commit 7532837d7b03b3ca5b9a63d77a5bd81dd23f3d9c
Author: Martin Sebor <msebor@redhat.com>
Date:   Wed Nov 15 17:39:59 2017 -0700

    The -Wstringop-truncation option new in GCC 8 detects common misuses
    of the strncat and strncpy function that may result in truncating
    the copied string before the terminating NUL.  To avoid false positive
    warnings for correct code that intentionally creates sequences of
    characters that aren't guaranteed to be NUL-terminated, arrays that
    are intended to store such sequences should be decorated with a new
    nonstring attribute.  This change add this attribute to Glibc and
    uses it to suppress such false positives.
    
    ChangeLog:
            * misc/sys/cdefs.h (__attribute_nonstring__): New macro.
            * sysdeps/gnu/bits/utmp.h (struct utmp): Use it.
            * sysdeps/unix/sysv/linux/s390/bits/utmp.h (struct utmp): Same.

---
 misc/sys/cdefs.h                         |    9 +++++++++
 sysdeps/gnu/bits/utmp.h                  |    9 ++++++---
 sysdeps/unix/sysv/linux/s390/bits/utmp.h |    9 ++++++---
 3 files changed, 21 insertions(+), 6 deletions(-)

--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -382,6 +382,15 @@
 # define __glibc_unlikely(cond) (cond)
 #endif
 
+#if __GNUC_PREREQ (8, 0)
+/* Describes a char array whose address can safely be passed as the first
+   argument to strncpy and strncat, as the char array is not necessarily
+   a NUL-terminated string.  */
+# define __attribute_nonstring__ __attribute__ ((__nonstring__))
+#else
+# define __attribute_nonstring__
+#endif
+
 #include <bits/wordsize.h>
 
 #if defined __LONG_DOUBLE_MATH_OPTIONAL && defined __NO_LONG_DOUBLE_MATH
--- a/sysdeps/gnu/bits/utmp.h
+++ b/sysdeps/gnu/bits/utmp.h
@@ -59,10 +59,13 @@
 {
   short int ut_type;		/* Type of login.  */
   pid_t ut_pid;			/* Process ID of login process.  */
-  char ut_line[UT_LINESIZE];	/* Devicename.  */
+  char ut_line[UT_LINESIZE]
+    __attribute_nonstring__;	/* Devicename.  */
   char ut_id[4];		/* Inittab ID.  */
-  char ut_user[UT_NAMESIZE];	/* Username.  */
-  char ut_host[UT_HOSTSIZE];	/* Hostname for remote login.  */
+  char ut_user[UT_NAMESIZE]
+    __attribute_nonstring__;	/* Username.  */
+  char ut_host[UT_HOSTSIZE]
+    __attribute_nonstring__;	/* Hostname for remote login.  */
   struct exit_status ut_exit;	/* Exit status of a process marked
 				   as DEAD_PROCESS.  */
 /* The ut_session and ut_tv fields must be the same size when compiled
--- a/sysdeps/unix/sysv/linux/s390/bits/utmp.h
+++ b/sysdeps/unix/sysv/linux/s390/bits/utmp.h
@@ -60,10 +60,13 @@
 {
   short int ut_type;		/* Type of login.  */
   pid_t ut_pid;			/* Process ID of login process.  */
-  char ut_line[UT_LINESIZE];	/* Devicename.  */
+  char ut_line[UT_LINESIZE]
+     __attribute_nonstring__;	/* Devicename.  */
   char ut_id[4];		/* Inittab ID.  */
-  char ut_user[UT_NAMESIZE];	/* Username.  */
-  char ut_host[UT_HOSTSIZE];	/* Hostname for remote login.  */
+  char ut_user[UT_NAMESIZE]
+     __attribute_nonstring__;	/* Username.  */
+  char ut_host[UT_HOSTSIZE]
+     __attribute_nonstring__;	/* Hostname for remote login.  */
   struct exit_status ut_exit;	/* Exit status of a process marked
 				   as DEAD_PROCESS.  */
 /* The ut_session and ut_tv fields must be the same size when compiled
