Add magic value

lib/Logging/Logging.cpp

@@ -8,14 +8,25 @@
 // Simple ring buffer log, useful for error reporting when we encounter a crash
 RTC_NOINIT_ATTR char logMessages[MAX_LOG_LINES][MAX_ENTRY_LEN];
 RTC_NOINIT_ATTR size_t logHead = 0;
+// Magic word written alongside logHead to detect uninitialized RTC memory.
+// RTC_NOINIT_ATTR is not zeroed on cold boot, so logHead may appear in-range
+// (0..MAX_LOG_LINES-1) by chance even though logMessages is garbage. The magic
+// value is only set by clearLastLogs(), so its absence means the buffer was
+// never properly initialized.
+RTC_NOINIT_ATTR uint32_t rtcLogMagic;
+static constexpr uint32_t fnv1a32(const char* s, uint32_t h = 2166136261u) {
+  return *s ? fnv1a32(s + 1, (h ^ static_cast<uint32_t>(*s)) * 16777619u) : h;
+}
+static constexpr uint32_t LOG_RTC_MAGIC = fnv1a32("crosspoint-reader");
 
 void addToLogRingBuffer(const char* message) {
-  // Add the message to the ring buffer, overwriting old messages if necessary
-  // If RTC_NOINIT_ATTR left logHead out of range on cold boot, all slots are
-  // garbage too — clear the entire buffer so subsequent reads are safe.
-  if (logHead >= MAX_LOG_LINES) {
+  // Add the message to the ring buffer, overwriting old messages if necessary.
+  // If the magic is wrong or logHead is out of range (RTC_NOINIT_ATTR garbage
+  // on cold boot), clear the entire buffer so subsequent reads are safe.
+  if (rtcLogMagic != LOG_RTC_MAGIC || logHead >= MAX_LOG_LINES) {
     memset(logMessages, 0, sizeof(logMessages));
     logHead = 0;
+    rtcLogMagic = LOG_RTC_MAGIC;
   }
   strncpy(logMessages[logHead], message, MAX_ENTRY_LEN - 1);
   logMessages[logHead][MAX_ENTRY_LEN - 1] = '\0';
@@ -81,7 +92,7 @@ std::string getLastLogs() {
 }
 
 bool sanitizeLogHead() {
-  if (logHead >= MAX_LOG_LINES) {
+  if (rtcLogMagic != LOG_RTC_MAGIC || logHead >= MAX_LOG_LINES) {
     logHead = 0;
     return true;
   }
@@ -93,4 +104,5 @@ void clearLastLogs() {
     logMessages[i][0] = '\0';
   }
   logHead = 0;
+  rtcLogMagic = LOG_RTC_MAGIC;
 }

lib/Logging/Logging.h

@@ -57,10 +57,10 @@ void logPrintf(const char* level, const char* origin, const char* format, ...);
 
 std::string getLastLogs();
 void clearLastLogs();
-// Clamps logHead into range. Returns true if logHead was out of range (repaired),
-// which also means logMessages is untrusted garbage. Callers should call
-// clearLastLogs() when this returns true so getLastLogs() does not dump corrupt
-// data into crash reports.
+// Validates the RTC log state (magic word + logHead range). Returns true if
+// corruption was detected (magic mismatch or logHead out of range), meaning
+// logMessages is untrusted garbage. Callers should call clearLastLogs() when
+// this returns true so getLastLogs() does not dump corrupt data into crash reports.
 bool sanitizeLogHead();
 
 class MySerialImpl : public Print {