Fault handling

Prerequisites

  1. Set up your machine.
  2. Capabilities tutorial.
  3. IPC Tutorial.

Initialising

# For instructions about obtaining the tutorial sources see https://docs.sel4.systems/Tutorials/#get-the-code
#
# Follow these instructions to initialise the tutorial
# initialising the build directory with a tutorial exercise
./init --tut fault-handlers
# building the tutorial exercise
cd fault-handlers_build
ninja

Outcomes

  1. Learn what a thread fault is.
  2. Understand that a thread fault is different from a processor hardware fault.
  3. Learn what a fault handler is.
  4. Understand what the kernel does to a thread which has faulted.
  5. Learn how to set the endpoint that the kernel will deliver fault messages on (master vs MCS).
  6. Learn how to resume threads after they have faulted.

Background: What is a fault, and what is a fault handler?

A fault handler is a separate instruction stream which the CPU can jump to in order to rectify an anomalous condition in the current thread and then return to the previous instruction stream.

In seL4, faults are modeled as separately programmer-designated “fault handler” threads. In monolithic kernels, faults are not usually delivered to a userspace handler, but they are handled by the monolithic kernel itself.

In general, attempting to resume execution of the faulted thread without rectifying the anomaly will simply re-trigger the fault ad infinitum until the anomaly is cleared away.

Thread faults vs other sources of faults

There are several sources of faults in a running system; they include:

  • Fault events generated by the CPU itself when it encounters anomalies in the instruction stream (aka, “processor exceptions”).
  • Fault events generated by hardware in the event of some hardware anomaly (such as a machine check or non-maskable interrupt).
  • Fault events generated by the seL4 kernel when it encounters anomalies in the current thread.

This tutorial is only concerned with those fault events generated by the seL4 kernel. We will call them “thread faults” from here onward to reduce ambiguity.

How does thread fault handling work?

In seL4, when a thread generates a thread fault, the kernel will block the faulting thread’s execution and attempt to deliver a message across a special endpoint associated with that thread, called its “fault handler” endpoint.

The only special thing about the fault handler endpoint is that a thread can only have one of them. Otherwise it is created and managed just the same way as any other kind of seL4 endpoint object.

The thread which is listening on the other end of the fault endpoint is called the “fault handler”. The kernel expects that the fault handler will correct the anomaly that ails the faulting thread and then tell the kernel when it is safe to try executing the faulting thread once again.

To tell the kernel to resume execution of the faulting thread, the fault handler can either:

  • Invoke a reply operation (with seL4_Reply()) on the fault handler endpoint and make sure that the label in the seL4_MessageInfo_t tag is set to 0;
  • Explicitly tell the kernel to resume executing the faulting thread using seL4_TCB_Resume().

Please note that if the handler sets message registers in the reply message, the kernel may interpret these as meaning something: some fault replies accept parameters. See the seL4 manual for the reply message format for all faults.

If the fault handler did not properly rectify the anomaly in the faulting thread, resuming the faulting thread will simply cause the kernel to re-generate the fault.

Reasons for thread faults:

Thread faults can be generated for different reasons. When a fault occurs the kernel will pass information describing the cause of the fault as an IPC message. At the time of writing, the following faults could be generated by the Master version of the seL4 kernel:

  • Cap fault: A fault triggered because of an invalid cap access.
  • VM fault: A fault triggered by incoherent page table state or incorrect memory accesses by a thread.
  • Unknown Syscall fault: Triggered by performing a syscall invocation that is unknown to the kernel.
  • Debug fault: Triggered when a breakpoint, watchpoint or single-step debug event occurs.

In addition, the following fault types are added by the MCS kernel:

  • Timeout fault: Triggered when a thread consumes all of its budget and still has further execution to do in the current period.

Thread fault messages:

When a fault is generated, the kernel will deliver an IPC message across the fault endpoint. This IPC message contains information that tells the fault handler why the fault occured as well as surrounding contextual information about the fault which might help the fault handler to rectify the anomaly.

Each anomaly has its own message format because the information needed to describe each anomaly will be different. For more information about the contents of the IPC message sent by the seL4 kernel for each fault anomaly, please see the seL4 Manual.

The rest of this tutorial will attempt to teach the reader how to receive and handle seL4 thread faults.

Setting up a fault endpoint for a thread:

In the scenario where a fault message is being delivered on a fault endpoint, the kernel acts as the IPC “sender” and the fault handler acts as a receiver.

This implies that when caps are being handed out to the fault endpoint object, one cap to the object must be given to the kernel and one cap to the object must be given to the handler.

Kernel end vs handler end:

Programmers specify the capability to use a fault handler for a thread when configuring a TCB. As a result the programmer can also set a badge on the kernel’s cap to the fault endpoint object.

When the kernel sends a fault IPC message using a badged endpoint cap, the badge is delivered to the receiver just the same way it is delivered for any other IPC where there is a badge on the sender’s cap.

A keen reader would probably have realized that this means that a badge on the kernel’s cap to a fault endpoint can be used to distinguish fault messages from different faulting threads, such that a single handler can handle faults from multiple threads. Please see the IPC Tutorial for a refresher on how badged fault endpoints work.

Differences between MCS and Master kernel:

There is a minor difference in the way that the kernel is informed of the cap to a fault endpoint, between the master and MCS kernels.

Regardless though, on both versions of the kernel, to inform the kernel of the fault endpoint for a thread, call the usual seL4_TCB_SetSpace().

See the MCS tutorial for more information.

Exercises

This tutorial has one address space set up by the CapDL loader, containing two threads which share the same CSpace. One of the threads is a fault handler while the other triggers a virtual memory fault.

You will be guided through the following broad steps:

  1. Badging and configuring a fault handler for the faulting thread.
  2. Having the faulting thread trigger a thread fault.
  3. Handling the fault in the fault handler.
  4. Resuming the execution of the faulting thread.

Description of the tutorial program:

The tutorial features two threads in different virtual address spaces. One thread is the “faulter” and the other is the “handler”. The faulter is going to generate a fault, and the handler will “handle” it.

In order for the handler to handle the fault, the handler must set up a fault-handling endpoint and tell the kernel to send all fault IPC messages generated by the faulter thread to itself. This is therefore the first step we take.

However, we have to ensure that the fault is only triggered after the handler thread has set up the fault-handling endpoint and is ready to receive the fault IPC message from the kernel.

If the faulter thread generates a fault and there is no thread to handle the the IPC message, the kernel will simply suspend the faulting thread.

For this reason we make the faulter thread seL4_call() the handler thread across an endpoint and tell it which slot the handler should place the fault handling endpoint cap into. After the handler has set up the handler endpoint, the handler will seL4_Reply() to the faulter to let it know that it the handler is ready to handle fault IPC messages.

After that we trigger a fault in the faulter, handle the fault in the handler, and then resume the faulter and that’s the end of the exercise.

Setting up the endpoint to be used for thread fault IPC messages.

The first exercise is to configure the TCB of the faulter with a fault endpoint. This exercise is meant to achieve two learning outcomes:

  1. Explain that the end of the endpoint that is given to the kernel can be badged, and the kernel will return that badge value when it sends a fault IPC message.
  2. Explain the differences between the Master and MCS kernels when it comes to telling the kernel about the fault endpoint.

Right now the faulter thread is blocked on an Endpoint, waiting for the handler to tell it where to put the fault handler endpoint within its own (the faulter’s) CSpace (for the Master kernel).

To set up the fault handler endpoint, we will to first badge it so that when the kernel sends us a fault IPC message, we will be able to identify the faulter. Fault handlers can handle faults from multiple threads, so a badge enables handlers to identify the faulters they are handling.

To badge the endpoint, use the seL4_CNode_Mint() syscall:

    error = seL4_CNode_Mint(
        handler_cspace_root,
        badged_faulter_fault_ep_cap,
        seL4_WordBits,
        handler_cspace_root,
        faulter_fault_ep_cap,
        seL4_WordBits,
        seL4_AllRights, FAULTER_BADGE_VALUE);

Since we are using the Master kernel, you will also need to copy the badged cap into the faulter’s CSpace (See the MCS tutorial for an explanation of the differences between the Master and MCS kernel when configuring fault endpoints):

    error = seL4_CNode_Copy(
        faulter_cspace_root,
        foreign_badged_faulter_empty_slot_cap,
        seL4_WordBits,
        handler_cspace_root,
        badged_faulter_fault_ep_cap,
        seL4_WordBits,
        seL4_AllRights);

Finally, we tell the kernel the cap address of the fault endpoint so that the kernel can deliver fault IPC messages to the handler. Since we’re using the Master kernel, we need to pass a CPtr that can be resolved from within the CSpace of the faulter thread:

    error = seL4_TCB_SetSpace(
        faulter_tcb_cap,
        foreign_badged_faulter_empty_slot_cap,
        faulter_cspace_root,
        0,
        faulter_vspace_root,
        0);

Receiving the IPC message from the kernel:

The kernel will deliver the IPC message to any thread waiting on the fault endpoint. To wait for a fault IPC message simply seL4_Recv(), the same way you’d wait for any other IPC message:

    foreign_faulter_capfault_cap = seL4_GetMR(seL4_CapFault_Addr);

Finding out information about the generated thread fault:

In the thread fault IPC message, the kernel will send information about the fault including the capability address whose access triggered the thread fault. The seL4 manual gives detailed information on which message registers in the IPC buffer contain information about the fault and if you’re so inclined, the libsel4 source code also has the exact code values as well.

In our example here, our sample code generated a Cap Fault, so according to the seL4 manual, we can find out the cap fault address using at offset seL4_CapFault_Addr in the IPC message, as you see above in the code snippet.

Handling a thread fault:

Now that we know the cap address that generated a fault in the faulting thread, we can “handle” the fault by putting a random cap into that slot and then when the faulter thread re-tries to access that slot, it will succeed this time and no thread fault will be generated.

So here we’ll copy an endpoint cap into the faulting slot:

    error = seL4_CNode_Copy(
        faulter_cspace_root,
        foreign_faulter_capfault_cap,
        seL4_WordBits,
        handler_cspace_root,
        sequencing_ep_cap,
        seL4_WordBits,
        seL4_AllRights);

Resuming a faulting thread:

Finally, to have the faulter thread wake up and try to execute again, we seL4_Reply() to it:

    seL4_Reply(seL4_MessageInfo_new(0, 0, 0, 0));

Further exercises

If you’d like to challenge yourself, make sure to set up the fault handling on both versions of the kernel: master and MCS.


Getting help

Stuck? See the resources below.