encoding/base64: Decode panics on short dst

[a-h]

What version of Go are you using (go version)?

• go version go1.17.5 linux/arm64
• go version go1.17.3 darwin/arm64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""

GOARCH="arm64"

GOBIN=""

GOCACHE="/Users/adrian/Library/Caches/go-build"

GOENV="/Users/adrian/Library/Application Support/go/env"

GOEXE=""

GOEXPERIMENT=""

GOFLAGS=""

GOHOSTARCH="arm64"

GOHOSTOS="darwin"

GOINSECURE=""

GOMODCACHE="/Users/adrian/go/pkg/mod"

GONOPROXY=""

GONOSUMDB="*"

GOOS="darwin"

GOPATH="/Users/adrian/go"

GOPRIVATE=""

GOPROXY="https://proxy.golang.org,direct"

GOROOT="/nix/store/346ngr086m8gdyarx2f61j7mrrg3zwzf-go-1.17.3/opt/go"

GOSUMDB="sum.golang.org"

GOTMPDIR=""

GOTOOLDIR="/nix/store/346ngr086m8gdyarx2f61j7mrrg3zwzf-go-1.17.3/opt/go/pkg/tool/darwin_arm64"

GOVCS=""

GOVERSION="go1.17.3"

GCCGO="gccgo"

AR="ar"

CC="clang"

CXX="clang++"

CGO_ENABLED="1"

GOMOD="/Users/adrian/github.com/a-h/gobase64panic/go.mod"

CGO_CFLAGS="-g -O2"

CGO_CPPFLAGS=""

CGO_CXXFLAGS="-g -O2"

CGO_FFLAGS="-g -O2"

CGO_LDFLAGS="-g -O2"

PKG_CONFIG="pkg-config"

GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/41/f9l_y7sd0y99gnh9dt2z8sd80000gn/T/go-build1137988266=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

package main

import (
	"encoding/base64"
	"fmt"
)

func main() {
	input := base64.RawStdEncoding.EncodeToString([]byte("12345"))
	var output []byte
	_, err := base64.RawStdEncoding.Decode(output, []byte(input))
	if err != nil {
		fmt.Println("Received expected error")
	}
}

• https://go.dev/play/p/OuONhRn31xY

What did you expect to see?

An error returned by the Decode function.

What did you see instead?

A panic. The documentation of the function doesn't say to expect a panic on invalid data.

[a-h]

Same issue with StdEncoding: https://go.dev/play/p/iGdwQ30aeng

[seankhliao]

It's not invalid data, it's the short/nil output buffer. Maybe it can return io.ErrShortBuffer instead?

cc @rsc

[ianlancetaylor]

Many standard library functions panic on invalid nil parameter values. I don't think there is anything to do here.