vt(4): Skip vt_flush() for nested panics

[Why]
If there is a problem with DRM drivers or in their integration with
vt(4) and displaying something on the console triggers a panic, there is
a high chance that displaying that panic will trigger another one,
recursively.

[How]
If vt_flush() is called and it detects is is called resursively from
another panic, it return immediately, doing nothing, to avoid the risk
of triggering another panic.

Reviewed by:	manu
Approved by:	manu
Differential Revision:  https://reviews.freebsd.org/D42056

1 files changed, 15 insertions, 0 deletions

diff --git a/sys/dev/vt/vt_core.c b/sys/dev/vt/vt_core.c
index c97cb80ca3f1..406472a8fbf6 100644
--- a/sys/dev/vt/vt_core.c
+++ b/sys/dev/vt/vt_core.c
@@ -274,6 +274,8 @@ SYSINIT(vt_update_static, SI_SUB_KMEM, SI_ORDER_ANY, vt_update_static,
 SYSINIT(vt_early_cons, SI_SUB_INT_CONFIG_HOOKS, SI_ORDER_ANY, vt_upgrade,
     &vt_consdev);
 
+static bool inside_vt_flush = false;
+
 /* Initialize locks/mem depended members. */
 static void
 vt_update_static(void *dummy)
@@ -1344,6 +1346,9 @@ vt_flush(struct vt_device *vd)
 	int cursor_was_shown, cursor_moved;
 #endif
 
+	if (inside_vt_flush && KERNEL_PANICKED())
+		return (0);
+
 	vw = vd->vd_curwindow;
 	if (vw == NULL)
 		return (0);
@@ -1357,6 +1362,8 @@ vt_flush(struct vt_device *vd)
 
 	vtbuf_lock(&vw->vw_buf);
 
+	inside_vt_flush = true;
+
 #ifndef SC_NO_CUTPASTE
 	cursor_was_shown = vd->vd_mshown;
 	cursor_moved = (vd->vd_mx != vd->vd_mx_drawn ||
@@ -1414,10 +1421,12 @@ vt_flush(struct vt_device *vd)
 
 	if (tarea.tr_begin.tp_col < tarea.tr_end.tp_col) {
 		vd->vd_driver->vd_bitblt_text(vd, vw, &tarea);
+		inside_vt_flush = false;
 		vtbuf_unlock(&vw->vw_buf);
 		return (1);
 	}
 
+	inside_vt_flush = false;
 	vtbuf_unlock(&vw->vw_buf);
 	return (0);
 }
@@ -1444,6 +1453,9 @@ vtterm_pre_input(struct terminal *tm)
 {
 	struct vt_window *vw = tm->tm_softc;
 
+	if (inside_vt_flush && KERNEL_PANICKED())
+		return;
+
 	vtbuf_lock(&vw->vw_buf);
 }
 
@@ -1452,6 +1464,9 @@ vtterm_post_input(struct terminal *tm)
 {
 	struct vt_window *vw = tm->tm_softc;
 
+	if (inside_vt_flush && KERNEL_PANICKED())
+		return;
+
 	vtbuf_unlock(&vw->vw_buf);
 	vt_resume_flush_timer(vw, 0);
 }