[glibc] nis: Fix stringop-truncation warning with -O3 in nis_local_host.

[Stefan Liebler <stli@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=1e0c8356f591a62df9725b6c9387da78002ba412

commit 1e0c8356f591a62df9725b6c9387da78002ba412
Author: Stefan Liebler <stli@linux.ibm.com>
Date:   Tue Feb 28 13:37:35 2023 +0100

    nis: Fix stringop-truncation warning with -O3 in nis_local_host.
    
    When building with -O3 on s390x/x86_64, I get this stringop-truncation warning
    which leads to a build fail:
    
    In function ‘nis_local_host’,
        inlined from ‘nis_local_host’ at nis_local_names.c:147:1:
    nis_local_names.c:171:11: error: ‘strncpy’ output may be truncated copying between 0 and 1023 bytes from a string of length 1024 [-Werror=stringop-truncation]
    171 |           strncpy (cp, nis_local_directory (), NIS_MAXNAMELEN - len -1);
           |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
    We can just ignore this warning as the hostname + '.' + directory-name + '\0' always fits
    in __nishostname with length of (NIS_MAXNAMELEN + 1) as there is the runtime check above.
    Furthermore as we already know the length of the directory-name, we can also just use
    memcpy to copy the directory-name inclusive the NUL-termination.
    
    Note: This werror was introduced with commit
    32c7acd46401530fdbd4e98508c9baaa705f8b53
    "Replace rawmemchr (s, '\0') with strchr"
    
    Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

Diff:
---
 nis/nis_local_names.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/nis/nis_local_names.c b/nis/nis_local_names.c
index e685255300..699ca04e66 100644
--- a/nis/nis_local_names.c
+++ b/nis/nis_local_names.c
@@ -161,15 +161,19 @@ nis_local_host (void)
 	  if (cp[-1] == '.')
 	    return __nishostname;
 
-	  if (len + strlen (nis_local_directory ()) + 1 > NIS_MAXNAMELEN)
+	  nis_name local_directory = nis_local_directory ();
+	  size_t local_directory_len = strlen (local_directory);
+	  if (len + 1 + local_directory_len > NIS_MAXNAMELEN)
 	    {
 	      __nishostname[0] = '\0';
 	      return __nishostname;
 	    }
 
+	  /* We have enough space in __nishostname with length of
+	     (NIS_MAXNAMELEN + 1) for
+	     hostname + '.' + directory-name + '\0'.  */
 	  *cp++ = '.';
-	  strncpy (cp, nis_local_directory (), NIS_MAXNAMELEN - len -1);
-	  __nishostname[NIS_MAXNAMELEN] = '\0';
+	  memcpy (cp, local_directory, local_directory_len + 1);
 	}
     }