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encoding/asn1: When slicing bytes specify capacity or allocate new slice #14882

rolandshoemaker opened this issue Mar 19, 2016 · 3 comments


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@rolandshoemaker rolandshoemaker commented Mar 19, 2016

parseField should use triple slice assignments ([x:y:z]) to specify the capacity of slices when dealing with RawContent and RawValue fields. Defining the capacity of these slices will prevent other fields in a struct that reference the same underlying byte slice from inadvertently being mutated by operations that attempt to expand the slice (i.e. append).

  1. What version of Go are you using (go version)?
  2. What operating system and processor architecture are you using (go env)?
    Ubuntu 15.10 amd64
  3. What did you do?
    Basic example:
package main

import "fmt"
import "encoding/asn1"

type outer struct {
    Raw     asn1.RawContent
    Version int `asn1:"optional,default:1,tag:0"`
    Inner   inner
    Other   asn1.RawValue


type inner struct {
    Raw     asn1.RawContent
    Version int `asn1:"optional,default:2,tag:2"`

func main() {
    a := outer{Version: 1, Inner: inner{Version:2}, Other: asn1.RawValue{1, 1, false, []byte{10}, []byte{1, 1, 10}}}
    out, _ := asn1.Marshal(a)
    var b outer
    asn1.Unmarshal(out, &b)

    // Since b.Inner.Raw and b.Raw share the shame underlying slice when
    // append extends b.Inner.Raw (since it has no capacity) it is extending
    // into the part of the underlying slice which is referenced by b.Raw and in
    // doing so overwrites a section of that slice
    _ = append((&b).Inner.Raw, []byte{255, 255, 255}...)
  1. What did you expect to see?
    In above example b.Raw shouldn't be affected by using append
  2. What did you see instead?
    The last three bytes of b.Raw are overwritten.
@rolandshoemaker rolandshoemaker changed the title encoding/asn1: When slicing bytes specify capacity encoding/asn1: When slicing bytes specify capacity or allocate new slice Mar 20, 2016
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@rolandshoemaker rolandshoemaker commented Mar 20, 2016

Thinking about it setting the capacity may not be the right approach since it means those arrays can never be grown without causing a panic. Perhaps it would be better (although less efficient) to create new slices for each field and copy into them...

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@bradfitz bradfitz commented Mar 20, 2016

Or we just document that RawContent aliases other []byte fields.

I'm not sure there's precedent for setting slice capacity in Go APIs as a safety measure.

/cc @agl in case he has an opinion.

@bradfitz bradfitz added this to the Unplanned milestone Mar 20, 2016
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@gopherbot gopherbot commented May 9, 2020

Change mentions this issue: encoding/asn1: document RawContent aliases input

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