bin/56443: dd conv=swab doesn't always work

[rvp%SDF.ORG@localhost]

>Number:         56443
>Category:       bin
>Synopsis:       dd conv=swab doesn't always work
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Oct 06 07:35:00 +0000 2021
>Originator:     RVP
>Release:        NetBSD/amd64 9.99.90
>Organization:
>Environment:
NetBSD x202e.localdomain 9.99.90 NetBSD 9.99.90 (MYKERNEL) #0: Wed Oct  6 03:29:01 UTC 2021  bld@x202e.localdomain:/usr/obj/sys/arch/amd64/compile/MYKERNEL amd64
>Description:
dd conv=swab doesn't swap the final pair of bytes at the end of input or
of the buffer:

$ printf 'abcdefgh' | dd conv=swab msgfmt=quiet
badcfegh
$ printf 'abcdefgh' | dd conv=swab msgfmt=quiet bs=4
bacdfegh
$
>How-To-Repeat:
See above, and also run this test script:

---START dd_test.sh---
#!/bin/sh

set -eu -o pipefail
opt="conv=swab msgfmt=quiet"
# make a ~1K buffer 
data=$(jot -b abcdefghijklmnopqrstuvwxyz 40 | tr -d '\n')
for n in $(jot 521)	# a prime no. > def. block size
do	in=$(printf '%.*s' $n "$data" | ./swab)
	out=$(printf '%.*s' $n "$data" | dd $opt)
	if [ "$in" != "$out" ]
	then	echo >&2 "$0: mismatch @ len $n:"
		echo >&2 "exp: $in"
		echo >&2 "got: $out"
		exit 1
	fi
done
---END dd_test.sh---

which needs this program to be compiled:

---START swab.c---
/**
 * swap byte-pairs
 */
#include <stdio.h>

int
main(void)
{
	int c1, c2;

	for (;;) {
		if ((c1 = getchar()) == EOF)
			break;
		if ((c2 = getchar()) == EOF) {
			printf("%c", c1);
			break;
		}
		printf("%c%c", c2, c1);
	}
	return 0;
}
---END swab.c---
>Fix:
diff -urN dd.orig/dd_swab.c dd/dd_swab.c
--- dd.orig/dd_swab.c	2019-10-04 08:57:38.000000000 +0000
+++ dd/dd_swab.c	2021-09-18 09:18:12.127706233 +0000
@@ -71,7 +71,7 @@
 	}
 
 	/* round to multiple of 8 */
-	while ((--len % 8) != 0)
+	for ( ;(len % 8) != 0; --len)
 		STEP;
 	len /= 8;
 	if (len == 0)