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.

1 files changed, 9 insertions, 0 deletions

diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index cfd39d5302..a603cb9df0 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -407,6 +407,15 @@
 # endif
 #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
+
 #if (!defined _Static_assert && !defined __cplusplus \
      && (defined __STDC_VERSION__ ? __STDC_VERSION__ : 0) < 201112 \
      && (!__GNUC_PREREQ (4, 6) || defined __STRICT_ANSI__))