firewire: cdev: fix 32 bit userland on 64 bit kernel compat corner cases

Clemens points out that we need to use compat_ptr() in order to safely
cast from u64 to addresses of a 32-bit usermode client.

Before, our conversion went wrong
  - in practice if the client cast from pointer to integer such that
    sign-extension happened, (libraw1394 and libdc1394 at least were not
    doing that, IOW were not affected)
or
  - in theory on s390 (which doesn't have FireWire though) and on the
    tile architecture, regardless of what the client does.
The bug would usually be observed as the initial get_info ioctl failing
with "Bad address" (EFAULT).
Reported-by: Carl Karsten <carl@personnelware.com>
Reported-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/core-cdev.c

@@ -216,15 +216,33 @@ struct inbound_phy_packet_event {
 	struct fw_cdev_event_phy_packet phy_packet;
 };
 
-static inline void __user *u64_to_uptr(__u64 value)
+#ifdef CONFIG_COMPAT
+static void __user *u64_to_uptr(u64 value)
+{
+	if (is_compat_task())
+		return compat_ptr(value);
+	else
+		return (void __user *)(unsigned long)value;
+}
+
+static u64 uptr_to_u64(void __user *ptr)
+{
+	if (is_compat_task())
+		return ptr_to_compat(ptr);
+	else
+		return (u64)(unsigned long)ptr;
+}
+#else
+static inline void __user *u64_to_uptr(u64 value)
 {
 	return (void __user *)(unsigned long)value;
 }
 
-static inline __u64 uptr_to_u64(void __user *ptr)
+static inline u64 uptr_to_u64(void __user *ptr)
 {
-	return (__u64)(unsigned long)ptr;
+	return (u64)(unsigned long)ptr;
 }
+#endif /* CONFIG_COMPAT */
 
 static int fw_device_op_open(struct inode *inode, struct file *file)
 {