Checking Interface Usage with Vault

1. Checking Interface Usagewith Vault Rob DeLine 23 May 2001 CS 590 MD

2. Interfaces have (unenforced) usage rules • Usage rules are in the documentation — we hope! • “Call A(x) before calling B(x)” • “If you call A(x), you must eventually call B(x)” • “Call A(x) before accessing variable y” • “After calling B(x), don’t access variable y” • Disobeying a rule often leads to a crash

3. Vault enforces interface usage rules • Vault is a new programming language • Interface provider records usage rules • Client code is rejected if it violates a rule (type error) • Every violation is guaranteed to be found • Vault’s type system tracks individual objects • Recall our rule: “Call A(x) before calling B(x)” • We track an object’s availability (can I access it?) • We track an object’s symbolic state (what can I call on it?)

4. Tracking individual objects • Rule 1: “Close every file that you open.” • Rule 2: “Do not read from a file after closing it.” void ReadFromFiles (FILE f1, FILE f2) { char x = fgetc(f1); fclose(f1); char y = fgetc(f2); fclose(f2); } • Are we obeying the rules?

5. Tracking individual objects • Rule 1: “Close every file that you open.” • Rule 2: “Do not read from a file after closing it.” void ReadFromFiles (FILE f1, FILE f2) { char x = fgetc(f1); fclose(f1); char y = fgetc(f2); fclose(f2); } • Are we obeying the rules? • Maybe not, consider ReadFromFiles(x,x)

6. Aliases make it tricky to enforce rules • Vault uses keys to spell out aliasing • Key = compile-time name for run-time object • Same key Û same object • Tracked type associates a key with an object • Tracked types (only) have aliasing restrictions • void ReadFromFiles1 (tracked(K) FILE f1, tracked(K) FILE f2) • // f1 and f2 must be aliases • void ReadFromFiles2 (tracked(K) FILE f1, tracked(J) FILE f2) • // f1 and f2 cannot be aliases

7. Using keys to track object availability and state • At each program point, Vault computes... • Keys held (which objects are available) • Each key’s state (symbolic state of each object) • Vault function signature includes effects on keys • Pre- and post-condition combined as one change spec • Add key tracked(K) T foo() [new K]; • Delete key void foo(tracked(K) T) [-K]; • Change key state void foo(tracked(K) T) [K@a->b]; • Allocation adds new key to key set tracked(K) Point p = new tracked Point {x=1;y=2;};

8. Controlling data access with keys • Guarded types control data access K:T x • Can read/write variable x only when K is in the key set • Allows us to correlate the lifetimes of objects type region; tracked(R) region make() [new R]; void delete(tracked(R) region) [-R]; R:byte[] alloc(tracked(R) region, int) [R];

9. {E} fork {E} {} signal wait {} {E} Specializing types to particular keys • Type can have type parm list<T:type> • Type can also have key parm mutex<K:key> type KEVENT<R:key>; KEVENT<E> KeInitializeEvent<T> (tracked(E) T) [E]; NTSTATUS KeSignalEvent(KEVENT<E>) [-E]; NTSTATUS KeWaitForEvent(KEVENT<E>) [+E];

10. Holding keys conditionally • Datatypes (variants) divide values into cases • Syntactic tags distinguish cases (constructors, pattern matching) • Vault datatypes can hold keys, as well as values tracked variant OptKey<K:key> [ `NoKey | `SomeKey{K} ]; void foo(tracked(X) T x) [X] { {X} tracked OptKey<X> y = `SomeKey{X}; {} ... {} switch (y) { {} case `SomeKey{X}: ... {X} case `NoKey: ... {} }

11. Vault checks keys for each function • Calculate function body’s effect on keys • Create CFG for body • Take initial key set from precondition • Ensure needed keys held at each node (guards,preconds) • Ensure final key set matches postcondition • Unify key sets at join points { } { } y=new tracked T y=new tracked T i++ y=new tracked T { K23 } { K23 } { } { K56 } { K98 }

12. Checking Windows 2000 device drivers • Driver handles requests from the kernel • e.g. start, read, write, shutdown, ... • driver exports a function for each request type • lifetime of request ¹ lifetime of function call • Request is encapsulated in a data structure • I/O Request Packet (IRP) • Driver handles request by side-effecting IRP • IRP ownership and lifetime are important

13. KERNEL file system driver storage class driver floppy driver bus driver Drivers form a stack • Kernel sends IRP to top driver in stack • Driver may... • handle IRP itself • pass IRP down • pass new IRP(s) down

14. IRP Ownership IoCompleteRequest VOID IoCompleteRequest( IN PIRPIrp, IN CCHARPriorityBoost); IoCompleteRequest indicates the caller has completed all processing for a given I/O request and is returning the given IRP to the I/O Manager. Parameters Irp Points to the IRP to be completed. PriorityBoost Specifies a system-defined constant by which to increment the runtime priority of the original thread that requested the operation. This value is IO_NO_INCREMENT if the original thread requested an operation the driver could complete quickly (so the requesting thread is not compensated for its assumed wait on I/O) or if the IRP is completed with an error. Otherwise, the set of PriorityBoost constants are device-type-specific. See ntddk.h or wdm.h for these constants. Comments When a driver has finished all processing for a given IRP, it calls IoCompleteRequest. The I/O Manager checks the IRP to determine whether any higher-level drivers have set up an IoCompletion routine for the IRP. If so, each IoCompletion routine is called, in turn, until every layered driver in the chain has completed the IRP. When all drivers have completed a given IRP, the I/O Manger returns status to the original requestor of the operation. Note that a higher-level driver that sets up a driver-created IRP must supply an IoCompletion routine to release the IRP it created. Callers of IoCompleteRequest must be running at IRQL <= DISPATCH_LEVEL. See Also IoSetCompletionRoutine “IoCompleteRequest indicates the caller has completed all processing for a given I/O request and is returning the given IRP to the I/O Manager.”

15. IRP Ownership IoCompleteRequest VOID IoCompleteRequest( IN PIRPIrp, IN CCHARPriorityBoost); IoCompleteRequest indicates the caller has completed all processing for a given I/O request and is returning the given IRP to the I/O Manager. Parameters Irp Points to the IRP to be completed. PriorityBoost Specifies a system-defined constant by which to increment the runtime priority of the original thread that requested the operation. This value is IO_NO_INCREMENT if the original thread requested an operation the driver could complete quickly (so the requesting thread is not compensated for its assumed wait on I/O) or if the IRP is completed with an error. Otherwise, the set of PriorityBoost constants are device-type-specific. See ntddk.h or wdm.h for these constants. Comments When a driver has finished all processing for a given IRP, it calls IoCompleteRequest. The I/O Manager checks the IRP to determine whether any higher-level drivers have set up an IoCompletion routine for the IRP. If so, each IoCompletion routine is called, in turn, until every layered driver in the chain has completed the IRP. When all drivers have completed a given IRP, the I/O Manger returns status to the original requestor of the operation. Note that a higher-level driver that sets up a driver-created IRP must supply an IoCompletion routine to release the IRP it created. Callers of IoCompleteRequest must be running at IRQL <= DISPATCH_LEVEL. See Also IoSetCompletionRoutine void IoCompleteRequest(tracked(I) IRP, CHAR) [-I];

16. IRP Ownership IoCallDriver NTSTATUS IoCallDriver( IN PDEVICE_OBJECTDeviceObject, IN OUT PIRPIrp); IoCallDriver sends an IRP to the next-lower-level driver after the caller has set up the I/O stack location in the IRP for that driver. Parameters DeviceObject Points to the next-lower driver's device object, representing the target device for the requested I/O operation. Irp Points to the IRP. Return Value IoCallDriver returns the NTSTATUS value that a lower driver set in the I/O status block for the given request or STATUS_PENDING if the request was queued for additional processing. Comments IoCallDriver assigns the DeviceObject input parameter to the device object field of the IRP stack location for the next lower driver. An IRP passed in a call to IoCallDriver becomes inaccessible to the higher-level driver, unless the higher-level driver has set up its IoCompletion routine for the IRP with IoSetCompletionRoutine. If it does, the IRP input to the driver-supplied IoCompletion routine has its I/O status block set by the lower driver(s) and all lower-level driver(s)' I/O stack locations filled with zeros. Drivers must not use IoCallDriver to pass power IRPs (IRP_MJ_POWER). Use PoCallDriver instead. Callers of IoCallDriver must be running at IRQL <= DISPATCH_LEVEL. See Also IoAllocateIrp, IoBuildAsynchronousFsdRequest, IoBuildDeviceIoControlRequest, IoBuildSynchronousFsdRequest, IoSetCompletionRoutine, PoCallDriver “An IRP passed in a call to IoCallDriver becomes inaccessible to the higher-level driver, …”

17. IRP Ownership IoCallDriver NTSTATUS IoCallDriver( IN PDEVICE_OBJECTDeviceObject, IN OUT PIRPIrp); IoCallDriver sends an IRP to the next-lower-level driver after the caller has set up the I/O stack location in the IRP for that driver. Parameters DeviceObject Points to the next-lower driver's device object, representing the target device for the requested I/O operation. Irp Points to the IRP. Return Value IoCallDriver returns the NTSTATUS value that a lower driver set in the I/O status block for the given request or STATUS_PENDING if the request was queued for additional processing. Comments IoCallDriver assigns the DeviceObject input parameter to the device object field of the IRP stack location for the next lower driver. An IRP passed in a call to IoCallDriver becomes inaccessible to the higher-level driver, unless the higher-level driver has set up its IoCompletion routine for the IRP with IoSetCompletionRoutine. If it does, the IRP input to the driver-supplied IoCompletion routine has its I/O status block set by the lower driver(s) and all lower-level driver(s)' I/O stack locations filled with zeros. Drivers must not use IoCallDriver to pass power IRPs (IRP_MJ_POWER). Use PoCallDriver instead. Callers of IoCallDriver must be running at IRQL <= DISPATCH_LEVEL. See Also IoAllocateIrp, IoBuildAsynchronousFsdRequest, IoBuildDeviceIoControlRequest, IoBuildSynchronousFsdRequest, IoSetCompletionRoutine, PoCallDriver void IoCallDriver(DEVICE_OBJECT,tracked(I) IRP) [-I];

18. Example: Driver request • NTSTATUS Read(DEVICE_OBJECT Dev, tracked(I) IRP Irp) [-I] { • if (GetRequestLength(Irp) == 0) { • NTSTATUS status = `STATUS_SUCCESS(`TransferBytes(0)); • IoCompleteRequest(Irp, status); • return status; • } else • return IoCallDriver(NextDriver,Irp); • }

19. Example: Driver request • NTSTATUS Read(DEVICE_OBJECT Dev, tracked(I) IRP Irp) [-I] { {I} • if (GetRequestLength(Irp) == 0) { {I} • NTSTATUS status = `STATUS_SUCCESS(`TransferBytes(0)); {I} • IoCompleteRequest(Irp, status); {} • return status; {} • } else {I} • return IoCallDriver(NextDriver,Irp); {} • }

20. IRP completion routines • Getting IRP ownership back • Driver A hands IRP to B and wants it back after B is done • Driver A sets “completion routine” on IRP • void IoSetCompletionRoutine(tracked(K) IRP Irp, • COMPLETION_ROUTINE<K> Fun) [K]; • type COMPLETION_ROUTINE<key K> = • tracked COMPLETION_RESULT<K>(DEVICE_OBJECT Dev, • tracked(K) IRP Irp) [-K]; • tracked variant COMPLETION_RESULT<key K> [ • | `MoreProcessingRequired • | `Finished(NTSTATUS) {K} ];

21. Completion routine example • NTSTATUS PlugPlay(DEVICE_OBJECT Dev, tracked(R) IRP Irp) [-R] { • KEVENT<R> DoneEvent = KeInitializeEvent(Irp); • tracked COMPLETION_RESULT<I> • CompletePnP(DEVICE_OBJECT Dev, tracked(I) IRP Irp) [-I] { • KeSignalEvent(DoneEvent); • return `MoreProcessingRequired; • } • IoSetCompletionRoutine(Irp, CompletePnP); • CALL_RESULT<R> result = IoCallDriver(lowerDriver, Irp); • KeWaitForEvent(DoneEvent); • ... • }

22. Completion routine example • NTSTATUS PlugPlay(DEVICE_OBJECT Dev, tracked(R) IRP Irp) [-R] { {R} • KEVENT<R> DoneEvent = KeInitializeEvent(Irp); {R} • tracked COMPLETION_RESULT<I> • CompletePnP(DEVICE_OBJECT Dev, tracked(I) IRP Irp) [-I] { {I=R} • KeSignalEvent(DoneEvent); {} • return `MoreProcessingRequired; {} • } • {R} • IoSetCompletionRoutine(Irp, CompletePnP); {R} • CALL_RESULT<R> result = IoCallDriver(lowerDriver, Irp); {} • KeWaitForEvent(DoneEvent); {R} • ... • }

23. Interrupt levels (IRQLs) KeSetPriorityThread KPRIORITY KeSetPriorityThread( IN PKTHREADThread, IN KPRIORITYPriority); KeSetPriorityThread sets the run-time priority of a driver-created thread. Parameters Thread Pointer to the driver-created thread. Priority Specifies the priority of the driver-created thread, usually to the real-time priority value, LOW_REALTIME_PRIORITY. The value LOW_PRIORITY is reserved for system use. Return Value KeSetPriorityThread returns the old priority of the thread. Comments If a call to KeSetPriorityThread resets the thread's priority to a lower value, execution of the thread can be rescheduled even if it is currently running or is about to be dispatched for execution. Callers of KeSetPriorityThread must be running at IRQL PASSIVE_LEVEL. See Also KeGetCurrentThread, KeQueryPriorityThread, KeSetBasePriorityThread “Callers of KeSetPriorityThread must be running at IRQL PASSIVE_LEVEL.”

24. Interrupt levels (IRQLs) KeReleaseSemaphore LONG KeReleaseSemaphore( IN PRKSEMAPHORESemaphore, IN KPRIORITYIncrement, IN LONGAdjustment, IN BOOLEANWait); Parameters Semaphore Pointer to an initialized semaphore object for which the caller provides the storage. Increment Specifies the priority increment to be applied if releasing the semaphore causes a wait to be satisfied. Adjustment Specifies a value to be added to the current semaphore count. This value must be positive. Wait Specifies whether the call to KeReleaseSemaphore is to be followed immediately by a call to one of the KeWaitXxx. Return Value If the return value is zero, the previous state of the semaphore object is not signaled. Comments Callers of KeReleaseSemaphore must be running at IRQL <= DISPATCH_LEVEL provided that Wait is set to FALSE. Otherwise, the caller must be running at IRQL PASSIVE_LEVEL. Releasing a semaphore object causes the semaphore count to be augmented by the value of the Adjustment parameter. If the resulting value is greater than the limit of the semaphore object, the count is not adjusted and an exception, STATUS_SEMAPHORE_COUNT_EXCEEDED, is raised. Augmenting the semaphore object count causes the semaphore to attain a signaled state, and an attempt is made to satisfy as many waits as possible on the semaphore object. If the value of the Wait parameter is TRUE, the return to the caller is executed without lowering IRQL or releasing the dispatcher database spin lock. Therefore, the call to KeReleaseSemaphoremust be followed immediately by a call to one of the KeWaitXxx. “Callers of KeReleaseSemaphore must be running at IRQL <= DISPATCH_LEVEL…”

25. Interrupt levels (IRQLs) • KeAcquireSpinLock • VOID • KeAcquireSpinLock( • IN PKSPIN_LOCKSpinLock, • OUT PKIRQLOldIrql); • KeAcquireSpinLock acquires a spin lock so the caller can synchronize access to shared data in a multiprocessor-safe way by raising IRQL. • Parameters • SpinLock Pointer to an initialized spin lock for which the caller provides the storage. • OldIrql Pointer to a variable that is set to the current IRQL when this call occurs. • Comments • Callers of KeAcquireSpinLock must be running at IRQL <= DISPATCH_LEVEL. • The current IRQL is saved in OldIrql. Then, the current IRQL is reset to DISPATCH_LEVEL, and the specified spin lock is acquired. • The OldIrql value must be specified when the spin lock is released with KeReleaseSpinLock. • Spin locks can cause serious problems if not used judiciously. In particular, no deadlock protection is performed and dispatching is disabled while the spin lock is held. Therefore: • The code within a critical region guarded by an spin lock must neither be pageable nor make any references to pageable data. • The code within a critical region guarded by a spin lock can neither call any external function that might access pageable data or raise an exception, nor can it generate any exceptions. • The caller should release the spin lock with KeReleaseSpinLock as quickly as possible. • See Also • KeAcquireSpinLockAtDpcLevel, KeInitializeSpinLock, KeReleaseSpinLock “Callers of KeAcquireSpinLock must be running at IRQL <= DISPATCH_LEVEL. The current IRQL is saved in OldIrql. Then, the current IRQL is reset to DISPATCH_LEVEL, and the specified spin lock is acquired.”

26. Checking interrupt levels • stateset IRQ_LEVEL = [ PASSIVE_LEVEL < APC_LEVEL < DISPATCH_LEVEL < DIRQL ]; • key IRQL @ IRQ_LEVEL; • KPRIORITY • KeSetPriorityThread(KTHREAD Thread, KPRIORITY Priority) • [ IRQL @ PASSIVE_LEVEL ] ; • LONG • KeReleaseSemaphore(KSEMAPHORE Sem,KPRIORITY Pri,LONG Adj) • [ IRQL <= DISPATCH_LEVEL ] ; • KIRQL<$current> • KeAcquireSpinLock(KSPIN_LOCK SpinLock) • [ IRQL @ $current <= DISPATCH_LEVEL -> DISPATCH_LEVEL ] ;

27. Vault project goals • Commit to a verification approach • MS mostly relies on testing for improving quality (incomplete) • Type checking offers exhaustive analysis • Consider cost effectiveness • Focus on bugs that are hard to repro (e.g., scribbles, races) • Make developers fix bugs before check-in, when cheapest • Focus on real software, real developers • Modeled actual rules of Windows 2000 device drivers • Built a working floppy disk driver (~5 kloc) • Sweat the details of syntax/conceptual model

28. Future work • Does Vault check useful interface rules? • Try interfaces beyond drivers (DirectX?) • Does Vault support convenient programming? • Currently, cannot type imperative loops over tracked data • Key annotations for regions are too burdensome • Can we add Vault features to existing languages? • Going to add keys to C#/CIL • research.microsoft.com/vault