[posix] avoid CLI buffer overflow (#6578)

script/check-posix-pty

@@ -134,6 +134,20 @@ do_check()
         sudo "${OT_CTL}" -I "${NETIF_NAME}" factoryreset
         # sleep a while for daemon ready
         sleep 2
+
+        readonly OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH=640
+        local -r kMaxStringLength="$((OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH - 1))"
+
+        # verify success if command length doesn't exceed the limit
+        for len in $(seq 1 ${kMaxStringLength}); do
+            sudo "${OT_CTL}" -I "${NETIF_NAME}" "$(printf '1%.0s' $(seq 1 "${len}"))"
+        done
+
+        # verify failure if command length exceeds the limit
+        len=${OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH}
+        if sudo "${OT_CTL}" -I "${NETIF_NAME}" "$(printf '1%.0s' $(seq 1 "${len}"))"; then
+            die
+        fi
         OT_CLI_CMD="${OT_CTL} -I ${NETIF_NAME}"
     else
         OT_CLI="$PWD/build/posix/src/posix/ot-cli"