[LSF/MM/BPF TOPIC] Multi-kfunc sets / restricted scoping

[David Vernet <void@manifault.com>]

Hi all,

I would like to propose discussing a potential new kfunc-related feature
at LSF/MM/BPF: Enabling kfuncs to be restricted to only being callable
from a subset of specific BPF programs, e.g. from only a subset of
callbacks defined in a struct_ops struct, rather than from any
struct_ops program.

Some kfuncs may not be safe or logical to call from all contexts. For
example, the backend kernel implementation which is invoking a
struct_ops callback may set some global state before calling into BPF,
and may thus expect that the state is set when the program calls back
into the kernel from that struct_ops callback, via a kfunc. If the kfunc
can't actually rely on that expectation, whether for safety reasons or
correctness reasons, it has to implement its own methodology for
ensuring it was called from the right context.

Providing developers with an ability to specify the specific programs
that a kfunc should be invokable from would address this problem, and
would avoid every kfunc implementation from having to implement its own
scope checking / validation where required.

I would like to discuss possible design approaches, UX approaches, etc.

Thoughts?

Thanks,
David