net_tstamp: Add SIOCGHWTSTAMP ioctl to match SIOCSHWTSTAMP

SIOCSHWTSTAMP returns the real configuration to the application
using it, but there is currently no way for any other
application to find out the configuration non-destructively.
Add a new ioctl for this, making it unprivileged.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

Documentation/networking/timestamping.txt

@@ -85,7 +85,7 @@ Filled in if SOF_TIMESTAMPING_SYS_HARDWARE is set. Requires support
 by the network device and will be empty without that support.
 
 
-SIOCSHWTSTAMP:
+SIOCSHWTSTAMP, SIOCGHWTSTAMP:
 
 Hardware time stamping must also be initialized for each device driver
 that is expected to do hardware time stamping. The parameter is defined in
@@ -115,6 +115,10 @@ Only a processes with admin rights may change the configuration. User
 space is responsible to ensure that multiple processes don't interfere
 with each other and that the settings are reset.
 
+Any process can read the actual configuration by passing this
+structure to ioctl(SIOCGHWTSTAMP) in the same way.  However, this has
+not been implemented in all drivers.
+
 /* possible values for hwtstamp_config->tx_type */
 enum {
 	/*
@@ -157,7 +161,8 @@ DEVICE IMPLEMENTATION
 
 A driver which supports hardware time stamping must support the
 SIOCSHWTSTAMP ioctl and update the supplied struct hwtstamp_config with
-the actual values as described in the section on SIOCSHWTSTAMP.
+the actual values as described in the section on SIOCSHWTSTAMP.  It
+should also support SIOCGHWTSTAMP.
 
 Time stamps for received packets must be stored in the skb. To get a pointer
 to the shared time stamp structure of the skb call skb_hwtstamps(). Then

include/uapi/linux/net_tstamp.h

@@ -26,17 +26,17 @@ enum {
 };
 
 /**
- * struct hwtstamp_config - %SIOCSHWTSTAMP parameter
+ * struct hwtstamp_config - %SIOCGHWTSTAMP and %SIOCSHWTSTAMP parameter
  *
- * @flags:	no flags defined right now, must be zero
+ * @flags:	no flags defined right now, must be zero for %SIOCSHWTSTAMP
  * @tx_type:	one of HWTSTAMP_TX_*
  * @rx_filter:	one of HWTSTAMP_FILTER_*
  *
- * %SIOCSHWTSTAMP expects a &struct ifreq with a ifr_data pointer to
- * this structure.  If the driver or hardware does not support the
- * requested @rx_filter value, the driver may use a more general
- * filter mode.  In this case @rx_filter will indicate the actual mode
- * on return.
+ * %SIOCGHWTSTAMP and %SIOCSHWTSTAMP expect a &struct ifreq with a
+ * ifr_data pointer to this structure.  For %SIOCSHWTSTAMP, if the
+ * driver or hardware does not support the requested @rx_filter value,
+ * the driver may use a more general filter mode.  In this case
+ * @rx_filter will indicate the actual mode on return.
  */
 struct hwtstamp_config {
 	int flags;

include/uapi/linux/sockios.h

@@ -125,7 +125,8 @@
 #define SIOCBRDELIF	0x89a3		/* remove interface from bridge */
 
 /* hardware time stamping: parameters in linux/net_tstamp.h */
-#define SIOCSHWTSTAMP   0x89b0
+#define SIOCSHWTSTAMP	0x89b0		/* set and get config		*/
+#define SIOCGHWTSTAMP	0x89b1		/* get config			*/
 
 /* Device private ioctl calls */
 

net/core/dev_ioctl.c

@@ -327,6 +327,7 @@ static int dev_ifsioc(struct net *net, struct ifreq *ifr, unsigned int cmd)
 		    cmd == SIOCBRADDIF ||
 		    cmd == SIOCBRDELIF ||
 		    cmd == SIOCSHWTSTAMP ||
+		    cmd == SIOCGHWTSTAMP ||
 		    cmd == SIOCWANDEV) {
 			err = -EOPNOTSUPP;
 			if (ops->ndo_do_ioctl) {
@@ -546,6 +547,7 @@ int dev_ioctl(struct net *net, unsigned int cmd, void __user *arg)
 	 */
 	default:
 		if (cmd == SIOCWANDEV ||
+		    cmd == SIOCGHWTSTAMP ||
 		    (cmd >= SIOCDEVPRIVATE &&
 		     cmd <= SIOCDEVPRIVATE + 15)) {
 			dev_load(net, ifr.ifr_name);

net/socket.c

@@ -3231,6 +3231,7 @@ static int compat_sock_ioctl_trans(struct file *file, struct socket *sock,
 	case SIOCBONDSLAVEINFOQUERY:
 	case SIOCBONDINFOQUERY:
 	case SIOCSHWTSTAMP:
+	case SIOCGHWTSTAMP:
 		return compat_ifr_data_ioctl(net, cmd, argp);
 
 	case FIOSETOWN: