tty: warn on deprecated serial flags

When somebody calls TIOCSSERIAL ioctl with serial flags to set one of
* ASYNC_SESSION_LOCKOUT
* ASYNC_PGRP_LOCKOUT
* ASYNC_CALLOUT_NOHUP
* ASYNC_AUTOPROBE
nothing happens. We actually ignore the flags for over a decade at
least (I checked 2.6.0).

So start yelling at users who use those flags, that they shouldn't.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -2760,6 +2760,24 @@ static int tty_tiocgicount(struct tty_struct *tty, void __user *arg)
 	return 0;
 }
 
+static void tty_warn_deprecated_flags(struct serial_struct __user *ss)
+{
+	static DEFINE_RATELIMIT_STATE(depr_flags,
+			DEFAULT_RATELIMIT_INTERVAL,
+			DEFAULT_RATELIMIT_BURST);
+	char comm[TASK_COMM_LEN];
+	int flags;
+
+	if (get_user(flags, &ss->flags))
+		return;
+
+	flags &= ASYNC_DEPRECATED;
+
+	if (flags && __ratelimit(&depr_flags))
+		pr_warning("%s: '%s' is using deprecated serial flags (with no effect): %.8x\n",
+				__func__, get_task_comm(comm, current), flags);
+}
+
 /*
  * if pty, return the slave side (real_tty)
  * otherwise, return self
@@ -2903,6 +2921,9 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			break;
 		}
 		break;
+	case TIOCSSERIAL:
+		tty_warn_deprecated_flags(p);
+		break;
 	}
 	if (tty->ops->ioctl) {
 		retval = tty->ops->ioctl(tty, cmd, arg);

include/uapi/linux/tty_flags.h

@@ -64,6 +64,8 @@
 #define ASYNC_MAGIC_MULTIPLIER	(1U << ASYNCB_MAGIC_MULTIPLIER)
 
 #define ASYNC_FLAGS		((1U << (ASYNCB_LAST_USER + 1)) - 1)
+#define ASYNC_DEPRECATED	(ASYNC_SESSION_LOCKOUT | ASYNC_PGRP_LOCKOUT | \
+		ASYNC_CALLOUT_NOHUP | ASYNC_AUTOPROBE)
 #define ASYNC_USR_MASK		(ASYNC_SPD_MASK|ASYNC_CALLOUT_NOHUP| \
 		ASYNC_LOW_LATENCY)
 #define ASYNC_SPD_CUST		(ASYNC_SPD_HI|ASYNC_SPD_VHI)