[RFC PATCH] libvduse: Do not truncate terminating NUL character with strncpy()

["Philippe Mathieu-Daudé via" <qemu-devel@nongnu.org>]

GCC 8 added a -Wstringop-truncation warning:

  The -Wstringop-truncation warning added in GCC 8.0 via r254630 for
  bug 81117 is specifically intended to highlight likely unintended
  uses of the strncpy function that truncate the terminating NUL
  character from the source string.

Here the next line indeed unconditionally zeroes the last byte, so
we can call strncpy() on the buffer size less the last byte. This
fixes when using gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0:

  [42/666] Compiling C object subprojects/libvduse/libvduse.a.p/libvduse.c.o
  FAILED: subprojects/libvduse/libvduse.a.p/libvduse.c.o
  cc -m64 -mcx16 -Isubprojects/libvduse/libvduse.a.p -Isubprojects/libvduse -I../../subprojects/libvduse [...] -o subprojects/libvduse/libvduse.a.p/libvduse.c.o -c ../../subprojects/libvduse/libvduse.c
  In file included from /usr/include/string.h:495,
                   from ../../subprojects/libvduse/libvduse.c:24:
  In function ‘strncpy’,
      inlined from ‘vduse_dev_create’ at ../../subprojects/libvduse/libvduse.c:1312:5:
  /usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 256 equals destination size [-Werror=stringop-truncation]
    106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors
  ninja: build stopped: cannot make progress due to previous errors.

Fixes: d9cf16c0be ("libvduse: Replace strcpy() with strncpy()")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
Cc: Xie Yongji <xieyongji@bytedance.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>

RFC: Any better idea? We can't use strpadcpy() because libvduse
doesn't depend on QEMU.
---
 subprojects/libvduse/libvduse.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/subprojects/libvduse/libvduse.c b/subprojects/libvduse/libvduse.c
index 1a5981445c..e460780ce3 100644
--- a/subprojects/libvduse/libvduse.c
+++ b/subprojects/libvduse/libvduse.c
@@ -1309,7 +1309,7 @@ VduseDev *vduse_dev_create(const char *name, uint32_t device_id,
         goto err_dev;
     }
 
-    strncpy(dev_config->name, name, VDUSE_NAME_MAX);
+    strncpy(dev_config->name, name, VDUSE_NAME_MAX - 1);
     dev_config->name[VDUSE_NAME_MAX - 1] = '\0';
     dev_config->device_id = device_id;
     dev_config->vendor_id = vendor_id;
-- 
2.37.3