[PATCH v2] net/tap-win32: Fix gcc 14 format truncation errors

[Bernhard Beschow <shentey@gmail.com>]

The patch fixes the following errors generated by GCC 14.2:

../src/net/tap-win32.c:343:19: error: '%s' directive output may be truncated writing up to 255 bytes into a region of size 176 [-Werror=format-truncation=]
  343 |              "%s\\%s\\Connection",
      |                   ^~
  344 |              NETWORK_CONNECTIONS_KEY, enum_name);
      |                                       ~~~~~~~~~

../src/net/tap-win32.c:341:9: note: 'snprintf' output between 92 and 347 bytes into a destination of size 256
  341 |         snprintf(connection_string,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  342 |              sizeof(connection_string),
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~
  343 |              "%s\\%s\\Connection",
      |              ~~~~~~~~~~~~~~~~~~~~~
  344 |              NETWORK_CONNECTIONS_KEY, enum_name);
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

../src/net/tap-win32.c:242:58: error: '%s' directive output may be truncated writing up to 255 bytes into a region of size 178 [-Werror=format-truncation=]
  242 |         snprintf (unit_string, sizeof(unit_string), "%s\\%s",
      |                                                          ^~
  243 |                   ADAPTER_KEY, enum_name);
      |                                ~~~~~~~~~

../src/net/tap-win32.c:242:9: note: 'snprintf' output between 79 and 334 bytes into a destination of size 256
  242 |         snprintf (unit_string, sizeof(unit_string), "%s\\%s",
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  243 |                   ADAPTER_KEY, enum_name);
      |                   ~~~~~~~~~~~~~~~~~~~~~~~

../src/net/tap-win32.c:620:52: error: '%s' directive output may be truncated writing up to 255 bytes into a region of size 245 [-Werror=format-truncation=]
  620 |     snprintf (device_path, sizeof(device_path), "%s%s%s",
      |                                                    ^~
  621 |               USERMODEDEVICEDIR,
  622 |               device_guid,
      |               ~~~~~~~~~~~
../src/net/tap-win32.c:620:5: note: 'snprintf' output between 16 and 271 bytes into a destination of size 256
  620 |     snprintf (device_path, sizeof(device_path), "%s%s%s",
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  621 |               USERMODEDEVICEDIR,
      |               ~~~~~~~~~~~~~~~~~~
  622 |               device_guid,
      |               ~~~~~~~~~~~~
  623 |               TAPSUFFIX);
      |               ~~~~~~~~~~

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2607
Cc: qemu-stable@nongnu.org

--

This patch was just compile-tested (which fixes my issue). Testing TAP
networking under Windows apparently requires extra drivers which I don't want to
install (not my computer). So it would be nice if someone could give this patch
a test ride. Thanks!

Changes since v1:
* Use g_autofree and g_strdup_printf() rather than fixed size arrays (Peter)
---
 net/tap-win32.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/net/tap-win32.c b/net/tap-win32.c
index 7edbd71633..671dee970f 100644
--- a/net/tap-win32.c
+++ b/net/tap-win32.c
@@ -214,7 +214,7 @@ static int is_tap_win32_dev(const char *guid)
 
     for (;;) {
         char enum_name[256];
-        char unit_string[256];
+        g_autofree char *unit_string = NULL;
         HKEY unit_key;
         char component_id_string[] = "ComponentId";
         char component_id[256];
@@ -239,8 +239,7 @@ static int is_tap_win32_dev(const char *guid)
             return FALSE;
         }
 
-        snprintf (unit_string, sizeof(unit_string), "%s\\%s",
-                  ADAPTER_KEY, enum_name);
+        unit_string = g_strdup_printf("%s\\%s", ADAPTER_KEY, enum_name);
 
         status = RegOpenKeyEx(
             HKEY_LOCAL_MACHINE,
@@ -315,7 +314,7 @@ static int get_device_guid(
     while (!stop)
     {
         char enum_name[256];
-        char connection_string[256];
+        g_autofree char *connection_string = NULL;
         HKEY connection_key;
         char name_data[256];
         DWORD name_type;
@@ -338,9 +337,7 @@ static int get_device_guid(
             return -1;
         }
 
-        snprintf(connection_string,
-             sizeof(connection_string),
-             "%s\\%s\\Connection",
+        connection_string = g_strdup_printf("%s\\%s\\Connection",
              NETWORK_CONNECTIONS_KEY, enum_name);
 
         status = RegOpenKeyEx(
@@ -595,7 +592,7 @@ static void tap_win32_free_buffer(tap_win32_overlapped_t *overlapped,
 static int tap_win32_open(tap_win32_overlapped_t **phandle,
                           const char *preferred_name)
 {
-    char device_path[256];
+    g_autofree char *device_path = NULL;
     char device_guid[0x100];
     int rc;
     HANDLE handle;
@@ -617,7 +614,7 @@ static int tap_win32_open(tap_win32_overlapped_t **phandle,
     if (rc)
         return -1;
 
-    snprintf (device_path, sizeof(device_path), "%s%s%s",
+    device_path = g_strdup_printf("%s%s%s",
               USERMODEDEVICEDIR,
               device_guid,
               TAPSUFFIX);
-- 
2.47.0