Fix sector size calculation for Disk images

Fix for: DiscUtils#247
LTRData · Feb 24, 2024 · ca3b9c3 · ca3b9c3
1 parent 8ddb30a
commit ca3b9c3
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Showing 2 changed files with 97 additions and 5 deletions.
diff --git a/Library/DiscUtils.Iso9660/CDBuilder.cs b/Library/DiscUtils.Iso9660/CDBuilder.cs
@@ -261,22 +261,32 @@ protected override List<BuilderExtent> FixExtents(out long totalLength)
         long bootCatalogPos = 0;
         if (_bootEntry != null)
         {
+            // Boot catalog MUST be at beginning
+            bootCatalogPos = focus;
+            focus += IsoUtilities.SectorSize;
             var bootImagePos = focus;
             var realBootImage = PatchBootImage(_bootImage, (uint)(DiskStart / IsoUtilities.SectorSize),
                 (uint)(bootImagePos / IsoUtilities.SectorSize));
             var bootImageExtent = new BuilderStreamExtent(focus, realBootImage);
             fixedRegions.Add(bootImageExtent);
             focus += MathUtilities.RoundUp(bootImageExtent.Length, IsoUtilities.SectorSize);
 
-            bootCatalogPos = focus;
             var bootCatalog = new byte[IsoUtilities.SectorSize];
             var bve = new BootValidationEntry();
             bve.WriteTo(bootCatalog, 0x00);
             _bootEntry.ImageStart = (uint)MathUtilities.Ceil(bootImagePos, IsoUtilities.SectorSize);
-            _bootEntry.SectorCount = (ushort)MathUtilities.Ceil(_bootImage.Length, Sizes.Sector);
+            if (_bootEntry.BootMediaType != BootDeviceEmulation.NoEmulation)
+            {
+                _bootEntry.SectorCount = 1;
+            }
+            else
+            {
+                _bootEntry.SectorCount = (ushort)MathUtilities.Ceil(_bootImage.Length, Sizes.Sector);
+            }
             _bootEntry.WriteTo(bootCatalog, 0x20);
             fixedRegions.Add(new BuilderBufferExtent(bootCatalogPos, bootCatalog));
-            focus += IsoUtilities.SectorSize;
+
+            // Don't add to focus, we already skipped the length of the bootCatalog
         }
 
         // ####################################################################
@@ -552,7 +562,7 @@ private BuildDirectoryInfo TryGetDirectory(ReadOnlyMemory<char>[] path, int path
             }
             else
             {
-                if (!(next is BuildDirectoryInfo nextAsBuildDirectoryInfo))
+                if (next is not BuildDirectoryInfo nextAsBuildDirectoryInfo)
                 {
                     throw new IOException("File with conflicting name exists");
                 }

diff --git a/Library/DiscUtils.Iso9660/VfsCDReader.cs b/Library/DiscUtils.Iso9660/VfsCDReader.cs
@@ -420,13 +420,95 @@ public UnixFileSystemInfo GetUnixFileInfo(string path)
         return file.UnixFileInfo;
     }
 
+    /// <summary>
+    /// Detects size of FAT file system from a disk's boot sector.
+    /// </summary>
+    /// <param name="stream">The stream to inspect.</param>
+    /// <returns><c>Sector count</c> if the stream appears to be a FAT file system, else <c>0</c>.</returns>
+    public static ushort DetectSectorCountFromBootSector(Stream stream)
+    {
+        if (stream.Length < 512)
+        {
+            return 0;
+        }
+
+        stream.Position = 0;
+        var bytes = stream.ReadExactly(512);
+        var bpbBytesPerSec = EndianUtilities.ToUInt16LittleEndian(bytes, 11);
+        if (bpbBytesPerSec != 512)
+        {
+            return 0;
+        }
+
+        var bpbNumFATs = bytes[16];
+        if (bpbNumFATs is 0 or > 2)
+        {
+            return 0;
+        }
+
+        var bpbTotSec16 = EndianUtilities.ToUInt16LittleEndian(bytes, 19);
+        var bpbTotSec32 = EndianUtilities.ToUInt32LittleEndian(bytes, 32);
+
+        if (!((bpbTotSec16 == 0) ^ (bpbTotSec32 == 0)))
+        {
+            return 0;
+        }
+
+        var totalSectors = bpbTotSec16 + bpbTotSec32;
+        // Can't be greater than unsigned short
+        if (totalSectors > 65535)
+        {
+            return 0;
+        }
+        return (ushort)totalSectors;
+    }
+
+
+    /// <summary>
+    /// Returns the actual sector count for a boot image (vs. reported).
+    /// 
+    /// Boot Images with MBR or floppy boot sectors will have boot Entries that 
+    /// list their sector count as 1.
+    /// This is incorrect and is either determined by the emulation type or what
+    /// the boot sector lists the partition as having.
+    /// </summary>
+    /// <returns>int: sector count in 512 bytes</returns>
+    public long GetActualBootImageSectorCount()
+    {
+        switch (BootEmulation)
+        {
+            case BootDeviceEmulation.Diskette1440KiB:
+                return 1440 * 1024 / Sizes.Sector;
+            case BootDeviceEmulation.Diskette1200KiB:
+                return 1200 * 1024 / Sizes.Sector;
+            case BootDeviceEmulation.Diskette2880KiB:
+                return 2880 * 1024 / Sizes.Sector;
+            case BootDeviceEmulation.HardDisk:
+            case BootDeviceEmulation.NoEmulation:
+            default:
+                var initialEntry = GetBootInitialEntry();
+                var BytesToStart = initialEntry.ImageStart * IsoUtilities.SectorSize;
+                var sectorCount = DetectSectorCountFromBootSector(
+                    new SubStream(_data, BytesToStart, Sizes.Sector)
+                    );
+                // Invalid length read from BootSector
+                if (sectorCount == 0 || sectorCount * Sizes.Sector + BytesToStart > _data.Length)
+                {
+                    sectorCount = initialEntry.SectorCount;
+                }
+                return sectorCount;
+        }
+
+    }
+
     public Stream OpenBootImage()
     {
         var initialEntry = GetBootInitialEntry();
+        var sectorCount = GetActualBootImageSectorCount();
         if (initialEntry != null)
         {
             return new SubStream(_data, initialEntry.ImageStart * IsoUtilities.SectorSize,
-                initialEntry.SectorCount * Sizes.Sector);
+                sectorCount * Sizes.Sector);
         }
         throw new InvalidOperationException("No valid boot image");
     }