Define LOG_FILEPATH_SIZE constant for maintainability

- Add LOG_FILEPATH_SIZE constant (256) in both header and cpp
- Update LogEntry.filepath to use the constant
- Improve comments explaining width specifier relationship
- Addresses code review feedback about magic numbers

Co-authored-by: dakejahl <37091262+dakejahl@users.noreply.github.com>

src/modules/mavlink/mavlink_log_handler.cpp

@@ -54,9 +54,13 @@ static const char *kLogDir = PX4_STORAGEDIR "/log";
 #define PX4_MAX_FILEPATH 	PATH_MAX
 #endif
 
+// Size for log file path buffers - must match MavlinkLogHandler::LOG_FILEPATH_SIZE
+#define LOG_FILEPATH_SIZE 256
+#define LOG_FILEPATH_MAX_CHARS (LOG_FILEPATH_SIZE - 1)  // Max chars that can be read (leave room for null terminator)
+
 // Ensure PX4_MAX_FILEPATH is large enough for our buffer sizes
-// LogEntry.filepath is 256 bytes, so PX4_MAX_FILEPATH must be at least 256
-static_assert(PX4_MAX_FILEPATH >= 256, "PX4_MAX_FILEPATH must be at least 256 bytes for log file paths");
+static_assert(PX4_MAX_FILEPATH >= LOG_FILEPATH_SIZE,
+	      "PX4_MAX_FILEPATH must be at least LOG_FILEPATH_SIZE bytes for log file paths");
 
 MavlinkLogHandler::MavlinkLogHandler(Mavlink &mavlink)
 	: _mavlink(mavlink)
@@ -178,8 +182,9 @@ void MavlinkLogHandler::state_listing()
 		char filepath[PX4_MAX_FILEPATH];
 
 		// If parsed lined successfully, send the entry
-		// Note: Using %255s to safely read into filepath buffer (max 255 chars + null terminator)
-		// This is conservative but safe across all platforms regardless of PX4_MAX_FILEPATH value
+		// Note: Using width specifier to prevent buffer overflow
+		// %255s reads max 255 chars + null terminator = 256 bytes (LOG_FILEPATH_SIZE)
+		// This is safe across all platforms (static_assert ensures PX4_MAX_FILEPATH >= LOG_FILEPATH_SIZE)
 		if (sscanf(line, "%" PRIu32 " %" PRIu32 " %255s", &time_utc, &size_bytes, filepath) != 3) {
 			PX4_DEBUG("sscanf failed");
 			continue;
@@ -512,6 +517,8 @@ bool MavlinkLogHandler::log_entry_from_id(uint16_t log_id, LogEntry *entry)
 			continue;
 		}
 
+		// Parse log entry with width specifier to prevent buffer overflow
+		// %255s = max 255 chars + null = 256 bytes (LOG_FILEPATH_SIZE)
 		if (sscanf(line, "%" PRIu32 " %" PRIu32 " %255s", &(entry->time_utc), &(entry->size_bytes), entry->filepath) != 3) {
 			PX4_DEBUG("sscanf failed");
 			continue;

src/modules/mavlink/mavlink_log_handler.h

@@ -48,12 +48,14 @@ public:
 	void handle_message(const mavlink_message_t *msg);
 
 private:
+	static constexpr size_t LOG_FILEPATH_SIZE = 256;  // Buffer size for log file paths
+
 	struct LogEntry {
 		uint16_t id{0xffff};
 		uint32_t time_utc{};
 		uint32_t size_bytes{};
 		FILE *fp{nullptr};
-		char filepath[256];
+		char filepath[LOG_FILEPATH_SIZE];
 		uint32_t offset{};
 	};