From eda45b4df2f8f9adf2ab79f7b8e2b2180cdf7a63 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Beat=20K=C3=BCng?= <beat-kueng@gmx.net>
Date: Fri, 14 Dec 2018 12:11:16 +0100
Subject: [PATCH] sbus: add time-based hardening (only for IO and NuttX)
Since SBUS does not have CRC, we can use timing information to improve
parsing reliability and reject unexpected bytes.
---
src/lib/rc/sbus.cpp | 55 ++++++++++++++++++++++++++++-----------------
1 file changed, 34 insertions(+), 21 deletions(-)
diff --git a/src/lib/rc/sbus.cpp b/src/lib/rc/sbus.cpp
index 7b70044c91..a765460e94 100644
--- a/src/lib/rc/sbus.cpp
+++ b/src/lib/rc/sbus.cpp
@@ -52,6 +52,8 @@
#include "common_rc.h"
#include <drivers/drv_hrt.h>
+using namespace time_literals;
+
#define SBUS_DEBUG_LEVEL 0 /* Set debug output level */
#if defined(__PX4_POSIX_OCPOC)
@@ -280,8 +282,21 @@ bool
sbus_input(int sbus_fd, uint16_t *values, uint16_t *num_values, bool *sbus_failsafe, bool *sbus_frame_drop,
uint16_t max_channels)
{
- int ret = 1;
- hrt_abstime now;
+ /*
+ * Fetch bytes, but no more than we would need to complete
+ * a complete frame.
+ */
+ uint8_t buf[SBUS_FRAME_SIZE * 2];
+
+ int ret = read(sbus_fd, &buf[0], SBUS_FRAME_SIZE);
+
+ /* if the read failed for any reason, just give up here */
+ if (ret < 1) {
+ return false;
+ }
+
+ const hrt_abstime now = hrt_absolute_time();
+#ifdef __PX4_NUTTX /* limit time-based hardening to RTOS's where we have reliable timing */
/*
* The S.BUS protocol doesn't provide reliable framing,
@@ -298,32 +313,30 @@ sbus_input(int sbus_fd, uint16_t *values, uint16_t *num_values, bool *sbus_fails
* provides a degree of protection. Of course, it would be better
* if we didn't drop bytes...
*/
- now = hrt_absolute_time();
-
- /*
- * Fetch bytes, but no more than we would need to complete
- * a complete frame.
- */
- uint8_t buf[SBUS_FRAME_SIZE * 2];
- bool sbus_decoded = false;
-
- ret = read(sbus_fd, &buf[0], SBUS_FRAME_SIZE);
+ if (now - last_rx_time > 3_ms) {
+ if (partial_frame_count > 0) {
+ partial_frame_count = 0;
+ sbus_decode_state = SBUS2_DECODE_STATE_DESYNC;
+#if defined(SBUS_DEBUG_LEVEL) && SBUS_DEBUG_LEVEL > 0
+ printf("SBUS: RESET (TIME LIM)\n");
+#endif
+ }
+ }
- /* if the read failed for any reason, just give up here */
- if (ret < 1) {
+ if (partial_frame_count == 0 && buf[0] != SBUS_START_SYMBOL) {
+ /* don't bother going through the buffer if we don't get the
+ * expected start symbol as a first byte */
+ sbus_decode_state = SBUS2_DECODE_STATE_DESYNC;
return false;
}
+#endif /* __PX4_NUTTX */
+
/*
* Try to decode something with what we got
*/
- if (sbus_parse(now, &buf[0], ret, values, num_values, sbus_failsafe,
- sbus_frame_drop, &sbus_frame_drops, max_channels)) {
-
- sbus_decoded = true;
- }
-
- return sbus_decoded;
+ return sbus_parse(now, &buf[0], ret, values, num_values, sbus_failsafe,
+ sbus_frame_drop, &sbus_frame_drops, max_channels);
}
bool
--
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