daemon: detect and reject too-long paths

When we are checking the path via path_ok(), we use some
fixed PATH_MAX buffers. We write into them via snprintf(),
so there's no possibility of overflow, but it does mean we
may silently truncate the path, leading to potentially
confusing errors when the partial path does not exist.

We're better off to reject the path explicitly.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

daemon.c

@@ -161,6 +161,7 @@ static const char *path_ok(const char *directory, struct hostinfo *hi)
 {
 	static char rpath[PATH_MAX];
 	static char interp_path[PATH_MAX];
+	size_t rlen;
 	const char *path;
 	const char *dir;
 
@@ -188,8 +189,12 @@ static const char *path_ok(const char *directory, struct hostinfo *hi)
 			namlen = slash - dir;
 			restlen -= namlen;
 			loginfo("userpath <%s>, request <%s>, namlen %d, restlen %d, slash <%s>", user_path, dir, namlen, restlen, slash);
-			snprintf(rpath, PATH_MAX, "%.*s/%s%.*s",
-				 namlen, dir, user_path, restlen, slash);
+			rlen = snprintf(rpath, sizeof(rpath), "%.*s/%s%.*s",
+					namlen, dir, user_path, restlen, slash);
+			if (rlen >= sizeof(rpath)) {
+				logerror("user-path too large: %s", rpath);
+				return NULL;
+			}
 			dir = rpath;
 		}
 	}
@@ -208,7 +213,15 @@ static const char *path_ok(const char *directory, struct hostinfo *hi)
 
 		strbuf_expand(&expanded_path, interpolated_path,
 			      expand_path, &context);
-		strlcpy(interp_path, expanded_path.buf, PATH_MAX);
+
+		rlen = strlcpy(interp_path, expanded_path.buf,
+			       sizeof(interp_path));
+		if (rlen >= sizeof(interp_path)) {
+			logerror("interpolated path too large: %s",
+				 interp_path);
+			return NULL;
+		}
+
 		strbuf_release(&expanded_path);
 		loginfo("Interpolated dir '%s'", interp_path);
 
@@ -220,7 +233,11 @@ static const char *path_ok(const char *directory, struct hostinfo *hi)
 			logerror("'%s': Non-absolute path denied (base-path active)", dir);
 			return NULL;
 		}
-		snprintf(rpath, PATH_MAX, "%s%s", base_path, dir);
+		rlen = snprintf(rpath, sizeof(rpath), "%s%s", base_path, dir);
+		if (rlen >= sizeof(rpath)) {
+			logerror("base-path too large: %s", rpath);
+			return NULL;
+		}
 		dir = rpath;
 	}