lib/fs/readlink/: readlinknul(): Add function

Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/Makefile.am

@@ -103,6 +103,8 @@ libshadow_la_SOURCES = \
 	find_new_sub_gids.c \
 	find_new_sub_uids.c \
 	fputsx.c \
+	fs/readlink/readlinknul.c \
+	fs/readlink/readlinknul.h \
 	get_pid.c \
 	getdate.h \
 	getdate.y \

lib/fs/readlink/readlinknul.c

@@ -0,0 +1,13 @@
+// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#include <config.h>
+
+#include "fs/readlink/readlinknul.h"
+
+#include <stddef.h>
+
+
+extern inline int readlinknul(const char *restrict link, char *restrict buf,
+    size_t size);

lib/fs/readlink/readlinknul.h

@@ -0,0 +1,46 @@
+// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#ifndef SHADOW_INCLUDE_LIB_FS_READLINK_READLINKNUL_H_
+#define SHADOW_INCLUDE_LIB_FS_READLINK_READLINKNUL_H_
+
+
+#include <config.h>
+
+#include <errno.h>
+#include <stddef.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "attr.h"
+
+
+ATTR_STRING(1)
+inline int readlinknul(const char *restrict link, char *restrict buf,
+    size_t size);
+
+
+// Similar to readlink(2), but terminate the string.
+inline int
+readlinknul(const char *restrict link, char *restrict buf, size_t size)
+{
+	ssize_t  len;
+
+	len = readlink(link, buf, size);
+	if (len == -1)
+		return -1;
+
+	if (len == size) {
+		stpcpy(&buf[size-1], "");
+		errno = E2BIG;
+		return -1;
+	}
+
+	stpcpy(&buf[len], "");
+	return len;
+}
+
+
+#endif  // include guard