sysdeps: Reassure gcc 8 that we are not overflowing struct sockaddr_un

Using strncpy (buffer, str, strlen (str)) is a "code smell" that
might indicate a serious bug (it effectively turns strncpy into
strcpy), and gcc 8 now warns about it. In fact we avoided the bug
here, but it wasn't at all obvious.

We already checked that path_len is less than or equal to
_DBUS_MAX_SUN_PATH_LENGTH, which is 99, chosen to be strictly less
than the POSIX minimum sizeof(sun_path) >= 100, so we couldn't
actually be overflowing the available buffer.

The new static assertion in this commit matches a comment above the
definition of _DBUS_MAX_SUN_PATH_LENGTH: we define
_DBUS_MAX_SUN_PATH_LENGTH to 99, because POSIX says struct
sockaddr_un's sun_path member is at least 100 bytes (including space
for a \0 terminator). dbus will now fail to compile on
platforms that are non-POSIX-compliant in this way, except for Windows.

We zeroed the struct sockaddr_un before writing into it, so stopping
one byte short of the end of sun_path ensures that we get \0
termination.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=107350
Reviewed-by: Thiago Macieira <thiago@kde.org>
Reviewed-by: Philip Withnall <withnall@endlessm.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/dbus/dbus-sysdeps-unix.c b/dbus/dbus-sysdeps-unix.c
index 0af3e4a2..1b203373 100644
--- a/dbus/dbus-sysdeps-unix.c
+++ b/dbus/dbus-sysdeps-unix.c
@@ -913,6 +913,7 @@ _dbus_connect_unix_socket (const char     *path,
   int fd;
   size_t path_len;
   struct sockaddr_un addr;
+  _DBUS_STATIC_ASSERT (sizeof (addr.sun_path) > _DBUS_MAX_SUN_PATH_LENGTH);
 
   _DBUS_ASSERT_ERROR_IS_CLEAR (error);
 
@@ -945,7 +946,7 @@ _dbus_connect_unix_socket (const char     *path,
           return -1;
 	}
 
-      strncpy (&addr.sun_path[1], path, path_len);
+      strncpy (&addr.sun_path[1], path, sizeof (addr.sun_path) - 2);
       /* _dbus_verbose_bytes (addr.sun_path, sizeof (addr.sun_path)); */
 #else /* !__linux__ */
       dbus_set_error (error, DBUS_ERROR_NOT_SUPPORTED,
@@ -964,7 +965,7 @@ _dbus_connect_unix_socket (const char     *path,
           return -1;
 	}
 
-      strncpy (addr.sun_path, path, path_len);
+      strncpy (addr.sun_path, path, sizeof (addr.sun_path) - 1);
     }
 
   if (connect (fd, (struct sockaddr*) &addr, _DBUS_STRUCT_OFFSET (struct sockaddr_un, sun_path) + path_len) < 0)
@@ -1120,6 +1121,7 @@ _dbus_listen_unix_socket (const char     *path,
   int listen_fd;
   struct sockaddr_un addr;
   size_t path_len;
+  _DBUS_STATIC_ASSERT (sizeof (addr.sun_path) > _DBUS_MAX_SUN_PATH_LENGTH);
 
   _DBUS_ASSERT_ERROR_IS_CLEAR (error);
 
@@ -1154,7 +1156,7 @@ _dbus_listen_unix_socket (const char     *path,
           return -1;
 	}
 
-      strncpy (&addr.sun_path[1], path, path_len);
+      strncpy (&addr.sun_path[1], path, sizeof (addr.sun_path) - 2);
       /* _dbus_verbose_bytes (addr.sun_path, sizeof (addr.sun_path)); */
 #else /* !__linux__ */
       dbus_set_error (error, DBUS_ERROR_NOT_SUPPORTED,
@@ -1191,7 +1193,7 @@ _dbus_listen_unix_socket (const char     *path,
           return -1;
 	}
 
-      strncpy (addr.sun_path, path, path_len);
+      strncpy (addr.sun_path, path, sizeof (addr.sun_path) - 1);
     }
 
   if (bind (listen_fd, (struct sockaddr*) &addr, _DBUS_STRUCT_OFFSET (struct sockaddr_un, sun_path) + path_len) < 0)