Re: [PATCH bpf] bpf: Keep progs alive until the trampoline image calling them is freed

["Florent Revest" <florent.revest@linux.dev>]

On Mon Aug 31, 2026 at 2:43 AM UTC, Kumar Kartikeya Dwivedi wrote:
> On Sun Aug 30, 2026 at 3:21 PM CEST, Alexei Starovoitov wrote:
> > On Sun, Aug 30, 2026 at 3:41 AM Kumar Kartikeya Dwivedi
> > <memxor@gmail.com> wrote:
> >>
> >> Overall, looks good to me. Thanks for the fix!
> >>
> >> Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> >>
> >> Note for whoever applies this: please add Reported-by: tag for Sechang as well.
> >> Optionally, wordsmith the commit log with the suggestion above.
> >
> > Hold on. I don't think we can proceed with this fix.
> > It defeats the point of fexit jmp patching and keeps progs
> > pinned until a sleepable kernel function that were attached to
> > will return. Which means that the tracing prog attached to "unlucky" kernel
> > function that sleeps for an hour will stay pinned for an hour.
> > Let's think of a different way of fixing the race.
>
> I don't have background on the original commit being fixed, but is that really
> realistic? Or worrisome even if it happens in practice, since worst case the
> program refcounts remains raised for that duration?
>
> We have similar worst case for programs too (e.g. using bpf_copy_from_user on
> user controlled buffer in, say, LSM progs). At least here we won't be extending
> any RCU flavored GP.
>
> That said I will think about alternative fixes in the meantime, if we accept the
> premise that we don't want to pin program references in the image and keep their
> lifetimes decoupled.

How about having old trampolines skip freed programs instead of keeping
them alive ?

For example, bpf_tramp_image could hold one RCU managed prog pointer per
prog it calls and we could JIT trampolines that'd pass that slot to
__bpf_prog_enter*() instead of the prog itself.

The enter helpers could rcu_dereference() the prog under their
rcu_read_lock()/rcu_read_lock_trace() and return 0 when it is NULL.
Would that work ?