Re: Linux Kernel module notification bug

[Jessica Yu]

+++ Adam Zabrocki [10/01/21 18:54 +0100]:
> Hello,
>
> I believe that the following commit does introduce a gentle "functionality
> bug":
>
> "module: delay kobject uevent until after module init call":
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/kernel/module.c?id=38dc717e97153e46375ee21797aa54777e5498f3
>
> The official Linux Kernel API for the kernel module activities notification has
> been divided based on the readiness 'stage' for such module. We have the
> following stages:
>
>        MODULE_STATE_LIVE,      /* Normal state. */
>        MODULE_STATE_COMING,    /* Full formed, running module_init. */
>        MODULE_STATE_GOING,     /* Going away. */
>        MODULE_STATE_UNFORMED,  /* Still setting it up. */
>
> LIVE means that the kernel module is correctly running and all initialization
> work has been already done. Otherwise, we have event 'COMING' or 'UNFORMED'.
>
> In the described commit, creation of the KOBJECT has been moved after invoking
> a notficiation of the newly formed kernel module state (LIVE). That's somehow
> inconsistent from my understanding of the kernel modules notifiers logic.
> Creation of the new objects (like KOBJ) should be done before notification of
> the stage LIVE is invoked.

I'm confused. We're not creating any kobjects here. That is all done
in mod_sysfs_setup(), which is called while the module is still
COMING.  What that commit does is delay telling userspace about the
module (specifically, systemd/udev) until the module is basically
ready. Systemd was basically receiving the uevent too early, before
the module has initialized, hence we decided to delay the uevent [1].

> This commit breaks some of the projects that rely on the LIVE notification to
> monitor when the newly loaded module is ready.

Hm, could you please explain specifically what is the issue you're seeing?
What projects is it breaking?

Thanks,

Jessica

[1] https://github.com/systemd/systemd/issues/17586