Re: [seL4] Understanding Camkes Dataport Interface Access Rights

Thanks Damon and Matthew for the quick response. Further, I have following doubts if someone could clarify: a) As far as I read the camkes code, I understand there is a collapse function which merges the shared page in the two components and gives required capabilities to both the components on that page. If, it is same page, we can make it read only or write only at the hardware level as suggested. How would the other thread, write or read from that page for the communication to happen. b) However, if we consider we have one page each for the dataport in both components. Then, when one component writes on a dataport then that should also be written on the dataport of the other component. Can you point me towards that piece of code. c) I would like to see the piece of code which converts my read/writes of str/n/d in the client.c.template source file to the page level read and write in camkes/seL4. As far as I understand, these are just pointers. Some code must be there to convert them to page level pointers. d) I am curious to see the seL4/Camkes code which creates the page level mapping (as suggested by Matthew). e) In the entire conversation, I believe that the kernel stands for seL4 kernel and not the host system (Ubuntu in my case) kernel. Please correct me, if am wrong. -- Thanks and Regards, Amit Goyal On 2019-07-01 09:53, Matthew Fernandez wrote:

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) 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 on behalf of Amit Goyal 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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