compat_ioctl: handle PPPIOCGIDLE for 64-bit time_t

The ppp_idle structure is defined in terms of __kernel_time_t, which is
defined as 'long' on all architectures, and this usage is not affected
by the y2038 problem since it transports a time interval rather than an
absolute time.

However, the ppp user space defines the same structure as time_t, which
may be 64-bit wide on new libc versions even on 32-bit architectures.

It's easy enough to just handle both possible structure layouts on
all architectures, to deal with the possibility that a user space ppp
implementation comes with its own ppp_idle structure definition, as well
as to document the fact that the driver is y2038-safe.

Doing this also avoids the need for a special compat mode translation,
since 32-bit and 64-bit kernels now support the same interfaces.  The old
32-bit structure is also available on native 64-bit architectures now,
but this is harmless.

Cc: netdev@vger.kernel.org
Cc: linux-ppp@vger.kernel.org
Cc: Paul Mackerras <paulus@samba.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/uapi/linux/ppp_defs.h

@@ -142,10 +142,24 @@ struct ppp_comp_stats {
 /*
  * The following structure records the time in seconds since
  * the last NP packet was sent or received.
+ *
+ * Linux implements both 32-bit and 64-bit time_t versions
+ * for compatibility with user space that defines ppp_idle
+ * based on the libc time_t.
  */
 struct ppp_idle {
     __kernel_time_t xmit_idle;	/* time since last NP packet sent */
     __kernel_time_t recv_idle;	/* time since last NP packet received */
 };
 
+struct ppp_idle32 {
+    __s32 xmit_idle;		/* time since last NP packet sent */
+    __s32 recv_idle;		/* time since last NP packet received */
+};
+
+struct ppp_idle64 {
+    __s64 xmit_idle;		/* time since last NP packet sent */
+    __s64 recv_idle;		/* time since last NP packet received */
+};
+
 #endif /* _UAPI_PPP_DEFS_H_ */