[Qemu-devel] [PATCH] sockets: Fix stringop-truncation warning

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Compiling with clang-8 fails with:

    CC      util/qemu-sockets.o
  util/qemu-sockets.c: In function 'unix_connect_saddr':
  util/qemu-sockets.c:925:5: error: 'strncpy' specified bound 108 equals destination size [-Werror=stringop-truncation]
       strncpy(un.sun_path, saddr->path, sizeof(un.sun_path));
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  util/qemu-sockets.c: In function 'unix_listen_saddr':
  util/qemu-sockets.c:880:5: error: 'strncpy' specified bound 108 equals destination size [-Werror=stringop-truncation]
       strncpy(un.sun_path, path, sizeof(un.sun_path));
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Per the unix socket manpage:

  UNIX(7)

  Pathname sockets
  When binding a socket to a pathname, a few rules should be observed for maximum portability and ease of coding:
  *  The pathname in sun_path should be null-terminated.
  *  The length of the pathname, including the terminating null byte, should not exceed the size of sun_path.

Reduce the length of the unix socket path by 1 to hold the NUL byte.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 util/qemu-sockets.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
index 97050516900..935271d58c0 100644
--- a/util/qemu-sockets.c
+++ b/util/qemu-sockets.c
@@ -845,10 +845,10 @@ static int unix_listen_saddr(UnixSocketAddress *saddr,
         path = pathbuf = g_strdup_printf("%s/qemu-socket-XXXXXX", tmpdir);
     }
 
-    if (strlen(path) > sizeof(un.sun_path)) {
+    if (strlen(path) > sizeof(un.sun_path) - 1) {
         error_setg(errp, "UNIX socket path '%s' is too long", path);
         error_append_hint(errp, "Path must be less than %zu bytes\n",
-                          sizeof(un.sun_path));
+                          sizeof(un.sun_path) - 1);
         goto err;
     }
 
@@ -877,7 +877,7 @@ static int unix_listen_saddr(UnixSocketAddress *saddr,
 
     memset(&un, 0, sizeof(un));
     un.sun_family = AF_UNIX;
-    strncpy(un.sun_path, path, sizeof(un.sun_path));
+    strncpy(un.sun_path, path, sizeof(un.sun_path) - 1);
 
     if (bind(sock, (struct sockaddr*) &un, sizeof(un)) < 0) {
         error_setg_errno(errp, errno, "Failed to bind socket to %s", path);
@@ -913,16 +913,16 @@ static int unix_connect_saddr(UnixSocketAddress *saddr, Error **errp)
         return -1;
     }
 
-    if (strlen(saddr->path) > sizeof(un.sun_path)) {
+    if (strlen(saddr->path) > sizeof(un.sun_path) - 1) {
         error_setg(errp, "UNIX socket path '%s' is too long", saddr->path);
         error_append_hint(errp, "Path must be less than %zu bytes\n",
-                          sizeof(un.sun_path));
+                          sizeof(un.sun_path) - 1);
         goto err;
     }
 
     memset(&un, 0, sizeof(un));
     un.sun_family = AF_UNIX;
-    strncpy(un.sun_path, saddr->path, sizeof(un.sun_path));
+    strncpy(un.sun_path, saddr->path, sizeof(un.sun_path) - 1);
 
     /* connect to peer */
     do {
-- 
2.20.1