Re: [PATCH bpf-next 1/3] bpf: Make struct task_struct an RCU-safe type

[Alexei Starovoitov]

On Thu, Mar 30, 2023 at 07:57:31PM -0500, David Vernet wrote:
>  kernel/bpf/helpers.c                          | 11 ++-
>  kernel/bpf/verifier.c                         |  1 +
>  .../selftests/bpf/prog_tests/task_kfunc.c     |  2 +
>  .../selftests/bpf/progs/task_kfunc_common.h   |  5 +
>  .../selftests/bpf/progs/task_kfunc_failure.c  | 98 +++++++++++++++++--
>  .../selftests/bpf/progs/task_kfunc_success.c  | 52 +++++++++-
>  6 files changed, 153 insertions(+), 16 deletions(-)

See CI failures on gcc compiled kernel:
https://github.com/kernel-patches/bpf/actions/runs/4570493668/jobs/8068004031

>  __bpf_kfunc struct task_struct *bpf_task_acquire(struct task_struct *p)
>  {
> -	return get_task_struct(p);
> +	if (refcount_inc_not_zero(&p->rcu_users))
> +		return p;
> +	return NULL;
>  }

I wonder whether we should add a bit of safety net here.
Like do not allow acquire of tasks with PF_KTHREAD | PF_EXITING
or at least is_idle_task ?