d_path: lift -ENAMETOOLONG handling into callers of prepend_path()

The only negative value ever returned by prepend_path() is -ENAMETOOLONG
and callers can recognize that situation (overflow) by looking at the
sign of buflen.  Lift that into the callers; we already have the
same logics (buf if buflen is non-negative, ERR_PTR(-ENAMETOOLONG) otherwise)
in several places and that'll become a new primitive several commits down
the road.

Make prepend_path() return 0 instead of -ENAMETOOLONG.  That makes for
saner calling conventions (0/1/2/3/-ENAMETOOLONG is obnoxious) and
callers actually get simpler, especially once the aforementioned
primitive gets added.

In prepend_path() itself we switch prepending the / (in case of
empty path) to use of prepend() - no need to open-code that, compiler
will do the right thing.  It's exactly the same logics as in
__dentry_path().
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/d_path.c

@@ -127,8 +127,7 @@ static int prepend_path(const struct path *path,
 		}
 		parent = dentry->d_parent;
 		prefetch(parent);
-		error = prepend_name(&bptr, &blen, &dentry->d_name);
-		if (error)
+		if (unlikely(prepend_name(&bptr, &blen, &dentry->d_name) < 0))
 			break;
 
 		dentry = parent;
@@ -149,12 +148,9 @@ static int prepend_path(const struct path *path,
 	}
 	done_seqretry(&mount_lock, m_seq);
 
-	if (error >= 0 && bptr == *buffer) {
-		if (--blen < 0)
-			error = -ENAMETOOLONG;
-		else
-			*--bptr = '/';
-	}
+	if (blen == *buflen)
+		prepend(&bptr, &blen, "/", 1);
+
 	*buffer = bptr;
 	*buflen = blen;
 	return error;
@@ -181,16 +177,11 @@ char *__d_path(const struct path *path,
 	       char *buf, int buflen)
 {
 	char *res = buf + buflen;
-	int error;
 
 	prepend(&res, &buflen, "", 1);
-	error = prepend_path(path, root, &res, &buflen);
-
-	if (error < 0)
-		return ERR_PTR(error);
-	if (error > 0)
+	if (prepend_path(path, root, &res, &buflen) > 0)
 		return NULL;
-	return res;
+	return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
 }
 
 char *d_absolute_path(const struct path *path,
@@ -198,16 +189,11 @@ char *d_absolute_path(const struct path *path,
 {
 	struct path root = {};
 	char *res = buf + buflen;
-	int error;
 
 	prepend(&res, &buflen, "", 1);
-	error = prepend_path(path, &root, &res, &buflen);
-
-	if (error > 1)
-		error = -EINVAL;
-	if (error < 0)
-		return ERR_PTR(error);
-	return res;
+	if (prepend_path(path, &root, &res, &buflen) > 1)
+		return ERR_PTR(-EINVAL);
+	return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
 }
 
 static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
@@ -240,7 +226,6 @@ char *d_path(const struct path *path, char *buf, int buflen)
 {
 	char *res = buf + buflen;
 	struct path root;
-	int error;
 
 	/*
 	 * We have various synthetic filesystems that never get mounted.  On
@@ -263,12 +248,10 @@ char *d_path(const struct path *path, char *buf, int buflen)
 		prepend(&res, &buflen, " (deleted)", 11);
 	else
 		prepend(&res, &buflen, "", 1);
-	error = prepend_path(path, &root, &res, &buflen);
+	prepend_path(path, &root, &res, &buflen);
 	rcu_read_unlock();
 
-	if (error < 0)
-		res = ERR_PTR(error);
-	return res;
+	return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
 }
 EXPORT_SYMBOL(d_path);