[PATCH] console: Fix buffer copy on vc resize

On a vc resize, the contents of the old screen buffer are transferred to the
new screenbuffer.  If the new screenbuffer is smaller than the old one, only
the contents from the bottom are copied to new.  If the contents of the old
buffer are located at the top, then the contents will not be copied to the new
buffer resulting in a blank screen.

This bug will happen only if the vc in question is not in the foreground.
Doing an fbset -a or con2fbmap will trigger this bug.

To fix this problem, base the start of the copy from the location of the
current cursor.  If the cursor is near the top of the buffer, copy the
contents at the top, and if the cursor is near the bottom of the buffer, then
copy the contents at the bottom.  In the unlikely case where the new row size
is greater than 2x smaller than the old one, and the cursor is in the middle,
copy 1/2 screenful from the top and bottom of the cursor position.

Signed-off-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/vt.c

@@ -751,6 +751,7 @@ int vc_resize(struct vc_data *vc, unsigned int cols, unsigned int lines)
 	unsigned long old_origin, new_origin, new_scr_end, rlth, rrem, err = 0;
 	unsigned int old_cols, old_rows, old_row_size, old_screen_size;
 	unsigned int new_cols, new_rows, new_row_size, new_screen_size;
+	unsigned int end;
 	unsigned short *newscreen;
 
 	WARN_CONSOLE_UNLOCKED();
@@ -794,20 +795,44 @@ int vc_resize(struct vc_data *vc, unsigned int cols, unsigned int lines)
 	old_origin = vc->vc_origin;
 	new_origin = (long) newscreen;
 	new_scr_end = new_origin + new_screen_size;
-	if (new_rows < old_rows)
-		old_origin += (old_rows - new_rows) * old_row_size;
+
+	if (vc->vc_y > new_rows) {
+		if (old_rows - vc->vc_y < new_rows) {
+			/*
+			 * Cursor near the bottom, copy contents from the
+			 * bottom of buffer
+			 */
+			old_origin += (old_rows - new_rows) * old_row_size;
+			end = vc->vc_scr_end;
+		} else {
+			/*
+			 * Cursor is in no man's land, copy 1/2 screenful
+			 * from the top and bottom of cursor position
+			 */
+			old_origin += (vc->vc_y - new_rows/2) * old_row_size;
+			end = old_origin + new_screen_size;
+		}
+	} else
+		/*
+		 * Cursor near the top, copy contents from the top of buffer
+		 */
+		end = (old_rows > new_rows) ? old_origin + new_screen_size :
+			vc->vc_scr_end;
 
 	update_attr(vc);
 
-	while (old_origin < vc->vc_scr_end) {
-		scr_memcpyw((unsigned short *) new_origin, (unsigned short *) old_origin, rlth);
+	while (old_origin < end) {
+		scr_memcpyw((unsigned short *) new_origin,
+			    (unsigned short *) old_origin, rlth);
 		if (rrem)
-			scr_memsetw((void *)(new_origin + rlth), vc->vc_video_erase_char, rrem);
+			scr_memsetw((void *)(new_origin + rlth),
+				    vc->vc_video_erase_char, rrem);
 		old_origin += old_row_size;
 		new_origin += new_row_size;
 	}
 	if (new_scr_end > new_origin)
-		scr_memsetw((void *)new_origin, vc->vc_video_erase_char, new_scr_end - new_origin);
+		scr_memsetw((void *)new_origin, vc->vc_video_erase_char,
+			    new_scr_end - new_origin);
 	if (vc->vc_kmalloced)
 		kfree(vc->vc_screenbuf);
 	vc->vc_screenbuf = newscreen;