Add strncpytail_a(), and use it instead of its pattern

lib/Makefile.am

@@ -115,6 +115,8 @@ libshadow_la_SOURCES = \
 	lockpw.c \
 	loginprompt.c \
 	mail.c \
+	memory/memcpy/strncpytail.c \
+	memory/memcpy/strncpytail.h \
 	motd.c \
 	myname.c \
 	nss.c \

lib/memory/memcpy/strncpytail.c

@@ -0,0 +1,13 @@
+// SPDX-FileCopyrightText: 2025-2026, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#include "config.h"
+
+#include "memory/memcpy/strncpytail.h"
+
+#include <stddef.h>
+
+
+extern inline char *strncpytail(char *restrict dst, const char *restrict src,
+    size_t dsize);

lib/memory/memcpy/strncpytail.h

@@ -0,0 +1,38 @@
+// SPDX-FileCopyrightText: 2025-2026, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#ifndef SHADOW_INCLUDE_LIB_MEMORY_MEMCPY_STRNCPYTAIL_H_
+#define SHADOW_INCLUDE_LIB_MEMORY_MEMCPY_STRNCPYTAIL_H_
+
+
+#include "config.h"
+
+#include <memory.h>
+#include <stddef.h>
+#include <string.h>
+#include <sys/param.h>
+
+#include "attr.h"
+#include "sizeof.h"
+#include "string/strchr/strnul.h"
+
+
+// strncpytail_a - nonstring copy tail-of-string array
+#define strncpytail_a(dst, src)  strncpytail(dst, src, countof(dst))
+
+
+ATTR_STRING(2)
+inline char *strncpytail(char *restrict dst, const char *restrict src,
+    size_t dsize);
+
+
+// strncpytail - nonstring copy tail-of-string
+inline char *
+strncpytail(char *restrict dst, const char *restrict src, size_t dsize)
+{
+	return strncpy(dst, strnul(src) - MIN(strlen(src), dsize), dsize);
+}
+
+
+#endif  // include guard

lib/string/README

@@ -179,7 +179,7 @@ strcpy/ - String copying
     strncpytail()
 	Like strncpy(), but when truncating, the tail of the string is
 	kept instead of the beginning.  This is useful for ut_id.
-    STRNCPYTAIL()
+    strncpytail_a()
 	Like strncpytail, but takes an array.
 
     strncat_a()  // To be removed

lib/utmp.c

@@ -30,6 +30,7 @@
 #include "alloc/malloc.h"
 #include "attr.h"
 #include "io/syslog.h"
+#include "memory/memcpy/strncpytail.h"
 #include "sizeof.h"
 #include "string/strchr/strnul.h"
 #include "string/strcmp/streq.h"
@@ -285,12 +286,10 @@ prepare_utmp(const char *name, const char *line, const char *host,
 	utent->ut_type = USER_PROCESS;
 	utent->ut_pid = main_pid;
 	strncpy_a(utent->ut_line, line);
-	if (   (NULL != ut)
-	    && ('\0' != ut->ut_id[0])) {
+	if (NULL != ut && !strneq_a(ut->ut_id, ""))
 		strncpy_a(utent->ut_id, ut->ut_id);
-	} else {
-		strncpy_a(utent->ut_id, strnul(line) - MIN(strlen(line), countof(utent->ut_id)));
-	}
+	else
+		strncpytail_a(utent->ut_id, line);
 #if defined(HAVE_STRUCT_UTMPX_UT_NAME)
 	strncpy_a(utent->ut_name, name);
 #endif