encoding/hex: fix Decode output check regression

CL 461958 fixed a potential panic,
but also introduced an observable regression where
invalid input could be detected before the panic occurs.
Adjust the check to preserve prior behavior,
while also preventing the panic.

Change-Id: I52819f88a6a64883fbc9fd512697c38c29ca0ccd
Reviewed-on: https://go-review.googlesource.com/c/go/+/465855
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>

diff --git a/src/encoding/hex/hex.go b/src/encoding/hex/hex.go
index 5a8243ae80548d9a1de901911438bc63e0326b69..f69abb2f7fea766c6b0bcc55dee73491b62a89ca 100644 (file)
--- a/src/encoding/hex/hex.go
+++ b/src/encoding/hex/hex.go
@@ -75,9 +75,6 @@ func DecodedLen(x int) int { return x / 2 }
 // If the input is malformed, Decode returns the number
 // of bytes decoded before the error.
 func Decode(dst, src []byte) (int, error) {
-       if len(dst) < DecodedLen(len(src)) {
-               return 0, errors.New("encoding/hex: output buffer too small")
-       }
        i, j := 0, 1
        for ; j < len(src); j += 2 {
                p := src[j-1]
@@ -91,6 +88,9 @@ func Decode(dst, src []byte) (int, error) {
                if b > 0x0f {
                        return i, InvalidByteError(q)
                }
+               if i >= len(dst) {
+                       return i, errors.New("encoding/hex: output buffer too small")
+               }
                dst[i] = (a << 4) | b
                i++
        }

diff --git a/src/encoding/hex/hex_test.go b/src/encoding/hex/hex_test.go
index 1eb169cdee5bc6829d89f4d32a7d8b03dbe3693c..8d1ae7077443f3435bbd5a9127642dd973829fa1 100644 (file)
--- a/src/encoding/hex/hex_test.go
+++ b/src/encoding/hex/hex_test.go
@@ -55,13 +55,16 @@ func TestDecode(t *testing.T) {
        }
 }
 
-func TestDecode_tooFewDstBytes(t *testing.T) {
+func TestDecodeDstTooSmall(t *testing.T) {
        dst := make([]byte, 1)
        src := []byte{'0', '1', '2', '3'}
-       _, err := Decode(dst, src)
+       n, err := Decode(dst, src)
        if err == nil {
                t.Errorf("expected Decode to return an error, but it returned none")
        }
+       if !bytes.Equal(dst[:n], []byte{0x01}) {
+               t.Errorf("output mismatch: got %x, want 01", dst[:n])
+       }
 }
 
 func TestEncodeToString(t *testing.T) {