Re: [PATCH bpf-next v2 0/2] bpf: Fix sleepable check for tracing prog

On Thu, Jul 30, 2026 at 10:20 PM Leon Hwang <[email protected]> wrote:
>
> On 31/7/26 08:07, Andrii Nakryiko wrote:
> > On Sat, Jul 25, 2026 at 6:27 AM Leon Hwang <[email protected]> wrote:
> >>
> >> When CONFIG_FUNCTION_ERROR_INJECTION is disabled, a sleepable tracing prog
> >> is allowed to attach to '__x64_'-alike prefix symbols.
> >>
> >> It is because the verifier does not verify whether the symbol is a kernel
> >> function or a bpf prog. That said, a sleepable tracing prog is allowed to
> >> attach to a bpf prog target whose name has '__x64_'-alike prefix.
> >>
> >> For example, a sleepable fentry prog attaches to a '__x64_sys_nop' XDP
> >
> > we do have addr, so we should be able to distinguish between attaching
> > to kernel function vs BPF program, no?
>
>
> Yes, we can distinguish a bpf prog from a kernel function by addr.
>
> I'd prefer passing 'tgt_prog' to btf_id_allow_sleepable() as a simple
> change.
>

I don't think there is a need for new tgt_prog argument, we can just
check that supplied btf is not kernel/module BTF (see btf_is_kernel())

> >
> >> prog, and copies buffer from a user pointer with bpf_copy_from_user()
> >> helper. After attaching the XDP prog to lo interface, the kernel BUG
> >> could be triggered by 'ping -c 1 -W 1 127.0.0.1':
> >>
> >> [    3.460756] BUG: sleeping function called from invalid context at kernel/bpf/trampoline.c:1324
> >>
> >> Fix it by disallowing sleepable tracing prog always when its target is
> >> bpf prog.
> >
> > what happens when we freplace sleepable BPF program/subprogram with
> > another sleepable BPF subprogram? And same question for sleepable
> > fentry/fexit program attaching to sleepable BPF program? Is it
> > something that just cannot work or we can actually allow that?
>
>
> Currently, sleepable freplace/fentry/fexit progs are already disallowed
> from attaching to bpf prog, because btf_id_allow_sleepable() returns
> -EINVAL for them (except for this BUG).
>
> Since no one has proposed relaxing the restriction, I'd like to keep it
> as-is when fixing this BUG.
>
> I'm not sure whether allowing it would introduce any issue.
>
> Thanks,
> Leon
>
> > [...]
>
>