In ibrk and segattach there are two different expressions which check for
expanding or newly created segments overlapping an existing segment.  Neither
version was general enough to catch all possible cases.

Reference: /n/sources/patch/segment-overlap
Date: Tue Apr 19 13:16:06 CES 2016
Signed-off-by: miller@hamnavoe.com

--- /sys/src/9/port/segment.c	Tue Apr 19 13:06:56 2016
+++ /sys/src/9/port/segment.c	Tue Apr 19 13:06:53 2016
@@ -484,7 +486,7 @@
 		ns = up->seg[i];
 		if(ns == 0 || ns == s)
 			continue;
-		if(newtop >= ns->base && newtop < ns->top) {
+		if(newtop > ns->base && s->base < ns->top) {
 			qunlock(&s->lk);
 			error(Esoverlap);
 		}
@@ -586,8 +588,7 @@
 		ns = p->seg[i];
 		if(ns == 0)
 			continue;
-		if((newtop > ns->base && newtop <= ns->top) ||
-		   (va >= ns->base && va < ns->top))
+		if(newtop > ns->base && va < ns->top)
 			return ns;
 	}
 	return nil;