attr.c: avoid inappropriate access to strbuf "buf" member

This code sequence performs a strcpy into the buf member of a strbuf
struct.  The strcpy may move the position of the terminating nul of the
string and effectively change the length of string so that it does not
match the len member of the strbuf struct.

Currently, this sequence works since the strbuf was given a hint when it
was initialized to allocate enough space to accomodate the string that will
be strcpy'ed, but this is an implementation detail of strbufs, not a
guarantee.

So, lets rework this sequence so that the strbuf is only manipulated by
strbuf functions, and direct modification of its "buf" member is not
necessary.
Signed-off-by: NBrandon Casey <drafnel@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

attr.c

@@ -552,7 +552,6 @@ static void prepare_attr_stack(const char *path)
 {
 	struct attr_stack *elem, *info;
 	int dirlen, len;
-	struct strbuf pathbuf;
 	const char *cp;
 
 	cp = strrchr(path, '/');
@@ -561,8 +560,6 @@ static void prepare_attr_stack(const char *path)
 	else
 		dirlen = cp - path;
 
-	strbuf_init(&pathbuf, dirlen+2+strlen(GITATTRIBUTES_FILE));
-
 	/*
 	 * At the bottom of the attribute stack is the built-in
 	 * set of attribute definitions, followed by the contents
@@ -607,27 +604,28 @@ static void prepare_attr_stack(const char *path)
 	 * Read from parent directories and push them down
 	 */
 	if (!is_bare_repository() || direction == GIT_ATTR_INDEX) {
-		while (1) {
-			char *cp;
+		struct strbuf pathbuf = STRBUF_INIT;
 
+		while (1) {
 			len = strlen(attr_stack->origin);
 			if (dirlen <= len)
 				break;
-			strbuf_reset(&pathbuf);
-			strbuf_add(&pathbuf, path, dirlen);
+			cp = memchr(path + len + 1, '/', dirlen - len - 1);
+			if (!cp)
+				cp = path + dirlen;
+			strbuf_add(&pathbuf, path, cp - path);
 			strbuf_addch(&pathbuf, '/');
-			cp = strchr(pathbuf.buf + len + 1, '/');
-			strcpy(cp + 1, GITATTRIBUTES_FILE);
+			strbuf_addstr(&pathbuf, GITATTRIBUTES_FILE);
 			elem = read_attr(pathbuf.buf, 0);
-			*cp = '\0';
-			elem->origin = strdup(pathbuf.buf);
+			strbuf_setlen(&pathbuf, cp - path);
+			elem->origin = strbuf_detach(&pathbuf, NULL);
 			elem->prev = attr_stack;
 			attr_stack = elem;
 			debug_push(elem);
 		}
-	}
 
-	strbuf_release(&pathbuf);
+		strbuf_release(&pathbuf);
+	}
 
 	/*
 	 * Finally push the "info" one at the top of the stack.