Index: net/websockets/websocket_deflater.cc |
diff --git a/net/websockets/websocket_deflater.cc b/net/websockets/websocket_deflater.cc |
index 9144c0116dc1ab7a13084c45c2ad5534069df4ae..9e577ec33c34644e1b4d6f7b41c8d57a052f6838 100644 |
--- a/net/websockets/websocket_deflater.cc |
+++ b/net/websockets/websocket_deflater.cc |
@@ -31,11 +31,33 @@ bool WebSocketDeflater::Initialize(int window_bits) { |
DCHECK_LE(8, window_bits); |
DCHECK_GE(15, window_bits); |
+ |
+ // Use a negative value to compress a raw deflate stream. |
+ // |
+ // Upgrade window_bits = 8 to 9 because zlib is unable to compress at |
+ // window_bits = 8. Historically, zlib has silently increased the window size |
+ // during compression in this case, although this is no longer done for raw |
+ // deflate streams since zlib 1.2.9. |
+ // |
+ // Because of a zlib deflate quirk, back-references will not use the entire |
+ // range of 1 << window_bits, but will instead use a restricted range of (1 << |
+ // window_bits) - 262. With an increased window_bits = 9, back-references will |
+ // be within a range of 250. These can still be decompressed with window_bits |
+ // = 8 and the 256-byte window used there. |
+ // |
+ // Both the requirement to do this upgrade and the ability to compress with |
+ // window_bits = 9 while expecting a decompressor to function with window_bits |
+ // = 8 are quite specific to zlib's particular deflate implementation, but not |
+ // specific to any particular inflate implementation. |
+ // |
+ // See https://crbug.com/691074 |
+ window_bits = -std::max(window_bits, 9); |
+ |
memset(stream_.get(), 0, sizeof(*stream_)); |
int result = deflateInit2(stream_.get(), |
Z_DEFAULT_COMPRESSION, |
Z_DEFLATED, |
- -window_bits, // Negative value for raw deflate |
+ window_bits, |
8, // default mem level |
Z_DEFAULT_STRATEGY); |
if (result != Z_OK) { |