diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
index 504bcfe911ead..c7da2bd7c3286 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
@@ -15,6 +15,7 @@ sout=""
 cin=""
 cinfail=""
 cinsent=""
+tmpfile=""
 cout=""
 capout=""
 ns1=""
@@ -168,6 +169,7 @@ cleanup()
 {
 	rm -f "$cin" "$cout" "$sinfail"
 	rm -f "$sin" "$sout" "$cinsent" "$cinfail"
+	rm -f "$tmpfile"
 	cleanup_partial
 }
 
@@ -362,9 +364,16 @@ check_transfer()
 			fail_test
 			return 1
 		fi
-		bytes="--bytes=${bytes}"
+
+		# note: BusyBox's "cmp" command doesn't support --bytes
+		tmpfile=$(mktemp)
+		head --bytes="$bytes" "$in" > "$tmpfile"
+		mv "$tmpfile" "$in"
+		head --bytes="$bytes" "$out" > "$tmpfile"
+		mv "$tmpfile" "$out"
+		tmpfile=""
 	fi
-	cmp -l "$in" "$out" ${bytes} | while read -r i a b; do
+	cmp -l "$in" "$out" | while read -r i a b; do
 		local sum=$((0${a} + 0${b}))
 		if [ $check_invert -eq 0 ] || [ $sum -ne $((0xff)) ]; then
 			echo "[ FAIL ] $what does not match (in, out):"