOC_TYPE && cmd != SIOCGSKNS)) {
 		if (copy_from_user(&ifr, argp, ifreq_len))
 			return -EFAULT;
 	} else {
@@ -2821,6 +2824,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 	rtnl_lock();
 
 	tun = tun_get(tfile);
+	net = sock_net(&tfile->sk);
 	if (cmd == TUNSETIFF) {
 		ret = -EEXIST;
 		if (tun)
@@ -2828,7 +2832,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 
 		ifr.ifr_name[IFNAMSIZ-1] = '\0';
 
-		ret = tun_set_iff(sock_net(&tfile->sk), file, &ifr);
+		ret = tun_set_iff(net, file, &ifr);
 
 		if (ret)
 			goto unlock;
@@ -2850,6 +2854,14 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 		tfile->ifindex = ifindex;
 		goto unlock;
 	}
+	if (cmd == SIOCGSKNS) {
+		ret = -EPERM;
+		if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+			goto unlock;
+
+		ret = open_related_ns(&net->ns, get_net_ns);
+		goto unlock;
+	}
 
 	ret = -EBADFD;
 	if (!tun)