List command bug fix, easier debugging

- List command was generating bad data size because it was adding
directory entry char to buffer before testing buffer overflow.
- Added MAVLINK_FTP_DEBUG define to easily turn on/off noisier
debugging output

src/modules/mavlink/mavlink_ftp.cpp

@@ -39,6 +39,9 @@
 
 #include "mavlink_ftp.h"
 
+// Uncomment the line below to get better debug output. Never commit with this left on.
+//#define MAVLINK_FTP_DEBUG
+
 MavlinkFTP *MavlinkFTP::_server;
 
 MavlinkFTP *
@@ -125,7 +128,9 @@ MavlinkFTP::_worker(Request *req)
 		warnx("ftp: bad crc");
 	}
 
-	//printf("ftp: channel %u opc %u size %u offset %u\n", req->channel(), hdr->opcode, hdr->size, hdr->offset);
+#ifdef MAVLINK_FTP_DEBUG
+	printf("ftp: channel %u opc %u size %u offset %u\n", req->channel(), hdr->opcode, hdr->size, hdr->offset);
+#endif
 
 	switch (hdr->opcode) {
 	case kCmdNone:
@@ -172,7 +177,9 @@ out:
 	// handle success vs. error
 	if (errorCode == kErrNone) {
 		hdr->opcode = kRspAck;
-		//warnx("FTP: ack\n");
+#ifdef MAVLINK_FTP_DEBUG
+		warnx("FTP: ack\n");
+#endif
 	} else {
 		warnx("FTP: nak %u", errorCode);
 		hdr->opcode = kRspNak;
@@ -217,7 +224,9 @@ MavlinkFTP::_workList(Request *req)
 		return kErrNotDir;
 	}
     
-	//warnx("FTP: list %s offset %d", dirPath, hdr->offset);
+#ifdef MAVLINK_FTP_DEBUG
+	warnx("FTP: list %s offset %d", dirPath, hdr->offset);
+#endif
 
 	ErrorCode errorCode = kErrNone;
 	struct dirent entry, *result = nullptr;
@@ -247,11 +256,13 @@ MavlinkFTP::_workList(Request *req)
 
 		uint32_t fileSize = 0;
 		char buf[256];
+		char direntType;
 
-		// store the type marker
+		// Determine the directory entry type
 		switch (entry.d_type) {
 		case DTYPE_FILE:
-			hdr->data[offset++] = kDirentFile;
+			// For files we get the file size as well
+			direntType = kDirentFile;
 			snprintf(buf, sizeof(buf), "%s/%s", dirPath, entry.d_name);
 			struct stat st;
 			if (stat(buf, &st) == 0) {
@@ -259,29 +270,34 @@ MavlinkFTP::_workList(Request *req)
 			}
 			break;
 		case DTYPE_DIRECTORY:
-			hdr->data[offset++] = kDirentDir;
+			direntType = kDirentDir;
 			break;
 		default:
-			hdr->data[offset++] = kDirentUnknown;
+			direntType = kDirentUnknown;
 			break;
 		}
 		
 		if (entry.d_type == DTYPE_FILE) {
+			// Files send filename and file length
 			snprintf(buf, sizeof(buf), "%s\t%d", entry.d_name, fileSize);
 		} else {
+			// Everything else just sends name
 			strncpy(buf, entry.d_name, sizeof(buf));
 			buf[sizeof(buf)-1] = 0;
 		}
 		size_t nameLen = strlen(buf);
 
-		// name too big to fit?
+		// Do we have room for the name, the one char directory identifier and the null terminator?
-		if ((nameLen + offset + 2) > kMaxDataLength) {
+		if ((offset + nameLen + 2) > kMaxDataLength) {
 			break;
 		}
 		
-		// copy the name, which we know will fit
+		// Move the data into the buffer
+		hdr->data[offset++] = direntType;
 		strcpy((char *)&hdr->data[offset], buf);
-		//printf("FTP: list %s %s\n", dirPath, (char *)&hdr->data[offset-1]);
+#ifdef MAVLINK_FTP_DEBUG
+		printf("FTP: list %s %s\n", dirPath, (char *)&hdr->data[offset-1]);
+#endif
 		offset += nameLen + 1;
 	}
 
@@ -342,7 +358,9 @@ MavlinkFTP::_workRead(Request *req)
 	}
 
 	// Seek to the specified position
-	//warnx("seek %d", hdr->offset);
+#ifdef MAVLINK_FTP_DEBUG
+	warnx("seek %d", hdr->offset);
+#endif
 	if (lseek(_session_fds[session_index], hdr->offset, SEEK_SET) < 0) {
 		// Unable to see to the specified location
 		warnx("seek fail");

src/modules/mavlink/mavlink_ftp.h

@@ -154,15 +154,17 @@ private:
 
 			if (!success) {
 				warnx("FTP TX ERR");
-			} 
+			}
-			// else {
+#ifdef MAVLINK_FTP_DEBUG
-				// warnx("wrote: sys: %d, comp: %d, chan: %d, len: %d, checksum: %d",
+			else {
-				// 	_mavlink->get_system_id(),
+				warnx("wrote: sys: %d, comp: %d, chan: %d, len: %d, checksum: %d",
-				// 	_mavlink->get_component_id(),
+				      _mavlink->get_system_id(),
-				// 	_mavlink->get_channel(),
+				      _mavlink->get_component_id(),
-				// 	len,
+				      _mavlink->get_channel(),
-				// 	msg.checksum);
+				      len,
-			// }
+				      msg.checksum);
+			}
+#endif
 		}
 
 		uint8_t		*rawData() { return &_message.data[0]; }