encoding: reject negative runes in Encoding.WithPadding

A negative rune (other than NoPadding) makes no semantic sense.
Doing so relies on integer overflow of converting a rune to a byte
and would thus be equivalent to passing the positive byte value
of byte(padding).

This may cause existing code to panic.
An alternative is treat negative runes as equivalent to NoPadding.
However, the code already panics to report erroneous padding values,
so this is in line with the existing API.

Change-Id: I02499705519581598adc0c8525d90e25278dc056
Reviewed-on: https://go-review.googlesource.com/c/go/+/505236
Auto-Submit: Joseph Tsai <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Joseph Tsai <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: dsnet

src/encoding/base32/base32.go

@@ -79,13 +79,13 @@ var HexEncoding = NewEncoding(encodeHex)
 
 // WithPadding creates a new encoding identical to enc except
 // with a specified padding character, or NoPadding to disable padding.
-// The padding character must not be '\r' or '\n', must not
-// be contained in the encoding's alphabet and must be a rune equal or
-// below '\xff'.
+// The padding character must not be '\r' or '\n',
+// must not be contained in the encoding's alphabet,
+// must not be negative, and must be a rune equal or below '\xff'.
 // Padding characters above '\x7f' are encoded as their exact byte value
 // rather than using the UTF-8 representation of the codepoint.
 func (enc Encoding) WithPadding(padding rune) *Encoding {
-	if padding == '\r' || padding == '\n' || padding > 0xff {
+	if padding < NoPadding || padding == '\r' || padding == '\n' || padding > 0xff {
 		panic("invalid padding")
 	}
 

src/encoding/base64/base64.go

@@ -82,13 +82,13 @@ func NewEncoding(encoder string) *Encoding {
 
 // WithPadding creates a new encoding identical to enc except
 // with a specified padding character, or NoPadding to disable padding.
-// The padding character must not be '\r' or '\n', must not
-// be contained in the encoding's alphabet and must be a rune equal or
-// below '\xff'.
+// The padding character must not be '\r' or '\n',
+// must not be contained in the encoding's alphabet,
+// must not be negative, and must be a rune equal or below '\xff'.
 // Padding characters above '\x7f' are encoded as their exact byte value
 // rather than using the UTF-8 representation of the codepoint.
 func (enc Encoding) WithPadding(padding rune) *Encoding {
-	if padding == '\r' || padding == '\n' || padding > 0xff {
+	if padding < NoPadding || padding == '\r' || padding == '\n' || padding > 0xff {
 		panic("invalid padding")
 	}