Re: [RFC PATCH 0/2] livepatch: Add support for hybrid mode

[Song Liu]

On Wed, Feb 5, 2025 at 6:55 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
[...]
> > I think we should first understand why the trampoline is not
> > freed.
>
> IIUC, the fexit works as follows,
>
>   bpf_trampoline
>     + __bpf_tramp_enter
>        + percpu_ref_get(&tr->pcref);
>
>     + call do_exit()
>
>     + __bpf_tramp_exit
>        + percpu_ref_put(&tr->pcref);
>
> Since do_exit() never returns, the refcnt of the trampoline image is
> never decremented, preventing it from being freed.

Thanks for the explanation. In this case, I think it makes sense to
disallow attaching fexit programs on __noreturn functions. I am not
sure what is the best solution for it though.

Thanks,
Song


> >
> > > We could either add functions annotated as "__noreturn" to the deny
> > > list for fexit as follows, or we could explore a more generic
> > > solution, such as embedding the "noreturn" information into the BTF
> > > and extracting it when attaching fexit.
> >
> > I personally don't think this is really necessary. It is good to have.
> > But a reasonable user should not expect noreturn function to
> > generate fexit events.