12 KiB
ADR 3: Dynamic Capability Store
Changelog
- 12 December 2019: Initial version
Context
Full implementation of the IBC specification requires the ability to create and authenticate object-capability keys at runtime (i.e., during transaction execution), as described in ICS 5. In the IBC specification, capability keys are created for each newly initialised port & channel, and are used to authenticate future usage of the port or channel. Since channels and potentially ports can be initialised during transaction execution, the state machine must be able to create object-capability keys at this time.
At present, the Cosmos SDK does not have the ability to do this. Object-capability keys are currently pointers (memory addresses) of StoreKey structs created at application initialisation in app.go (example)
and passed to Keepers as fixed arguments (example). Keepers cannot create or store capability keys during transaction execution — although they could call NewKVStoreKey and take the memory address
of the returned struct, storing this in the Merklised store would result in a consensus fault, since the memory address will be different on each machine (this is intentional — were this not the case, the keys would be predictable and couldn't serve as object capabilities).
Keepers need a way to keep a private map of store keys which can be altered during transacton execution, along with a suitable mechanism for regenerating the unique memory addresses (capability keys) in this map whenever the application is started or restarted. This ADR proposes such an interface & mechanism.
Decision
The SDK will include a new CapabilityKeeper abstraction, which is responsible for provisioning, tracking, and authenticating capabilities at runtime. During application initialisation in app.go, the CapabilityKeeper will
be hooked up to modules through unique function references (by calling ScopeToModule, defined below) so that it can identify the calling module when later invoked. When the initial state is loaded from disk, the CapabilityKeeper's Initialise function will create new capability keys
for all previously allocated capability identifiers (allocated during execution of past transactions and assigned to particular modes), and keep them in a memory-only store while the chain is running. The SDK will include a new MemoryStore store type, similar
to the existing TransientStore but without erasure on Commit(), which this CapabilityKeeper will use to privately store capability keys.
The CapabilityKeeper will use two stores: a regular, persistent KVStore, which will track what capabilities have been created by each module, and an in-memory MemoryStore (described below), which will
store the actual capabilities. The CapabilityKeeper will define the following types & functions:
The Capability interface is similar to StoreKey, but has a globally unique Index() instead of a name. A String() method is provided for debugging.
type Capability interface {
Index() uint64
String() string
}
A CapabilityKey is simply a struct, the address of which is taken for the actual capability.
type CapabilityKey struct {
name string
}
A CapabilityKeeper contains a persistent store key, memory store key, and mapping of allocated module names.
type CapabilityKeeper struct {
persistentKey StoreKey
memoryKey MemoryStoreKey
moduleNames map[string]interface{}
sealed bool
}
The CapabilityKeeper provides the ability to create scoped sub-keepers which are tied to a particular module name. These ScopedCapabilityKeepers must be created at application
initialisation and passed to modules, which can then use them to claim capabilities they receive and retrieve capabilities which they own by name, in addition
to creating new capabilities & authenticating capabilities passed by other modules.
type ScopedCapabilityKeeper struct {
persistentKey StoreKey
memoryKey MemoryStoreKey
moduleName string
}
ScopeToModule is used to create a scoped sub-keeper with a particular name, which must be unique. It MUST be called before InitialiseAndSeal.
func (ck CapabilityKeeper) ScopeToModule(moduleName string) ScopedCapabilityKeeper {
if ck.sealed {
panic("capability keeper is sealed")
}
if _, present := ck.moduleNames[moduleName]; present {
panic("cannot create multiple scoped capability keepers for the same module name")
}
ck.moduleNames[moduleName] = struct{}{}
return ScopedCapabilityKeeper{
persistentKey: ck.persistentKey,
memoryKey: ck.memoryKey,
moduleName: moduleName
}
}
InitialiseAndSeal MUST be called exactly once, after loading the initial state and creating all necessary ScopedCapabilityKeepers,
in order to populate the memory store with newly-created capability keys in accordance with the keys previously claimed by particular modules
and prevent the creation of any new ScopedCapabilityKeepers.
func (ck CapabilityKeeper) InitialiseAndSeal(ctx Context) {
if ck.sealed {
panic("capability keeper is sealed")
}
persistentStore := ctx.KVStore(ck.persistentKey)
memoryStore := ctx.KVStore(ck.memoryKey)
// initialise memory store for all names in persistent store
for index, value := range persistentStore.Iter() {
capability = &CapabilityKey{index: index}
for moduleAndCapability := range value {
moduleName, capabilityName := moduleAndCapability.Split("/")
memoryStore.Set(moduleName + "/fwd/" + capability, capabilityName)
memoryStore.Set(moduleName + "/rev/" + capabilityName, capability)
}
}
ck.sealed = true
}
NewCapability can be called by any module to create a new unique, unforgeable object-capability
reference. The newly created capability is automatically persisted; the calling module need not
call ClaimCapability.
func (sck ScopedCapabilityKeeper) NewCapability(ctx Context, name string) (Capability, error) {
memoryStore := ctx.KVStore(sck.memoryKey)
// check name not taken in memory store
if memoryStore.Get("rev/" + name) != nil {
return nil, errors.New("name already taken")
}
// fetch the current index
index := persistentStore.Get("index")
// create a new capability
capability := &CapabilityKey{index: index}
// set persistent store
persistentStore.Set(index, Set.singleton(sck.moduleName + "/" + name))
// update the index
index++
persistentStore.Set("index", index)
// set forward mapping in memory store from capability to name
memoryStore.Set(sck.moduleName + "/fwd/" + capability, name)
// set reverse mapping in memory store from name to capability
memoryStore.Set(sck.moduleName + "/rev/" + name, capability)
// return the newly created capability
return capability
}
AuthenticateCapability can be called by any module to check that a capability
does in fact correspond to a particular name (the name can be untrusted user input)
with which the calling module previously associated it.
func (sck ScopedCapabilityKeeper) AuthenticateCapability(name string, capability Capability) bool {
memoryStore := ctx.KVStore(sck.memoryKey)
// return whether forward mapping in memory store matches name
return memoryStore.Get(sck.moduleName + "/fwd/" + capability) === name
}
ClaimCapability allows a module to claim a capability key which it has received from another module so that future GetCapability calls will succeed.
ClaimCapability MUST be called if a module which receives a capability wishes to access it by name in the future. Capabilities are multi-owner, so if multiple modules have a single Capability reference, they will all own it.
func (sck ScopedCapabilityKeeper) ClaimCapability(ctx Context, capability Capability, name string) error {
persistentStore := ctx.KVStore(sck.persistentKey)
memoryStore := ctx.KVStore(sck.memoryKey)
// set forward mapping in memory store from capability to name
memoryStore.Set(sck.moduleName + "/fwd/" + capability, name)
// set reverse mapping in memory store from name to capability
memoryStore.Set(sck.moduleName + "/rev/" + name, capability)
// update owner set in persistent store
owners := persistentStore.Get(capability.Index())
owners.add(sck.moduleName + "/" + name)
persistentStore.Set(capability.Index(), owners)
}
GetCapability allows a module to fetch a capability which it has previously claimed by name. The module is not allowed to retrieve capabilities which it does not own. If another module
claims a capability, the previously owning module will no longer be able to claim it.
func (sck ScopedCapabilityKeeper) GetCapability(ctx Context, name string) (Capability, error) {
memoryStore := ctx.KVStore(sck.memoryKey)
// fetch capability from memory store
capability := memoryStore.Get(sck.moduleName + "/rev/" + name)
// return the capability
return capability
}
Memory store
A new store key type, MemoryStoreKey, will be added to the store package. The MemoryStoreKeys work just like StoreKeys.
The memory store will work just like the current transient store, except that it will not create a new dbadapter.Store when Commit() is called, but instead retain the current one (so that state will persist across blocks).
Initially the memory store will only be used by the CapabilityKeeper, but it could be used by other modules in the future.
Usage patterns
Initialisation
Any modules which use dynamic capabilities must be provided a ScopedCapabilityKeeper in app.go:
ck := NewCapabilityKeeper(persistentKey, memoryKey)
mod1Keeper := NewMod1Keeper(ck.ScopeToModule("mod1"), ....)
mod2Keeper := NewMod2Keeper(ck.ScopeToModule("mod2"), ....)
// other initialisation logic ...
// load initial state...
ck.InitialiseAndSeal(initialContext)
Creating, passing, claiming and using capabilities
Consider the case where mod1 wants to create a capability, associate it with a resource (e.g. an IBC channel) by name, then pass it to mod2 which will use it later:
Module 1 would have the following code:
capability := scopedCapabilityKeeper.NewCapability(ctx, "resourceABC")
mod2Keeper.SomeFunction(ctx, capability, args...)
SomeFunction, running in module 2, could then claim the capability:
func (k Mod2Keeper) SomeFunction(ctx Context, capability Capability) {
k.sck.ClaimCapability(ctx, capability, "resourceABC")
// other logic...
}
Later on, module 2 can retrieve that capability by name and pass it to module 1, which will authenticate it against the resource:
func (k Mod2Keeper) SomeOtherFunction(ctx Context, name string) {
capability := k.sck.GetCapability(ctx, name)
mod1.UseResource(ctx, capability, "resourceABC")
}
Module 1 will then check that this capability key is authenticated to use the resource before allowing module 2 to use it:
func (k Mod1Keeper) UseResource(ctx Context, capability Capability, resource string) {
if !k.sck.AuthenticateCapability(name, capability) {
return errors.New("unauthenticated")
}
// do something with the resource
}
If module 2 passed the capability key to module 3, module 3 could then claim it and call module 1 just like module 2 did (in which case module 1, module 2, and module 3 would all be able to use this capability).
Status
Proposed.
Consequences
Positive
- Dynamic capability support.
Negative
- Requires an additional keeper.
- Some overlap with existing
StoreKeysystem (in the future they could be combined, since this is a superset functionality-wise).
Neutral
(none known)