diff --git a/lib/libarchive/archive_string.c b/lib/libarchive/archive_string.c
index 03186bfca808..120c12783feb 100644
--- a/lib/libarchive/archive_string.c
+++ b/lib/libarchive/archive_string.c
@@ -115,11 +115,11 @@ __archive_string_ensure(struct archive_string *as, size_t s)
 		as->buffer_length = 32;
 	else if (as->buffer_length < 8192)
 		/* Buffers under 8k are doubled for speed. */
-		as->buffer_length *= 2;
+		as->buffer_length += as->buffer_length;
 	else {
 		/* Buffers 8k and over grow by at least 25% each time. */
 		size_t old_length = as->buffer_length;
-		as->buffer_length = (as->buffer_length * 5) / 4;
+		as->buffer_length += as->buffer_length / 4;
 		/* Be safe: If size wraps, release buffer and return NULL. */
 		if (as->buffer_length < old_length) {
 			free(as->s);
@@ -142,10 +142,12 @@ __archive_string_ensure(struct archive_string *as, size_t s)
 }
 
 struct archive_string *
-__archive_strncat(struct archive_string *as, const char *p, size_t n)
+__archive_strncat(struct archive_string *as, const void *_p, size_t n)
 {
 	size_t s;
-	const char *pp;
+	const char *p, *pp;
+
+	p = (const char *)_p;
 
 	/* Like strlen(p), except won't examine positions beyond p[n]. */
 	s = 0;
diff --git a/lib/libarchive/archive_string.h b/lib/libarchive/archive_string.h
index 7960b8e07bb0..56ca7ca31805 100644
--- a/lib/libarchive/archive_string.h
+++ b/lib/libarchive/archive_string.h
@@ -99,8 +99,12 @@ __archive_string_ensure(struct archive_string *, size_t);
 #define	archive_string_ensure __archive_string_ensure
 
 /* Append C string, which may lack trailing \0. */
+/* The source is declared void * here because this gets used with
+ * "signed char *", "unsigned char *" and "char *" arguments.
+ * Declaring it "char *" as with some of the other functions just
+ * leads to a lot of extra casts. */
 struct archive_string *
-__archive_strncat(struct archive_string *, const char *, size_t);
+__archive_strncat(struct archive_string *, const void *, size_t);
 #define	archive_strncat  __archive_strncat
 
 /* Append a C string to an archive_string, resizing as necessary. */