mavlink_ftp: fix stack overflow & add root dir prefix

- change memory allocation from stack to a malloc'd buffer. This avoids
  increasing the stack size. And since FTP is rarely used, the buffers
  are only allocated upon use and freed after a time of 2s inactivity.
- adds PX4_ROOTFSDIR as root directory prefix. This does not change
  anything on NuttX, but in SITL it will avoid enumerating the whole
  disk tree when using QGC (which enumerates all files recursively).

src/modules/mavlink/mavlink_ftp.h

@@ -39,6 +39,7 @@
 #include <dirent.h>
 #include <queue.h>
 
+#include <px4_defines.h>
 #include <systemlib/err.h>
 #include <drivers/drv_hrt.h>
 
@@ -146,6 +147,12 @@ private:
 	uint8_t _getServerComponentId(void);
 	uint8_t _getServerChannel(void);
 
+	/**
+	 * make sure that the working buffers _work_buffer* are allocated
+	 * @return true if buffers exist, false if allocation failed
+	 */
+	bool _ensure_buffers_exist();
+
 	static const char	kDirentFile = 'F';	///< Identifies File returned from List command
 	static const char	kDirentDir = 'D';	///< Identifies Directory returned from List command
 	static const char	kDirentSkip = 'S';	///< Identifies Skipped entry from List command
@@ -173,6 +180,21 @@ private:
 	MavlinkFTP(const MavlinkFTP &);
 	MavlinkFTP operator=(const MavlinkFTP &);
 
+	/* work buffers: they're allocated as soon as we get the first request (lazy, since FTP is rarely used) */
+	char *_work_buffer1;
+	static constexpr int _work_buffer1_len = kMaxDataLength;
+	char *_work_buffer2;
+	static constexpr int _work_buffer2_len = 256;
+	hrt_abstime _last_work_buffer_access; ///< timestamp when the buffers were last accessed
+
+	// prepend a root directory to each file/dir access to avoid enumerating the full FS tree (e.g. on Linux).
+	// Note that requests can still fall outside of the root dir by using ../..
+#ifdef MAVLINK_FTP_UNIT_TEST
+	static constexpr const char _root_dir[] = "";
+#else
+	static constexpr const char _root_dir[] = PX4_ROOTFSDIR;
+#endif
+	static constexpr const int _root_dir_len = sizeof(_root_dir) - 1;
 
 	// Mavlink test needs to be able to call send
 	friend class MavlinkFtpTest;