Limit the max number of splitting.

[a2ush]

What type of PR is this?
/kind feature

What this PR does / why we need it:

Currently, kubernetes api-server authenticates a request that has header that includes "bearer", "token", and " strings"(space + strings).
For example, we can get the information form the api-server when we do the bellow command in a pod with serviceAccount.

root@test:/# for i in {0..10000};do echo -n $i' ' >> 10k; done
root@test:/# KK=`cat 10k`
root@test:/# TOKEN=`cat /var/run/secrets/kubernetes.io/serviceaccount/token`
root@test:/# curl -H "Authorization: Bearer ${TOKEN} ${KK}" --cacert /var/run/secrets/kubernetes.io/serviceaccount/ca.crt https://kubernetes/api/v1/namespaces/default/pods
{
  "kind": "PodList",
  "apiVersion": "v1",
  "metadata": {
...

If there is the header including many spaces and strings, unnecessary slicing will occur, and processing time will be late.

I confirm that strings.Split() and strings.SplitN() are doing the same[1][2].
So I think it is better to limit the max number of splitting with strings.SplitN().

And also, as far as I read the RFC[3], I think the format "Authorization: Bearer token" is the expected to be specify to the header.
Even if there is a value(space + strings) after the token, this value is not used in any place.

In addition, if we change the condition from len(parts) > 2 to len(parts)!= 2, we can prevent the api-server from doing a authentication process when a invalid value inputs after token.
In a typical web server (apache, nginx and so on), the maximum size of HTTP header values is about 8K.(That is, they have the limit of the header length.)

So, I think it is better to set the limit to header, and if there is the format except "Authorization: Bearer token" in the header, kubernetes api-server should not perform the authentication process.

Special notes for your reviewer:

I think this PR is close to the specification change.
If you have opinions about this change (especially changing the condition from len(parts) > 2 to len(parts)!= 2), could you tell me that?

Does this PR introduce a user-facing change?:

None

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:
[1] https://golang.org/src/strings/strings.go?s=7164:7198#L289
[2] https://golang.org/src/strings/strings.go?h=func+genSplit#236
[3] https://tools.ietf.org/html/rfc6750#section-2.1

[k8s-ci-robot]

Hi @azush26. Thanks for your PR.

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[a2ush]

/assign @mikedanese

[liggitt]

/hold

Since this is a breaking change, the scope of the impact and the benefit needs to be made clear and justify the break.

[liggitt]

/hold cancel
/ok-to-test

go ahead and squash to a single commit and add a unit test showing this behaves the same if there are trailing spaces

[fedebongio]

/remove-sig api-machinery

[a2ush]

go ahead and squash to a single commit and add a unit test showing this behaves the same if there are trailing spaces

I squashed the previous commit to a single one, and added a unit test.
Could you confirm that?

[liggitt]

/lgtm
/approve
/milestone v1.20

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: azush26, liggitt

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