change semantics of ldisc ->compat_ioctl()

First of all, make it return int.  Returning long when native method
had never allowed that is ridiculous and inconvenient.

More importantly, change the caller; if ldisc ->compat_ioctl() is NULL
or returns -ENOIOCTLCMD, tty_compat_ioctl() will try to feed cmd and
compat_ptr(arg) to ldisc's native ->ioctl().

That simplifies ->compat_ioctl() instances quite a bit - they only
need to deal with ioctls that are neither generic tty ones (those
would get shunted off to tty_ioctl()) nor simple compat pointer ones.

Note that something like TCFLSH won't reach ->compat_ioctl(),
even if ldisc ->ioctl() does handle it - it will be recognized
earlier and passed to tty_ioctl() (and ultimately - ldisc ->ioctl()).

For many ldiscs it means that NULL ->compat_ioctl() does the
right thing.  Those where it won't serve (see e.g. n_r3964.c) are
also easily dealt with - we need to handle the numeric-argument
ioctls (calling the native instance) and, if such would exist,
the ioctls that need layout conversion, etc.

All in-tree ldiscs dealt with.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 8 insertions, 2 deletions

diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index 840894ca3fc0..b1e6043e9917 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -54,11 +54,17 @@
  *	low-level driver can "grab" an ioctl request before the line
  *	discpline has a chance to see it.
  *
- * long	(*compat_ioctl)(struct tty_struct * tty, struct file * file,
+ * int	(*compat_ioctl)(struct tty_struct * tty, struct file * file,
  *		        unsigned int cmd, unsigned long arg);
  *
  *	Process ioctl calls from 32-bit process on 64-bit system
  *
+ *	NOTE: only ioctls that are neither "pointer to compatible
+ *	structure" nor tty-generic.  Something private that takes
+ *	an integer or a pointer to wordsize-sensitive structure
+ *	belongs here, but most of ldiscs will happily leave
+ *	it NULL.
+ *
  * void	(*set_termios)(struct tty_struct *tty, struct ktermios * old);
  *
  *	This function notifies the line discpline that a change has
@@ -184,7 +190,7 @@ struct tty_ldisc_ops {
 			 const unsigned char *buf, size_t nr);
 	int	(*ioctl)(struct tty_struct *tty, struct file *file,
 			 unsigned int cmd, unsigned long arg);
-	long	(*compat_ioctl)(struct tty_struct *tty, struct file *file,
+	int	(*compat_ioctl)(struct tty_struct *tty, struct file *file,
 				unsigned int cmd, unsigned long arg);
 	void	(*set_termios)(struct tty_struct *tty, struct ktermios *old);
 	__poll_t (*poll)(struct tty_struct *, struct file *,