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Date:   Fri, 31 Mar 2023 10:05:04 -0700
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     David Vernet <void@manifault.com>
Cc:     bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net,
        andrii@kernel.org, martin.lau@linux.dev, song@kernel.org,
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        sdf@google.com, haoluo@google.com, jolsa@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@meta.com
Subject: Re: [PATCH bpf-next 1/3] bpf: Make struct task_struct an RCU-safe
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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 ?