Respecting fileHeader Offset, skipping bytes if necessary (#819)

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -1103,6 +1103,17 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
             this.infoHeader.VerifyDimensions();
 
+            int skipAmount = this.fileHeader.Offset - (int)this.stream.Position;
+            if ((skipAmount + (int)this.stream.Position) > this.stream.Length)
+            {
+                BmpThrowHelper.ThrowImageFormatException($"Invalid fileheader offset found. Offset is greater than the stream length.");
+            }
+
+            if (skipAmount > 0)
+            {
+                this.stream.Skip(skipAmount);
+            }
+
             return bytesPerColorMapEntry;
         }
     }

tests/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs

@@ -150,6 +150,18 @@ namespace SixLabors.ImageSharp.Tests
             }
         }
 
+        [Theory]
+        [WithFile(Pal8Offset, PixelTypes.Rgba32)]
+        public void BmpDecoder_RespectsFileHeaderOffset<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new BmpDecoder()))
+            {
+                image.DebugSave(provider);
+                image.CompareToOriginal(provider);
+            }
+        }
+
         [Theory]
         [WithFile(F, CommonNonDefaultPixelTypes)]
         public void BmpDecoder_IsNotBoundToSinglePixelType<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -218,6 +218,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Bit8Palette4 = "Bmp/pal8-0.bmp";
             public const string Os2v2Short = "Bmp/pal8os2v2-16.bmp";
             public const string Os2v2 = "Bmp/pal8os2v2.bmp";
+            public const string Pal8Offset = "Bmp/pal8offs.bmp";
 
             // Bitmap images with compression type BITFIELDS
             public const string Rgb32bfdef = "Bmp/rgb32bfdef.bmp";