capnproto

membrane.h (15500B)

---

 // Copyright (c) 2015 Sandstorm Development Group, Inc. and contributors
 // Licensed under the MIT License:
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
 // of this software and associated documentation files (the "Software"), to deal
 // in the Software without restriction, including without limitation the rights
 // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 // copies of the Software, and to permit persons to whom the Software is
 // furnished to do so, subject to the following conditions:
 //
 // The above copyright notice and this permission notice shall be included in
 // all copies or substantial portions of the Software.
 //
 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
 #pragma once
 // In capability theory, a "membrane" is a wrapper around a capability which (usually) forwards
 // calls but recursively wraps capabilities in those calls in the same membrane. The purpose of a
 // membrane is to enforce a barrier between two capabilities that cannot be bypassed by merely
 // introducing new objects.
 //
 // The most common use case for a membrane is revocation: Say Alice wants to give Bob a capability
 // to access Carol, but wants to be able to revoke this capability later. Alice can accomplish this
 // by wrapping Carol in a revokable wrapper which passes through calls until such a time as Alice
 // indicates it should be revoked, after which all calls through the wrapper will throw exceptions.
 // However, a naive wrapper approach has a problem: if Bob makes a call to Carol and sends a new
 // capability in that call, or if Carol returns a capability to Bob in the response to a call, then
 // the two are now able to communicate using this new capability, which Alice cannot revoke. In
 // order to avoid this problem, Alice must use not just a wrapper but a "membrane", which
 // recursively wraps all objects that pass through it in either direction. Thus, all connections
 // formed between Bob and Carol (originating from Alice's original introduction) can be revoked
 // together by revoking the membrane.
 //
 // Note that when a capability is passed into a membrane and then passed back out, the result is
 // the original capability, not a double-membraned capability. This means that in our revocation
 // example, if Bob uses his capability to Carol to obtain another capability from her, then send
 // it back to her, the capability Carol receives back will NOT be revoked when Bob's access to
 // Carol is revoked. Thus Bob can create long-term irrevocable connections. In most practical use
 // cases, this is what you want. APIs commonly rely on the fact that a capability obtained and then
 // passed back can be recognized as the original capability.
 //
 // Mark Miller on membranes: http://www.eros-os.org/pipermail/e-lang/2003-January/008434.html
 
 #include "capability.h"
 
 namespace capnp {
 
 class MembranePolicy {
   // Applications may implement this interface to define a membrane policy, which allows some
   // calls crossing the membrane to be blocked or redirected.
 
 public:
   virtual kj::Maybe<Capability::Client> inboundCall(
       uint64_t interfaceId, uint16_t methodId, Capability::Client target) = 0;
   // Given an inbound call (a call originating "outside" the membrane destined for an object
   // "inside" the membrane), decides what to do with it. The policy may:
   //
   // - Return null to indicate that the call should proceed to the destination. All capabilities
   //   in the parameters or result will be properly wrapped in the same membrane.
   // - Return a capability to have the call redirected to that capability. Note that the redirect
   //   capability will be treated as outside the membrane, so the params and results will not be
   //   auto-wrapped; however, the callee can easily wrap the returned capability in the membrane
   //   itself before returning to achieve this effect.
   // - Throw an exception to cause the call to fail with that exception.
   //
   // `target` is the underlying capability (*inside* the membrane) for which the call is destined.
   // Generally, the only way you should use `target` is to wrap it in some capability which you
   // return as a redirect. The redirect capability may modify the call in some way and send it to
   // `target`. Be careful to use `copyIntoMembrane()` and `copyOutOfMembrane()` as appropriate when
   // copying parameters or results across the membrane.
   //
   // Note that since `target` is inside the capability, if you were to directly return it (rather
   // than return null), the effect would be that the membrane would be broken: the call would
   // proceed directly and any new capabilities introduced through it would not be membraned. You
   // generally should not do that.
 
   virtual kj::Maybe<Capability::Client> outboundCall(
       uint64_t interfaceId, uint16_t methodId, Capability::Client target) = 0;
   // Like `inboundCall()`, but applies to calls originating *inside* the membrane and terminating
   // outside.
   //
   // Note: It is strongly recommended that `outboundCall()` returns null in exactly the same cases
   //   that `inboundCall()` return null. Conversely, for any case where `inboundCall()` would
   //   redirect or throw, `outboundCall()` should also redirect or throw. Otherwise, you can run
   //   into inconsistent behavion when a promise is returned across a membrane, and that promise
   //   later resolves to a capability on the other side of the membrane: calls on the promise
   //   will enter and then exit the membrane, but calls on the eventual resolution will not cross
   //   the membrane at all, so it is important that these two cases behave the same.
 
   virtual kj::Own<MembranePolicy> addRef() = 0;
   // Return a new owned pointer to the same policy.
   //
   // Typically an implementation of MembranePolicy should also inherit kj::Refcounted and implement
   // `addRef()` as `return kj::addRef(*this);`.
   //
   // Note that the membraning system considers two membranes created with the same MembranePolicy
   // object actually to be the *same* membrane. This is relevant when an object passes into the
   // membrane and then back out (or out and then back in): instead of double-wrapping the object,
   // the wrapping will be removed.
 
   virtual kj::Maybe<kj::Promise<void>> onRevoked() { return nullptr; }
   // If this returns non-null, then it is a promise that will reject (throw an exception) when the
   // membrane should be revoked. On revocation, all capabilities pointing across the membrane will
   // be dropped and all outstanding calls canceled. The exception thrown by the promise will be
   // propagated to all these calls. It is an error for the promise to resolve without throwing.
   //
   // After the revocation promise has rejected, inboundCall() and outboundCall() will still be
   // invoked for new calls, but the `target` passed to them will be a capability that always
   // rethrows the revocation exception.
 
   virtual bool shouldResolveBeforeRedirecting() { return true; }
   // If this returns true, then when inboundCall() or outboundCall() returns a redirect, but the
   // original target is a promise, then the membrane will discard the redirect and instead wait
   // for the promise to become more resolved and try again.
   //
   // This behavior is important in particular when implementing a membrane that wants to intercept
   // calls that would otherwise terminate inside the membrane, but needs to be careful not to
   // intercept calls that might be reflected back out of the membrane. If the promise eventually
   // resolves to a capability outside the membrane, then the call will be forwarded to that
   // capability without applying the policy at all.
   //
   // However, some membranes don't need this behavior, and may be negatively impacted by the
   // unnecessary waiting. Such membranes should override this to return false.
   //
   // TODO(cleanup): Consider a backwards-incompatible revamp of the MembranePolicy API with a
   //   better design here. Maybe we should more carefully distinguish between MembranePolicies
   //   which are reversible vs. those which are one-way?
 
   // ---------------------------------------------------------------------------
   // Control over importing and exporting.
   //
   // Most membranes should not override these methods. The default behavior is that a capability
   // that crosses the membrane is wrapped in it, and if the wrapped version crosses back the other
   // way, it is unwrapped.
 
   virtual Capability::Client importExternal(Capability::Client external);
   // An external capability is crossing into the membrane. Returns the capability that should
   // substitute for it when called from the inside.
   //
   // The default implementation creates a capability that invokes this MembranePolicy. E.g. all
   // calls will invoke outboundCall().
   //
   // Note that reverseMembrane(cap, policy) normally calls policy->importExternal(cap), unless
   // `cap` itself was originally returned by the default implementation of exportInternal(), in
   // which case importInternal() is called instead.
 
   virtual Capability::Client exportInternal(Capability::Client internal);
   // An internal capability is crossing out of the membrane. Returns the capability that should
   // substitute for it when called from the outside.
   //
   // The default implementation creates a capability that invokes this MembranePolicy. E.g. all
   // calls will invoke inboundCall().
   //
   // Note that membrane(cap, policy) normally calls policy->exportInternal(cap), unless `cap`
   // itself was originally returned by the default implementation of exportInternal(), in which
   // case importInternal() is called instead.
 
   virtual MembranePolicy& rootPolicy() { return *this; }
   // If two policies return the same value for rootPolicy(), then a capability imported through
   // one can be exported through the other, and vice versa. `importInternal()` and
   // `exportExternal()` will always be called on the root policy, passing the two child policies
   // as parameters. If you don't override rootPolicy(), then the policy references passed to
   // importInternal() and exportExternal() will always be references to *this.
 
   virtual Capability::Client importInternal(
       Capability::Client internal, MembranePolicy& exportPolicy, MembranePolicy& importPolicy);
   // An internal capability which was previously exported is now being re-imported, i.e. a
   // capability passed out of the membrane and then back in.
   //
   // The default implementation simply returns `internal`.
 
   virtual Capability::Client exportExternal(
       Capability::Client external, MembranePolicy& importPolicy, MembranePolicy& exportPolicy);
   // An external capability which was previously imported is now being re-exported, i.e. a
   // capability passed into the membrane and then back out.
   //
   // The default implementation simply returns `external`.
 };
 
 Capability::Client membrane(Capability::Client inner, kj::Own<MembranePolicy> policy);
 // Wrap `inner` in a membrane specified by `policy`. `inner` is considered "inside" the membrane,
 // while the returned capability should only be called from outside the membrane.
 
 Capability::Client reverseMembrane(Capability::Client outer, kj::Own<MembranePolicy> policy);
 // Like `membrane` but treat the input capability as "outside" the membrane, and return a
 // capability appropriate for use inside.
 //
 // Applications typically won't use this directly; the membraning code automatically sets up
 // reverse membranes where needed.
 
 template <typename ClientType>
 ClientType membrane(ClientType inner, kj::Own<MembranePolicy> policy);
 template <typename ClientType>
 ClientType reverseMembrane(ClientType inner, kj::Own<MembranePolicy> policy);
 // Convenience templates which return the same interface type as the input.
 
 template <typename ServerType>
 typename ServerType::Serves::Client membrane(
     kj::Own<ServerType> inner, kj::Own<MembranePolicy> policy);
 template <typename ServerType>
 typename ServerType::Serves::Client reverseMembrane(
     kj::Own<ServerType> inner, kj::Own<MembranePolicy> policy);
 // Convenience templates which input a capability server type and return the appropriate client
 // type.
 
 template <typename Reader>
 Orphan<typename kj::Decay<Reader>::Reads> copyIntoMembrane(
     Reader&& from, Orphanage to, kj::Own<MembranePolicy> policy);
 // Copy a Cap'n Proto object (e.g. struct or list), adding the given membrane to any capabilities
 // found within it. `from` is interpreted as "outside" the membrane while `to` is "inside".
 
 template <typename Reader>
 Orphan<typename kj::Decay<Reader>::Reads> copyOutOfMembrane(
     Reader&& from, Orphanage to, kj::Own<MembranePolicy> policy);
 // Like copyIntoMembrane() except that `from` is "inside" the membrane and `to` is "outside".
 
 // =======================================================================================
 // inline implementation details
 
 template <typename ClientType>
 ClientType membrane(ClientType inner, kj::Own<MembranePolicy> policy) {
   return membrane(Capability::Client(kj::mv(inner)), kj::mv(policy))
       .castAs<typename ClientType::Calls>();
 }
 template <typename ClientType>
 ClientType reverseMembrane(ClientType inner, kj::Own<MembranePolicy> policy) {
   return reverseMembrane(Capability::Client(kj::mv(inner)), kj::mv(policy))
       .castAs<typename ClientType::Calls>();
 }
 
 template <typename ServerType>
 typename ServerType::Serves::Client membrane(
     kj::Own<ServerType> inner, kj::Own<MembranePolicy> policy) {
   return membrane(Capability::Client(kj::mv(inner)), kj::mv(policy))
       .castAs<typename ServerType::Serves>();
 }
 template <typename ServerType>
 typename ServerType::Serves::Client reverseMembrane(
     kj::Own<ServerType> inner, kj::Own<MembranePolicy> policy) {
   return reverseMembrane(Capability::Client(kj::mv(inner)), kj::mv(policy))
       .castAs<typename ServerType::Serves>();
 }
 
 namespace _ {  // private
 
 OrphanBuilder copyOutOfMembrane(PointerReader from, Orphanage to,
                                 kj::Own<MembranePolicy> policy, bool reverse);
 OrphanBuilder copyOutOfMembrane(StructReader from, Orphanage to,
                                 kj::Own<MembranePolicy> policy, bool reverse);
 OrphanBuilder copyOutOfMembrane(ListReader from, Orphanage to,
                                 kj::Own<MembranePolicy> policy, bool reverse);
 
 }  // namespace _ (private)
 
 template <typename Reader>
 Orphan<typename kj::Decay<Reader>::Reads> copyIntoMembrane(
     Reader&& from, Orphanage to, kj::Own<MembranePolicy> policy) {
   return _::copyOutOfMembrane(
       _::PointerHelpers<typename kj::Decay<Reader>::Reads>::getInternalReader(from),
       to, kj::mv(policy), true);
 }
 
 template <typename Reader>
 Orphan<typename kj::Decay<Reader>::Reads> copyOutOfMembrane(
     Reader&& from, Orphanage to, kj::Own<MembranePolicy> policy) {
   return _::copyOutOfMembrane(
       _::PointerHelpers<typename kj::Decay<Reader>::Reads>::getInternalReader(from),
       to, kj::mv(policy), false);
 }
 
 } // namespace capnp