Fix FAT delete of items with long name (#21528)

Marlin/src/sd/SdBaseFile.cpp

@@ -1049,6 +1049,20 @@ int16_t SdBaseFile::read(void *buf, uint16_t nbyte) {
   return nbyte;
 }
 
+/**
+ * Calculate a checksum for an 8.3 filename
+ *
+ * \param name The 8.3 file name to calculate
+ *
+ * \return The checksum byte
+ */
+uint8_t lfn_checksum(const uint8_t *name) {
+  uint8_t sum = 0;
+  for (uint8_t i = 11; i; i--)
+    sum = ((sum & 1) << 7) + (sum >> 1) + *name++;
+  return sum;
+}
+
 /**
  * Read the next entry in a directory.
  *
@@ -1065,36 +1079,47 @@ int8_t SdBaseFile::readDir(dir_t *dir, char *longFilename) {
   // if not a directory file or miss-positioned return an error
   if (!isDir() || (0x1F & curPosition_)) return -1;
 
+  #define INVALIDATE_LONGNAME() (longFilename[0] = longFilename[1] = '\0')
+
   // If we have a longFilename buffer, mark it as invalid.
   // If a long filename is found it will be filled automatically.
-  if (longFilename) { longFilename[0] = '\0'; longFilename[1] = '\0'; }
+  if (longFilename) INVALIDATE_LONGNAME();
+
+  uint8_t checksum_error = 0xFF, checksum = 0;
 
   while (1) {
 
     n = read(dir, sizeof(dir_t));
     if (n != sizeof(dir_t)) return n ? -1 : 0;
 
-    // last entry if DIR_NAME_FREE
+    // Last entry if DIR_NAME_FREE
     if (dir->name[0] == DIR_NAME_FREE) return 0;
 
-    // skip deleted entry and entry for .  and ..
+    // Skip deleted entry and entry for . and ..
     if (dir->name[0] == DIR_NAME_DELETED || dir->name[0] == '.') {
-      if (longFilename) { longFilename[0] = '\0'; longFilename[1] = '\0'; } // Invalidate erased file long name, if any
+      if (longFilename) INVALIDATE_LONGNAME();   // Invalidate erased file long name, if any
       continue;
     }
 
+    if (longFilename) {
       // Fill the long filename if we have a long filename entry.
       // Long filename entries are stored before the short filename.
-    if (longFilename && DIR_IS_LONG_NAME(dir)) {
+      if (DIR_IS_LONG_NAME(dir)) {
         vfat_t *VFAT = (vfat_t*)dir;
         // Sanity-check the VFAT entry. The first cluster is always set to zero. And the sequence number should be higher than 0
         if (VFAT->firstClusterLow == 0) {
           const uint8_t seq = VFAT->sequenceNumber & 0x1F;
           if (WITHIN(seq, 1, MAX_VFAT_ENTRIES)) {
-          // TODO: Store the filename checksum to verify if a long-filename-unaware system modified the file table.
             n = (seq - 1) * (FILENAME_LENGTH);
+            if (n == 0) {
+              checksum = VFAT->checksum;
+              checksum_error = 0;
+            }
+            else if (checksum != VFAT->checksum) // orphan detected
+              checksum_error = 1;
+
             LOOP_L_N(i, FILENAME_LENGTH) {
-            uint16_t utf16_ch = (i < 5) ? VFAT->name1[i] : (i < 11) ? VFAT->name2[i - 5] : VFAT->name3[i - 11];
+              const uint16_t utf16_ch = (i >= 11) ? VFAT->name3[i - 11] : (i >= 5) ? VFAT->name2[i - 5] : VFAT->name1[i];
               #if ENABLED(UTF_FILENAME_SUPPORT)
                 // We can't reconvert to UTF-8 here as UTF-8 is variable-size encoding, but joining LFN blocks
                 // needs static bytes addressing. So here just store full UTF-16LE words to re-convert later.
@@ -1107,10 +1132,16 @@ int8_t SdBaseFile::readDir(dir_t *dir, char *longFilename) {
               #endif
             }
             // If this VFAT entry is the last one, add a NUL terminator at the end of the string
-          if (VFAT->sequenceNumber & 0x40) longFilename[(n + FILENAME_LENGTH) * LONG_FILENAME_CHARSIZE] = '\0';
+            if (VFAT->sequenceNumber & 0x40)
+                longFilename[(n + FILENAME_LENGTH) * LONG_FILENAME_CHARSIZE] = '\0';
           }
         }
       }
+      else {
+        if (!checksum_error && lfn_checksum(dir->name) != checksum) checksum_error = 1; // orphan detected
+        if (checksum_error) INVALIDATE_LONGNAME();
+      }
+    }
 
     // Post-process normal file or subdirectory longname, if any
     if (DIR_IS_FILE_OR_SUBDIR(dir)) {