[PATCH v1] tools: fix usage of strncpy

[Olaf Hering <olaf@xxxxxxxxx>]

In case of truncation no trailing zero will be added to the target
string. Reduce the amount of bytes to copy by one to make sure a
trailing zero always exists.

In file included from /usr/include/string.h:495,
                 from libxl_internal.h:38,
                 from libxl_utils.c:20:
In function 'strncpy',
    inlined from 'libxl__prepare_sockaddr_un' at libxl_utils.c:1262:5:
/usr/include/bits/string_fortified.h:106:10: error: '__builtin_strncpy' 
specified bound 108 equals destination size [-Werror=stringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
---

gcc may not detect the off-by-one error in libxl__prepare_sockaddr_un, fix the 
strncpy usage anyway.

 tools/libvchan/vchan-socket-proxy.c | 8 ++++----
 tools/libxl/libxl_utils.c           | 2 +-
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/tools/libvchan/vchan-socket-proxy.c 
b/tools/libvchan/vchan-socket-proxy.c
index 13700c5d67..b312f05ca7 100644
--- a/tools/libvchan/vchan-socket-proxy.c
+++ b/tools/libvchan/vchan-socket-proxy.c
@@ -140,7 +140,7 @@ static int set_nonblocking(int fd, int nonblocking) {
 static int connect_socket(const char *path_or_fd) {
     int fd;
     char *endptr;
-    struct sockaddr_un addr;
+    struct sockaddr_un addr = {};
 
     fd = strtoll(path_or_fd, &endptr, 0);
     if (*endptr == '\0') {
@@ -153,7 +153,7 @@ static int connect_socket(const char *path_or_fd) {
         return -1;
 
     addr.sun_family = AF_UNIX;
-    strncpy(addr.sun_path, path_or_fd, sizeof(addr.sun_path));
+    strncpy(addr.sun_path, path_or_fd, sizeof(addr.sun_path) - 1);
     if (connect(fd, (const struct sockaddr *)&addr, sizeof(addr)) == -1) {
         close(fd);
         return -1;
@@ -167,7 +167,7 @@ static int connect_socket(const char *path_or_fd) {
 static int listen_socket(const char *path_or_fd) {
     int fd;
     char *endptr;
-    struct sockaddr_un addr;
+    struct sockaddr_un addr = {};
 
     fd = strtoll(path_or_fd, &endptr, 0);
     if (*endptr == '\0') {
@@ -180,7 +180,7 @@ static int listen_socket(const char *path_or_fd) {
         return -1;
 
     addr.sun_family = AF_UNIX;
-    strncpy(addr.sun_path, path_or_fd, sizeof(addr.sun_path));
+    strncpy(addr.sun_path, path_or_fd, sizeof(addr.sun_path) - 1);
     if (bind(fd, (const struct sockaddr *)&addr, sizeof(addr)) == -1) {
         close(fd);
         return -1;
diff --git a/tools/libxl/libxl_utils.c b/tools/libxl/libxl_utils.c
index f360f5e228..83592e829d 100644
--- a/tools/libxl/libxl_utils.c
+++ b/tools/libxl/libxl_utils.c
@@ -1259,7 +1259,7 @@ int libxl__prepare_sockaddr_un(libxl__gc *gc,
     }
     memset(un, 0, sizeof(struct sockaddr_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);
     return 0;
 }