Fix Int32 overflow bug in buffering logic

src/libraries/System.IO.FileSystem/tests/FileStream/Read.cs

@@ -14,5 +14,49 @@ public void NegativeReadRootThrows()
             Assert.Throws<UnauthorizedAccessException>(() =>
                 new FileStream(Path.GetPathRoot(Directory.GetCurrentDirectory()), FileMode.Open, FileAccess.Read));
         }
+
+        [Fact]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/45954", TestPlatforms.Browser)]
+        public void NoInt32OverflowInTheBufferingLogic()
+        {
+            const long position1 = 10;
+            const long position2 = (1L << 32) + position1;
+
+            string filePath = GetTestFilePath();
+            byte[] data1 = new byte[] { 1, 2, 3, 4, 5 };
+            byte[] data2 = new byte[] { 6, 7, 8, 9, 10 };
+            byte[] buffer = new byte[5];
+
+            using (var stream = new FileStream(filePath, FileMode.Create, FileAccess.Write))
+            {
+                stream.Seek(position1, SeekOrigin.Begin);
+                stream.Write(data1, 0, data1.Length);
+
+                stream.Seek(position2, SeekOrigin.Begin);
+                stream.Write(data2, 0, data2.Length);
+            }
+
+            using (var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read))
+            {
+                stream.Seek(position1, SeekOrigin.Begin);
+                Assert.Equal(buffer.Length, stream.Read(buffer));
+                Assert.Equal(data1, buffer);
+
+                stream.Seek(position2, SeekOrigin.Begin);
+                Assert.Equal(buffer.Length, stream.Read(buffer));
+                Assert.Equal(data2, buffer);
+            }
+
+            using (var stream = new BufferedStream(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.None, bufferSize: 0)))
+            {
+                stream.Seek(position1, SeekOrigin.Begin);
+                Assert.Equal(buffer.Length, stream.Read(buffer));
+                Assert.Equal(data1, buffer);
+
+                stream.Seek(position2, SeekOrigin.Begin);
+                Assert.Equal(buffer.Length, stream.Read(buffer));
+                Assert.Equal(data2, buffer);
+            }
+        }
     }
 }

src/libraries/System.Private.CoreLib/src/System/IO/BufferedStream.cs

@@ -1222,11 +1222,12 @@ public override long Seek(long offset, SeekOrigin origin)
             // Otherwise we will throw away the buffer. This can only happen on read, as we flushed write data above.
 
             // The offset of the new/updated seek pointer within _buffer:
-            _readPos = (int)(newPos - (oldPos - _readPos));
+            long readPos = (newPos - (oldPos - _readPos));
 
             // If the offset of the updated seek pointer in the buffer is still legal, then we can keep using the buffer:
-            if (0 <= _readPos && _readPos < _readLen)
+            if (0 <= readPos && readPos < _readLen)
             {
+                _readPos = (int)readPos;
                 // Adjust the seek pointer of the underlying stream to reflect the amount of useful bytes in the read buffer:
                 _stream.Seek(_readLen - _readPos, SeekOrigin.Current);
             }

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs

@@ -974,11 +974,12 @@ public override long Seek(long offset, SeekOrigin origin)
             // Otherwise we will throw away the buffer. This can only happen on read, as we flushed write data above.
 
             // The offset of the new/updated seek pointer within _buffer:
-            _readPos = (int)(newPos - (oldPos - _readPos));
+            long readPos = (newPos - (oldPos - _readPos));
 
             // If the offset of the updated seek pointer in the buffer is still legal, then we can keep using the buffer:
-            if (0 <= _readPos && _readPos < _readLen)
+            if (0 <= readPos && readPos < _readLen)
             {
+                _readPos = (int)readPos;
                 // Adjust the seek pointer of the underlying stream to reflect the amount of useful bytes in the read buffer:
                 _strategy.Seek(_readLen - _readPos, SeekOrigin.Current);
             }