Re: [PATCH bpf-next v1 1/2] bpf: Summarize sleepable global subprogs

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2025-02-28 at 15:34 -0800, Andrii Nakryiko wrote:

[...]

> > There were two alternatives on the table last time:
> > - add support for tags on global functions;
> 
> I was supportive of this, I believe
> 
> > - verify global subprogram call tree in post-order,
> >   in order to have the flags ready when needed.
> 
> Remind me of the details here? we'd start validating the main prog,
> suspend that process when encountering global func, go validate global
> func, once done, come back to main prog, right?

Yes.
The tree can't be built statically if we account for dead code
elimination, as post-order might change.

> Alternatively, we could mark expected properties (restrictions) of
> global subprogs as we encounter them, right? E.g, if we come to global
> func call inside rcu_read_{lock,unlock}() region, we'd mark it
> internally as "needs to be non-sleepable".

For situation like below, suppose verification order is main, foo,
bar, buz:
- main() sleepable
  - foo()
  - bar()
- foo():
  - buz()
- bar():
  - foo() while holding lock
- buz():
  - calls something sleepable

I think, to handle this the call-tree needs to be built on the main
verification pass, and then checked for sleepable.
But that won't work for changes_pkt_data, as verdict has to be known
right-away to decide whether to invalidate packet pointers.

[...]