Patchwork [1/3] net: introduce SO_BINDTOIF sockopt

login
register
mail settings
Submitter David Herrmann
Date Jan. 10, 2019, 2:25 p.m.
Message ID <20190110142529.14315-1-dh.herrmann@gmail.com>
Download mbox | patch
Permalink /patch/696791/
State New
Headers show

Comments

David Herrmann - Jan. 10, 2019, 2:25 p.m.
This introduces a new generic SOL_SOCKET-level socket option called
SO_BINDTOIF. It behaves similar to SO_BINDTODEVICE, but takes a network
interface index as argument, rather than the network interface name.

User-space often refers to network-interfaces via their index, but has
to temporarily resolve it to a name for a call into SO_BINDTODEVICE.
This might pose problems when the network-device is renamed
asynchronously by other parts of the system. When this happens, the
SO_BINDTODEVICE might either fail, or worse, it might bind to the wrong
device.

In most cases user-space only ever operates on devices which they
either manage themselves, or otherwise have a guarantee that the device
name will not change (e.g., devices that are UP cannot be renamed).
However, particularly in libraries this guarantee is non-obvious and it
would be nice if that race-condition would simply not exist. It would
make it easier for those libraries to operate even in situations where
the device-name might change under the hood.

A real use-case that we recently hit is trying to start the network
stack early in the initrd but make it survive into the real system.
Existing distributions rename network-interfaces during the transition
from initrd into the real system. This, obviously, cannot affect
devices that are up and running (unless you also consider moving them
between network-namespaces). However, the network manager now has to
make sure its management engine for dormant devices will not run in
parallel to these renames. Particularly, when you offload operations
like DHCP into separate processes, these might setup their sockets
early, and thus have to resolve the device-name possibly running into
this race-condition.

By avoiding a call to resolve the device-name, we no longer depend on
the name and can run network setup of dormant devices in parallel to
the transition off the initrd. The SO_BINDTOIF ioctl plugs this race.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
---
 arch/alpha/include/uapi/asm/socket.h  |  2 ++
 arch/ia64/include/uapi/asm/socket.h   |  2 ++
 arch/mips/include/uapi/asm/socket.h   |  2 ++
 arch/parisc/include/uapi/asm/socket.h |  2 ++
 arch/s390/include/uapi/asm/socket.h   |  2 ++
 arch/sparc/include/uapi/asm/socket.h  |  2 ++
 arch/xtensa/include/uapi/asm/socket.h |  2 ++
 include/uapi/asm-generic/socket.h     |  2 ++
 net/core/sock.c                       | 46 +++++++++++++++++++++------
 9 files changed, 52 insertions(+), 10 deletions(-)
Tom Gundersen - Jan. 10, 2019, 2:39 p.m.
On Thu, Jan 10, 2019 at 3:25 PM David Herrmann <dh.herrmann@gmail.com> wrote:
>
> This introduces a new generic SOL_SOCKET-level socket option called
> SO_BINDTOIF. It behaves similar to SO_BINDTODEVICE, but takes a network
> interface index as argument, rather than the network interface name.
>
> User-space often refers to network-interfaces via their index, but has
> to temporarily resolve it to a name for a call into SO_BINDTODEVICE.
> This might pose problems when the network-device is renamed
> asynchronously by other parts of the system. When this happens, the
> SO_BINDTODEVICE might either fail, or worse, it might bind to the wrong
> device.
>
> In most cases user-space only ever operates on devices which they
> either manage themselves, or otherwise have a guarantee that the device
> name will not change (e.g., devices that are UP cannot be renamed).
> However, particularly in libraries this guarantee is non-obvious and it
> would be nice if that race-condition would simply not exist. It would
> make it easier for those libraries to operate even in situations where
> the device-name might change under the hood.
>
> A real use-case that we recently hit is trying to start the network
> stack early in the initrd but make it survive into the real system.
> Existing distributions rename network-interfaces during the transition
> from initrd into the real system. This, obviously, cannot affect
> devices that are up and running (unless you also consider moving them
> between network-namespaces). However, the network manager now has to
> make sure its management engine for dormant devices will not run in
> parallel to these renames. Particularly, when you offload operations
> like DHCP into separate processes, these might setup their sockets
> early, and thus have to resolve the device-name possibly running into
> this race-condition.
>
> By avoiding a call to resolve the device-name, we no longer depend on
> the name and can run network setup of dormant devices in parallel to
> the transition off the initrd. The SO_BINDTOIF ioctl plugs this race.
>
> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Tom Gundersen <teg@jklm.no>

> ---
>  arch/alpha/include/uapi/asm/socket.h  |  2 ++
>  arch/ia64/include/uapi/asm/socket.h   |  2 ++
>  arch/mips/include/uapi/asm/socket.h   |  2 ++
>  arch/parisc/include/uapi/asm/socket.h |  2 ++
>  arch/s390/include/uapi/asm/socket.h   |  2 ++
>  arch/sparc/include/uapi/asm/socket.h  |  2 ++
>  arch/xtensa/include/uapi/asm/socket.h |  2 ++
>  include/uapi/asm-generic/socket.h     |  2 ++
>  net/core/sock.c                       | 46 +++++++++++++++++++++------
>  9 files changed, 52 insertions(+), 10 deletions(-)
>
> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> index 065fb372e355..6e346e51eec7 100644
> --- a/arch/alpha/include/uapi/asm/socket.h
> +++ b/arch/alpha/include/uapi/asm/socket.h
> @@ -115,4 +115,6 @@
>  #define SO_TXTIME              61
>  #define SCM_TXTIME             SO_TXTIME
>
> +#define SO_BINDTOIF            62
> +
>  #endif /* _UAPI_ASM_SOCKET_H */
> diff --git a/arch/ia64/include/uapi/asm/socket.h b/arch/ia64/include/uapi/asm/socket.h
> index c872c4e6bafb..ece83ba17b9d 100644
> --- a/arch/ia64/include/uapi/asm/socket.h
> +++ b/arch/ia64/include/uapi/asm/socket.h
> @@ -117,4 +117,6 @@
>  #define SO_TXTIME              61
>  #define SCM_TXTIME             SO_TXTIME
>
> +#define SO_BINDTOIF            62
> +
>  #endif /* _ASM_IA64_SOCKET_H */
> diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> index 71370fb3ceef..27f7f761ace5 100644
> --- a/arch/mips/include/uapi/asm/socket.h
> +++ b/arch/mips/include/uapi/asm/socket.h
> @@ -126,4 +126,6 @@
>  #define SO_TXTIME              61
>  #define SCM_TXTIME             SO_TXTIME
>
> +#define SO_BINDTOIF            62
> +
>  #endif /* _UAPI_ASM_SOCKET_H */
> diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> index 061b9cf2a779..efd3917f23e1 100644
> --- a/arch/parisc/include/uapi/asm/socket.h
> +++ b/arch/parisc/include/uapi/asm/socket.h
> @@ -107,4 +107,6 @@
>  #define SO_TXTIME              0x4036
>  #define SCM_TXTIME             SO_TXTIME
>
> +#define SO_BINDTOIF            0x4037
> +
>  #endif /* _UAPI_ASM_SOCKET_H */
> diff --git a/arch/s390/include/uapi/asm/socket.h b/arch/s390/include/uapi/asm/socket.h
> index 39d901476ee5..c8ba542e69e6 100644
> --- a/arch/s390/include/uapi/asm/socket.h
> +++ b/arch/s390/include/uapi/asm/socket.h
> @@ -114,4 +114,6 @@
>  #define SO_TXTIME              61
>  #define SCM_TXTIME             SO_TXTIME
>
> +#define SO_BINDTOIF            62
> +
>  #endif /* _ASM_SOCKET_H */
> diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> index 7ea35e5601b6..50006bde7dc0 100644
> --- a/arch/sparc/include/uapi/asm/socket.h
> +++ b/arch/sparc/include/uapi/asm/socket.h
> @@ -104,6 +104,8 @@
>  #define SO_TXTIME              0x003f
>  #define SCM_TXTIME             SO_TXTIME
>
> +#define SO_BINDTOIF            0x0040
> +
>  /* Security levels - as per NRL IPv6 - don't actually do anything */
>  #define SO_SECURITY_AUTHENTICATION             0x5001
>  #define SO_SECURITY_ENCRYPTION_TRANSPORT       0x5002
> diff --git a/arch/xtensa/include/uapi/asm/socket.h b/arch/xtensa/include/uapi/asm/socket.h
> index 1de07a7f7680..a36241ffbd86 100644
> --- a/arch/xtensa/include/uapi/asm/socket.h
> +++ b/arch/xtensa/include/uapi/asm/socket.h
> @@ -119,4 +119,6 @@
>  #define SO_TXTIME              61
>  #define SCM_TXTIME             SO_TXTIME
>
> +#define SO_BINDTOIF            62
> +
>  #endif /* _XTENSA_SOCKET_H */
> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> index a12692e5f7a8..31fb8414ea4c 100644
> --- a/include/uapi/asm-generic/socket.h
> +++ b/include/uapi/asm-generic/socket.h
> @@ -110,4 +110,6 @@
>  #define SO_TXTIME              61
>  #define SCM_TXTIME             SO_TXTIME
>
> +#define SO_BINDTOIF            62
> +
>  #endif /* __ASM_GENERIC_SOCKET_H */
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 6aa2e7e0b4fb..df8f83bc22b3 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -520,20 +520,43 @@ struct dst_entry *sk_dst_check(struct sock *sk, u32 cookie)
>  }
>  EXPORT_SYMBOL(sk_dst_check);
>
> -static int sock_setbindtodevice(struct sock *sk, char __user *optval,
> -                               int optlen)
> +static int sock_setbindtodevice_locked(struct sock *sk, int ifindex)
>  {
>         int ret = -ENOPROTOOPT;
>  #ifdef CONFIG_NETDEVICES
>         struct net *net = sock_net(sk);
> -       char devname[IFNAMSIZ];
> -       int index;
>
>         /* Sorry... */
>         ret = -EPERM;
>         if (!ns_capable(net->user_ns, CAP_NET_RAW))
>                 goto out;
>
> +       ret = -EINVAL;
> +       if (ifindex < 0)
> +               goto out;
> +
> +       sk->sk_bound_dev_if = ifindex;
> +       if (sk->sk_prot->rehash)
> +               sk->sk_prot->rehash(sk);
> +       sk_dst_reset(sk);
> +
> +       ret = 0;
> +
> +out:
> +#endif
> +
> +       return ret;
> +}
> +
> +static int sock_setbindtodevice(struct sock *sk, char __user *optval,
> +                               int optlen)
> +{
> +       int ret = -ENOPROTOOPT;
> +#ifdef CONFIG_NETDEVICES
> +       struct net *net = sock_net(sk);
> +       char devname[IFNAMSIZ];
> +       int index;
> +
>         ret = -EINVAL;
>         if (optlen < 0)
>                 goto out;
> @@ -566,14 +589,9 @@ static int sock_setbindtodevice(struct sock *sk, char __user *optval,
>         }
>
>         lock_sock(sk);
> -       sk->sk_bound_dev_if = index;
> -       if (sk->sk_prot->rehash)
> -               sk->sk_prot->rehash(sk);
> -       sk_dst_reset(sk);
> +       ret = sock_setbindtodevice_locked(sk, index);
>         release_sock(sk);
>
> -       ret = 0;
> -
>  out:
>  #endif
>
> @@ -1055,6 +1073,10 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
>                 }
>                 break;
>
> +       case SO_BINDTOIF:
> +               ret = sock_setbindtodevice_locked(sk, val);
> +               break;
> +
>         default:
>                 ret = -ENOPROTOOPT;
>                 break;
> @@ -1399,6 +1421,10 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
>                                   SOF_TXTIME_REPORT_ERRORS : 0;
>                 break;
>
> +       case SO_BINDTOIF:
> +               v.val = sk->sk_bound_dev_if;
> +               break;
> +
>         default:
>                 /* We implement the SO_SNDLOWAT etc to not be settable
>                  * (1003.1g 7).
> --
> 2.20.1
>
David Ahern - Jan. 10, 2019, 4:38 p.m.
On 1/10/19 7:25 AM, David Herrmann wrote:
> This introduces a new generic SOL_SOCKET-level socket option called
> SO_BINDTOIF. It behaves similar to SO_BINDTODEVICE, but takes a network
> interface index as argument, rather than the network interface name.

SO_BINDTOIF is not very descriptive related to SO_BINDTODEVICE.

SO_BINDTOINDEX, SO_BINDTODEVINDEX or SO_BINDTODEVIDX would be clearer
about this option versus SO_BINDTODEVICE.
Roopa Prabhu - Jan. 10, 2019, 4:39 p.m.
On Thu, Jan 10, 2019 at 8:38 AM David Ahern <dsahern@gmail.com> wrote:
>
> On 1/10/19 7:25 AM, David Herrmann wrote:
> > This introduces a new generic SOL_SOCKET-level socket option called
> > SO_BINDTOIF. It behaves similar to SO_BINDTODEVICE, but takes a network
> > interface index as argument, rather than the network interface name.
>
> SO_BINDTOIF is not very descriptive related to SO_BINDTODEVICE.
>
> SO_BINDTOINDEX, SO_BINDTODEVINDEX or SO_BINDTODEVIDX would be clearer
> about this option versus SO_BINDTODEVICE.
>

+1 , also SO_BINDTOIFINDEX
David Herrmann - Jan. 10, 2019, 4:50 p.m.
Hi

On Thu, Jan 10, 2019 at 5:38 PM David Ahern <dsahern@gmail.com> wrote:
>
> On 1/10/19 7:25 AM, David Herrmann wrote:
> > This introduces a new generic SOL_SOCKET-level socket option called
> > SO_BINDTOIF. It behaves similar to SO_BINDTODEVICE, but takes a network
> > interface index as argument, rather than the network interface name.
>
> SO_BINDTOIF is not very descriptive related to SO_BINDTODEVICE.
>
> SO_BINDTOINDEX, SO_BINDTODEVINDEX or SO_BINDTODEVIDX would be clearer
> about this option versus SO_BINDTODEVICE.

I am open for these suggestions. I don't particularly have any
preference on the names, but I agree that BINDTOIF is not very easy to
read. For v2 I will pick SO_BINDTOIFINDEX (as suggested by Roopa),
unless there are any objections.

Thanks!
David
kbuild test robot - Jan. 13, 2019, 8:09 p.m.
Hi David,

I love your patch! Yet something to improve:

[auto build test ERROR on net/master]
[also build test ERROR on v5.0-rc1 next-20190111]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/David-Herrmann/net-introduce-SO_BINDTOIF-sockopt/20190111-124603
config: sparc-sparc64_defconfig (attached as .config)
compiler: sparc64-linux-gnu-gcc (Debian 7.2.0-11) 7.2.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=7.2.0 make.cross ARCH=sparc 

All errors (new ones prefixed by >>):

   net/core/sock.c: In function 'sock_getsockopt':
>> net/core/sock.c:1424:2: error: duplicate case value
     case SO_BINDTOIF:
     ^~~~
   net/core/sock.c:1258:2: note: previously used here
     case SO_PEERCRED:
     ^~~~

vim +1424 net/core/sock.c

  1403	
  1404		case SO_COOKIE:
  1405			lv = sizeof(u64);
  1406			if (len < lv)
  1407				return -EINVAL;
  1408			v.val64 = sock_gen_cookie(sk);
  1409			break;
  1410	
  1411		case SO_ZEROCOPY:
  1412			v.val = sock_flag(sk, SOCK_ZEROCOPY);
  1413			break;
  1414	
  1415		case SO_TXTIME:
  1416			lv = sizeof(v.txtime);
  1417			v.txtime.clockid = sk->sk_clockid;
  1418			v.txtime.flags |= sk->sk_txtime_deadline_mode ?
  1419					  SOF_TXTIME_DEADLINE_MODE : 0;
  1420			v.txtime.flags |= sk->sk_txtime_report_errors ?
  1421					  SOF_TXTIME_REPORT_ERRORS : 0;
  1422			break;
  1423	
> 1424		case SO_BINDTOIF:
  1425			v.val = sk->sk_bound_dev_if;
  1426			break;
  1427	
  1428		default:
  1429			/* We implement the SO_SNDLOWAT etc to not be settable
  1430			 * (1003.1g 7).
  1431			 */
  1432			return -ENOPROTOOPT;
  1433		}
  1434	
  1435		if (len > lv)
  1436			len = lv;
  1437		if (copy_to_user(optval, &v, len))
  1438			return -EFAULT;
  1439	lenout:
  1440		if (put_user(len, optlen))
  1441			return -EFAULT;
  1442		return 0;
  1443	}
  1444	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

Patch

diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
index 065fb372e355..6e346e51eec7 100644
--- a/arch/alpha/include/uapi/asm/socket.h
+++ b/arch/alpha/include/uapi/asm/socket.h
@@ -115,4 +115,6 @@ 
 #define SO_TXTIME		61
 #define SCM_TXTIME		SO_TXTIME
 
+#define SO_BINDTOIF		62
+
 #endif /* _UAPI_ASM_SOCKET_H */
diff --git a/arch/ia64/include/uapi/asm/socket.h b/arch/ia64/include/uapi/asm/socket.h
index c872c4e6bafb..ece83ba17b9d 100644
--- a/arch/ia64/include/uapi/asm/socket.h
+++ b/arch/ia64/include/uapi/asm/socket.h
@@ -117,4 +117,6 @@ 
 #define SO_TXTIME		61
 #define SCM_TXTIME		SO_TXTIME
 
+#define SO_BINDTOIF		62
+
 #endif /* _ASM_IA64_SOCKET_H */
diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
index 71370fb3ceef..27f7f761ace5 100644
--- a/arch/mips/include/uapi/asm/socket.h
+++ b/arch/mips/include/uapi/asm/socket.h
@@ -126,4 +126,6 @@ 
 #define SO_TXTIME		61
 #define SCM_TXTIME		SO_TXTIME
 
+#define SO_BINDTOIF		62
+
 #endif /* _UAPI_ASM_SOCKET_H */
diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
index 061b9cf2a779..efd3917f23e1 100644
--- a/arch/parisc/include/uapi/asm/socket.h
+++ b/arch/parisc/include/uapi/asm/socket.h
@@ -107,4 +107,6 @@ 
 #define SO_TXTIME		0x4036
 #define SCM_TXTIME		SO_TXTIME
 
+#define SO_BINDTOIF		0x4037
+
 #endif /* _UAPI_ASM_SOCKET_H */
diff --git a/arch/s390/include/uapi/asm/socket.h b/arch/s390/include/uapi/asm/socket.h
index 39d901476ee5..c8ba542e69e6 100644
--- a/arch/s390/include/uapi/asm/socket.h
+++ b/arch/s390/include/uapi/asm/socket.h
@@ -114,4 +114,6 @@ 
 #define SO_TXTIME		61
 #define SCM_TXTIME		SO_TXTIME
 
+#define SO_BINDTOIF		62
+
 #endif /* _ASM_SOCKET_H */
diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
index 7ea35e5601b6..50006bde7dc0 100644
--- a/arch/sparc/include/uapi/asm/socket.h
+++ b/arch/sparc/include/uapi/asm/socket.h
@@ -104,6 +104,8 @@ 
 #define SO_TXTIME		0x003f
 #define SCM_TXTIME		SO_TXTIME
 
+#define SO_BINDTOIF		0x0040
+
 /* Security levels - as per NRL IPv6 - don't actually do anything */
 #define SO_SECURITY_AUTHENTICATION		0x5001
 #define SO_SECURITY_ENCRYPTION_TRANSPORT	0x5002
diff --git a/arch/xtensa/include/uapi/asm/socket.h b/arch/xtensa/include/uapi/asm/socket.h
index 1de07a7f7680..a36241ffbd86 100644
--- a/arch/xtensa/include/uapi/asm/socket.h
+++ b/arch/xtensa/include/uapi/asm/socket.h
@@ -119,4 +119,6 @@ 
 #define SO_TXTIME		61
 #define SCM_TXTIME		SO_TXTIME
 
+#define SO_BINDTOIF		62
+
 #endif	/* _XTENSA_SOCKET_H */
diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
index a12692e5f7a8..31fb8414ea4c 100644
--- a/include/uapi/asm-generic/socket.h
+++ b/include/uapi/asm-generic/socket.h
@@ -110,4 +110,6 @@ 
 #define SO_TXTIME		61
 #define SCM_TXTIME		SO_TXTIME
 
+#define SO_BINDTOIF		62
+
 #endif /* __ASM_GENERIC_SOCKET_H */
diff --git a/net/core/sock.c b/net/core/sock.c
index 6aa2e7e0b4fb..df8f83bc22b3 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -520,20 +520,43 @@  struct dst_entry *sk_dst_check(struct sock *sk, u32 cookie)
 }
 EXPORT_SYMBOL(sk_dst_check);
 
-static int sock_setbindtodevice(struct sock *sk, char __user *optval,
-				int optlen)
+static int sock_setbindtodevice_locked(struct sock *sk, int ifindex)
 {
 	int ret = -ENOPROTOOPT;
 #ifdef CONFIG_NETDEVICES
 	struct net *net = sock_net(sk);
-	char devname[IFNAMSIZ];
-	int index;
 
 	/* Sorry... */
 	ret = -EPERM;
 	if (!ns_capable(net->user_ns, CAP_NET_RAW))
 		goto out;
 
+	ret = -EINVAL;
+	if (ifindex < 0)
+		goto out;
+
+	sk->sk_bound_dev_if = ifindex;
+	if (sk->sk_prot->rehash)
+		sk->sk_prot->rehash(sk);
+	sk_dst_reset(sk);
+
+	ret = 0;
+
+out:
+#endif
+
+	return ret;
+}
+
+static int sock_setbindtodevice(struct sock *sk, char __user *optval,
+				int optlen)
+{
+	int ret = -ENOPROTOOPT;
+#ifdef CONFIG_NETDEVICES
+	struct net *net = sock_net(sk);
+	char devname[IFNAMSIZ];
+	int index;
+
 	ret = -EINVAL;
 	if (optlen < 0)
 		goto out;
@@ -566,14 +589,9 @@  static int sock_setbindtodevice(struct sock *sk, char __user *optval,
 	}
 
 	lock_sock(sk);
-	sk->sk_bound_dev_if = index;
-	if (sk->sk_prot->rehash)
-		sk->sk_prot->rehash(sk);
-	sk_dst_reset(sk);
+	ret = sock_setbindtodevice_locked(sk, index);
 	release_sock(sk);
 
-	ret = 0;
-
 out:
 #endif
 
@@ -1055,6 +1073,10 @@  int sock_setsockopt(struct socket *sock, int level, int optname,
 		}
 		break;
 
+	case SO_BINDTOIF:
+		ret = sock_setbindtodevice_locked(sk, val);
+		break;
+
 	default:
 		ret = -ENOPROTOOPT;
 		break;
@@ -1399,6 +1421,10 @@  int sock_getsockopt(struct socket *sock, int level, int optname,
 				  SOF_TXTIME_REPORT_ERRORS : 0;
 		break;
 
+	case SO_BINDTOIF:
+		v.val = sk->sk_bound_dev_if;
+		break;
+
 	default:
 		/* We implement the SO_SNDLOWAT etc to not be settable
 		 * (1003.1g 7).