compress/lzw: reject writing bytes that don't fit into litWidth.

Fixes #11142.

Change-Id: Id772c4364c47776d6afe86b0939b9c6281e85edc
Reviewed-on: https://go-review.googlesource.com/11227
Reviewed-by: Russ Cox <rsc@golang.org>

diff --git a/src/compress/lzw/writer.go b/src/compress/lzw/writer.go
index e9314fc4740bdb7aef0177ea12d89357670db721..7367c29651d772b6a6f5eeba0ba0af5ac1959d57 100644 (file)
--- a/src/compress/lzw/writer.go
+++ b/src/compress/lzw/writer.go
@@ -138,16 +138,23 @@ func (e *encoder) Write(p []byte) (n int, err error) {
        if len(p) == 0 {
                return 0, nil
        }
+       if maxLit := uint8(1<<e.litWidth - 1); maxLit != 0xff {
+               for _, x := range p {
+                       if x > maxLit {
+                               e.err = errors.New("lzw: input byte too large for the litWidth")
+                               return 0, e.err
+                       }
+               }
+       }
        n = len(p)
-       litMask := uint32(1<<e.litWidth - 1)
        code := e.savedCode
        if code == invalidCode {
                // The first code sent is always a literal code.
-               code, p = uint32(p[0])&litMask, p[1:]
+               code, p = uint32(p[0]), p[1:]
        }
 loop:
        for _, x := range p {
-               literal := uint32(x) & litMask
+               literal := uint32(x)
                key := code<<8 | literal
                // If there is a hash table hit for this key then we continue the loop
                // and do not emit a code yet.

diff --git a/src/compress/lzw/writer_test.go b/src/compress/lzw/writer_test.go
index 3e4e6de2114772ebd51ced82f45ca5f2342b4269..c20d058f8d3df34d2f9abfb839643831539447b6 100644 (file)
--- a/src/compress/lzw/writer_test.go
+++ b/src/compress/lzw/writer_test.go
@@ -104,6 +104,16 @@ func TestWriterReturnValues(t *testing.T) {
        }
 }
 
+func TestSmallLitWidth(t *testing.T) {
+       w := NewWriter(ioutil.Discard, LSB, 2)
+       if _, err := w.Write([]byte{0x03}); err != nil {
+               t.Fatalf("write a byte < 1<<2: %v", err)
+       }
+       if _, err := w.Write([]byte{0x04}); err == nil {
+               t.Fatal("write a byte >= 1<<2: got nil error, want non-nil")
+       }
+}
+
 func benchmarkEncoder(b *testing.B, n int) {
        b.StopTimer()
        b.SetBytes(int64(n))