I haven’t worked with CAmkES for some time, so it’s possible I’m going to inject nothing but nonsense into this conversation, but Amit’s question stirred a memory of mine. I read Amit’s question as, “why does reading from a dataport with W permission trigger a fault?”

Whether you expect a read to write-only memory to trigger a fault or not depends on how you think of hardware. At one point, CAmkES was passing the user’s requested permissions directly to the kernel.¹ This meant if you asked for a write-only dataport, CAmkES asked the kernel for write-only memory. The platforms we ran on at the time (ARMv6 and 32-bit x86?) had no concept of write-only mappings, and the kernel’s logic silently downgraded this to a kernel-only mapping. When we realised this, we worked around it by coercing a write-only mapping into read/write during code generation. This seemed more consistent with the user’s expectation. I notice requesting a write-only mapping from seL4 now triggers a debug message (c.f. maskVMRights) so maybe it was in response to this.

As to why reading a write-only dataport now triggers a fault, I do not know. Maybe write-only dataports now really are write-only in some clever way? If I’ve misconstrued the question, please consider this a scenic detour and we will return to our regularly scheduled program.

¹ It’s possible this was actually in the CapDL loader not CAmkES dataports and I’m misremembering the scenario.

On Jun 30, 2019, at 17:47, Lee, Damon (Data61, Kensington NSW) Damon.Lee@data61.csiro.au wrote:

Hi Amit,

I'd like to apologise first for errors in the tutorials. The first hint for Task 8 of this tutorial should say: "use attributes <component>.<interface name>_access" instead.

As for the fault handler, I assume that you didn't get a fault where we intended it to (writing to 'd_typed').

The checks aren't really done by CAmkES or seL4 but rather, by the hardware. Depending on the permissions given to the shared memory dataports, pages are mapped in with different permissions. So writing to a read-only page will trigger a page fault and the kernel will defer it to the fault handlers of your components.

And as you've found out, the default handlers are found in 'component.template.c' of the CAmkES repository.

I hope this answers your question.

PS: Reference solutions for the tutorials can be accessed by running the init script with the --solution flag. E.g. `./init --tutorial hello-camkes-2 --solution` would get you the solutions for the 'hello-camkes-2' tutorial.

Sincerely, Damon From: Devel devel-bounces@sel4.systems on behalf of Amit Goyal amitgoyal@cse.iitb.ac.in Sent: Monday, 1 July 2019 9:16 AM To: devel@sel4.systems Subject: [seL4] Understanding Camkes Dataport Interface Access Rights

Hi All,

I was looking at hello-camkes-2 tutorial. I understand the concept of dataports and the access rights given on dataport interfaces. I changed the configuration in hello-camkes-2.camkes.template file by adding echo.d = "R"; client.d = "W";. I did some changes in the client.c.template file to read on the dataport interface d by trying to access its contents through str/n/d pointers. Although, it only has write access on "d" in the new configuration. This leads to the fault handler being called when I simulate the build. I was just curious to find out where are such checks being done at the backend in camkes/seL4. I could find the code of fault handler in component.template.c but I could not find anywhere the code which checks that the client only has write access on dataport d and thus calling the fault handler. Can someone point me towards that piece of code or explain what is exactly happening at the backend in camkes/seL4?

-- Thanks and Regards, Amit Goyal

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Camkes dataports on seL4 (seL4SharedData connector) are implemented as a shared memory mapping between the two components both pages are mapped to the same underlying frame of physical memory. The access rights are implemented via mapping attributes supported by the architecture. Your reads and writes through your dataport pointers are acting on this shared memory directly. The fault is generated by a hardware exception that seL4 handles by creating a fault and sending it to a registered fault handler (sendFaultIPC is the function in the kernel). There is a setting (that is enabled by default) that causes each Camkes component to set up a fault handler for all of its threads. If any of these threads cause a fault, such as by writing to read only memory, the fault handler will receive a message from the kernel.

These mapping attributes are set when the kernel creates the page mapping for each component using the relevant Map invocation [1]. The result of building a Camkes system for seL4 is a system spec that contains all of the seL4 kernel objects and capabilities described by the system. Within this spec, dataports are described as a capability to physical memory where each capability has the specified access rights. When the system initialiser (capdl-loader-app in our case) is started by seL4 as the root task, it constructs the system following the spec and starts each Camkes component. By this point, the seL4 invocations for setting up the shared data page mappings have been performed. You can look through the generated spec in the build directory by finding the file with the .cdl extension. If you search for the name of the dataport you can find the capabilities and objects to validate that the rights are as you would expect.

Hope this answers your questions.

@Matthew: Write only mappings are still coerced by the capdl-loader to read write mappings before calling the mapping invocation.

[1] https://docs.sel4.systems/ApiDoc.html#map-4