Introduce a hooks layer for the MPSAFE TTY layer.

One of the features that prevented us from fixing some of the TTY
consumers to work once again, was an interface that allowed consumers to
do the following:

- `Sniff' incoming data, which is used by the snp(4) driver.

- Take direct control of the input and output paths of a TTY, which is
  used by ng_tty(4), ppp(4), sl(4), etc.

There's no practical advantage in committing a hooks layer without
having any consumers. In P4 there is a preliminary port of snp(4) and
thompsa@ is busy porting ng_tty(4) to this interface. I already want to
have it in the tree, because this may stimulate others to work on the
remaining modules.

Discussed with:	thompsa
Obtained from:	//depot/projects/mpsafetty/...

1 files changed, 88 insertions, 2 deletions

diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index cd6a246..863b6ce 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -36,6 +36,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/conf.h>
 #include <sys/cons.h>
 #include <sys/fcntl.h>
+#include <sys/file.h>
 #include <sys/filio.h>
 #ifdef COMPAT_43TTY
 #include <sys/ioctl_compat.h>
@@ -59,6 +60,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/ttydefaults.h>
 #undef TTYDEFCHARS
 #include <sys/ucred.h>
+#include <sys/vnode.h>
 
 #include <machine/stdarg.h>
 
@@ -855,7 +857,7 @@ tty_alloc(struct ttydevsw *tsw, void *sc, struct mtx *mutex)
 
 	tp = malloc(sizeof(struct tty), M_TTY, M_WAITOK|M_ZERO);
 	tp->t_devsw = tsw;
-	tp->t_softc = sc;
+	tp->t_devswsoftc = sc;
 	tp->t_flags = tsw->tsw_flags;
 
 	tty_init_termios(tp);
@@ -923,7 +925,7 @@ tty_rel_free(struct tty *tp)
 	tty_lock_assert(tp, MA_OWNED);
 
 	if (tp->t_sessioncnt != 0 ||
-	    (tp->t_flags & (TF_GONE|TF_OPENED)) != TF_GONE) {
+	    (tp->t_flags & (TF_GONE|TF_OPENED|TF_HOOK)) != TF_GONE) {
 		/* TTY is still in use. */
 		tty_unlock(tp);
 		return;
@@ -1653,6 +1655,89 @@ tty_hiwat_in_unblock(struct tty *tp)
 		ttydevsw_inwakeup(tp);
 }
 
+static int
+ttyhook_defrint(struct tty *tp, char c, int flags)
+{
+
+	if (ttyhook_rint_bypass(tp, &c, 1) != 1)
+		return (-1);
+	
+	return (0);
+}
+
+int
+ttyhook_register(struct tty **rtp, struct thread *td, int fd,
+    struct ttyhook *th, void *softc)
+{
+	struct tty *tp;
+	struct file *fp;
+	struct cdev *dev;
+	struct cdevsw *cdp;
+	int error;
+
+	/* Validate the file descriptor. */
+	if (fget(td, fd, &fp) != 0)
+		return (EINVAL);
+	
+	/* Make sure the vnode is bound to a character device. */
+	error = EINVAL;
+	if (fp->f_type != DTYPE_VNODE || fp->f_vnode->v_type != VCHR ||
+	    fp->f_vnode->v_rdev == NULL)
+		goto done1;
+	dev = fp->f_vnode->v_rdev;
+
+	/* Make sure it is a TTY. */
+	cdp = dev_refthread(dev);
+	if (cdp == NULL)
+		goto done1;
+	if ((cdp->d_flags & D_TTY) == 0)
+		goto done2;
+	tp = dev->si_drv1;
+
+	/* Try to attach the hook to the TTY. */
+	error = EBUSY;
+	tty_lock(tp);
+	MPASS((tp->t_hook == NULL) == ((tp->t_flags & TF_HOOK) == 0));
+	if (tp->t_flags & TF_HOOK)
+		goto done3;
+
+	tp->t_flags |= TF_HOOK;
+	tp->t_hook = th;
+	tp->t_hooksoftc = softc;
+	*rtp = tp;
+	error = 0;
+
+	/* Maybe we can switch into bypass mode now. */
+	ttydisc_optimize(tp);
+
+	/* Silently convert rint() calls to rint_bypass() when possible. */
+	if (!ttyhook_hashook(tp, rint) && ttyhook_hashook(tp, rint_bypass))
+		th->th_rint = ttyhook_defrint;
+
+done3:	tty_unlock(tp);
+done2:	dev_relthread(dev);
+done1:	fdrop(fp, td);
+	return (error);
+}
+
+void
+ttyhook_unregister(struct tty *tp)
+{
+
+	tty_lock_assert(tp, MA_OWNED);
+	MPASS(tp->t_flags & TF_HOOK);
+
+	/* Disconnect the hook. */
+	tp->t_flags &= ~TF_HOOK;
+	tp->t_hook = NULL;
+
+	/* Maybe we need to leave bypass mode. */
+	ttydisc_optimize(tp);
+
+	/* Maybe deallocate the TTY as well. */
+	tty_rel_free(tp);
+}
+
 #include "opt_ddb.h"
 #ifdef DDB
 #include <ddb/ddb.h>
@@ -1686,6 +1771,7 @@ static struct {
 	{ TF_EXCLUDE,	'X' },
 	{ TF_BYPASS,	'l' },
 	{ TF_ZOMBIE,	'Z' },
+	{ TF_HOOK,	's' },
 
 	{ 0,	       '\0' },
 };