from brian again:

Fixes the once-in-a-blue-moon error of neglecting to add a null byte
to a file being read.  The only problem I've seen it actually cause is
preventing group access of the last group member on the last line of
/adm/users.  Of course, there may also be bigger problems that could
show up.  But I found this one this morning.

Reference: /n/atom/patch/applied/thetafsuid0
Date: Thu Feb 27 22:17:34 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/θfs/uid.c	Thu Feb 27 22:17:27 2014
+++ /sys/src/cmd/θfs/uid.c	Thu Feb 27 22:17:28 2014
@@ -130,13 +130,14 @@
 	if(getmetaint(-1, meta, "length", &len) == MTnone)
 		return nil;
 	fb = θmalloc(sizeof(Filebuf));
-	fb->buf = θmalloc(len);
+	fb->buf = θmalloc(len+1);
 	n = θpread(-1, qpath, fb->buf, len, 0);
 	if(n < len) {
 		free(fb->buf);
 		free(fb);
 		return nil;
 	}
+	fb->buf[len] = 0;
 	for(p = fb->buf; p < fb->buf + n; ++p)
 		if(*p == '\n')
 			++fb->nlines;