sysctl-s in a module should be accessible only when the module is initialized

[avg]

Summary

A sysctl can have a custom handler that may access data that is initialized
via SYSINIT(9) or via a module event handler (also invoked via SYSINIT).
Thus, it is not safe to allow access to the module's sysctl-s until
the initialization is performed. Likewise, we should not allow access
to teh sysctl-s after the module is uninitialized.
The latter is easy to achieve by properly ordering linker_file_unregister_sysctls
and linker_file_sysuninit.
The former is not as easy for two reasons:

• the initialization may depend on tunables which get set when sysctl-s are registered, so we need to set the tunables before running sysinit-s
• the initialization may try to dynamically add more sysctl-s under statically defined sysctl nodes

So, this change splits the sysctl setup into two phases. In the first phase
the sysctl-s are registered as before but they are disabled and hidden from
consumers. In the second phase, done after sysinit-s, normal access to the
sysctl-s is enabled.

The change should affect only dynamic module loading and unloading after
the system boot-up. Nothing changes for sysctl-s compiled into the kernel
and sysctl-s in preloaded modules.

[• hselasky]

sys/kern/kern_sysctl.c

494 ↗

(On Diff #33567)

Just a question about code factoring. Why is not CTLFLAG_DORMANT set by the wrapper before calling this function? Would save passing in "enable" argument to this function?

538 ↗

(On Diff #33567)

It might make sense to have:

sysctl_disable_oid() and use that to disable oid before calling sysctl_register_oid_impl()

--HPS

[kib]

I am fine with this approach. After you answer the Hans' questions, I do not see anything wrong.

[julian]

This comment was removed by julian.

[avg]

sys/kern/kern_sysctl.c

494 ↗

(On Diff #33567)

That's a very good suggestion, I will implement it.

538 ↗

(On Diff #33567)

Yes, that might make sense, thank you for the suggestion.
But I am somewhat reluctant to introduce a small internal function that has only a single consumer at this time.
I'll postpone this suggestion until the time when there is more than one use for sysctl_disable_oid.
Is that okay with you?

[avg]

implement sysctl_register_disabled_oid as a thin wrapper around sysctl_register_oid,
so that the latter does not need to be modified at all

[• hselasky]

sys/kern/kern_sysctl.c

538 ↗

(On Diff #33567)

Sure.

[avg]

enable sysctl-s only if the module file doesn't fail to load