Usermode Thread Futex write access denied

RE: Usermode Thread Futex write access denied

Susheel Nuguru — Wed, 03 Jan 2024 05:44:26 GMT

Thanks for this comment and the code snippet for giving access grant to the futex object. And Yes I think it is better to take this to Zephyr.

[quote user="mouthbreather"]globaly declared variables are in the default memory region for user mode threads.[/quote]

Just FYI, Reading setting up memory at boot, it says

Global variables, by default, are not accessible to user mode threads. Refer to Memory Domains and Partitions on how to use global variables in user mode threads, and on how to share data between user mode threads.

RE: Usermode Thread Futex write access denied

mouthbreather — Tue, 02 Jan 2024 20:01:21 GMT

Thanks for taking a look. I did some more fiddling and found out that even the main thread does not have access permission on the futex struct.Using k_object_is_valid(&running_f, K_OBJ_ANY) returns false. After k_object_grant_access to the main thread k_object_is_valid returns true. But the other threads still have no write permission. This kinda contradicts the documentation on the User Mode section of the zephyr project that states that globaly declared variables are in the default memory region for user mode threads.

Nonetheless I will take it to the offical zephyr discussion.

For sake of completness here are the code snippets and the output regarding k_object_is_valid (Mind that this does not compile in nrf Connect SDK 2.5.1 ; I used zephyr/west 3.5.99 to compile)

/*Main*/
        LOG_INF("Main: in %s mode", k_is_user_context() ? "user" : "kernel");
        LOG_INF("running_f is %s valid object", k_object_is_valid(&running_f, K_OBJ_ANY) ? "an" : "no");

        k_object_access_grant(&running_f, k_current_get());

        LOG_INF("running_f is %s valid object", k_object_is_valid(&running_f, K_OBJ_ANY) ? "an" : "no");

        LOG_INF("Start of main");

/*output*/
[00:00:00.584,564] <inf> futex: Main: in kernel mode
[00:00:00.584,625] <err> os: thread 0x20001070 (5) does not have permission on k_futex 0x200060b0
[00:00:00.584,655] <err> os: permission bitmap
                             00 00                                            |..               
[00:00:00.584,686] <inf> futex: running_f is no valid object
[00:00:00.584,716] <inf> futex: running_f is an valid object

RE: Usermode Thread Futex write access denied

Susheel Nuguru — Tue, 02 Jan 2024 13:09:10 GMT

It looks like you need to grant_access to both parent thread objects and also the kernel resources used by the child thread that was created in user mode. The K_INHERIT_PERMS as you said does not seem to be inheriting the parent threads access rights to the kernel objects.

I modified ncs\v2.5.0\zephyr\samples\userspace\prod_consumer\src\main.c to create treads in user mode like you sample and I see memory access violations here as well.

[quote user=""]I started reading on the usermode page of zehpyr and the memory partition part. Would this be the right way to go or is there an easier approach?[/quote]

It looks like running_f k_futex is not accessible by guard_thread. You might have to share access by putting this the sharable object and share access.

RE: Usermode Thread Futex write access denied

Susheel Nuguru — Tue, 02 Jan 2024 11:55:35 GMT

Hi,

These are very generic Zephyr RTOS questions that could have have been better asked in Zephyr forums. I haven't yet experimented with these options myself but I intend to do so this week. I will let you know with my observations.