attachproc curiously uses a constant variable set to ORDWR, a remnant of some earlier code.
instead of always forcing ORDWR for register access, why not try it, and if it can't be opened read-write, use read-only instead.

Reference: /n/sources/patch/applied/libmach-map-open
Date: Fri Mar  1 15:49:21 CET 2013
Signed-off-by: charles.forsyth@gmail.com

--- /sys/src/libmach/map.c	Fri Mar  1 15:47:45 2013
+++ /sys/src/libmach/map.c	Fri Mar  1 15:47:43 2013
@@ -84,7 +84,6 @@
 	int fd;
 	Map *map;
 	uvlong n;
-	int mode;
 
 	map = newmap(0, 4);
 	if (!map)
@@ -93,10 +92,11 @@
 		regs = "kregs";
 	else
 		regs = "regs";
-	mode = ORDWR;
 	if (mach->regsize) {
 		sprint(buf, "/proc/%d/%s", pid, regs);
-		fd = open(buf, mode);
+		fd = open(buf, ORDWR);
+		if(fd < 0)
+			fd = open(buf, OREAD);
 		if(fd < 0) {
 			free(map);
 			return 0;
@@ -105,7 +105,9 @@
 	}
 	if (mach->fpregsize) {
 		sprint(buf, "/proc/%d/fpregs", pid);
-		fd = open(buf, mode);
+		fd = open(buf, ORDWR);
+		if(fd < 0)
+			fd = open(buf, OREAD);
 		if(fd < 0) {
 			close(map->seg[0].fd);
 			free(map);