linux-kernel - Re: [PATCH v3 3/3] bpf/selftests: Add selftests for new task kfuncs

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Message-ID: <CAP01T77PTK+bD2mBrxJShKNPhEypT2+nSHcr3=uuJbrghv_wFg@mail.gmail.com>
Date:   Tue, 18 Oct 2022 07:23:23 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     David Vernet <void@...ifault.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, tj@...nel.org
Subject: Re: [PATCH v3 3/3] bpf/selftests: Add selftests for new task kfuncs

On Sat, 15 Oct 2022 at 01:45, David Vernet <void@...ifault.com> wrote:
>
> A previous change added a series of kfuncs for storing struct
> task_struct objects as referenced kptrs. This patch adds a new
> task_kfunc test suite for validating their expected behavior.
>
> Signed-off-by: David Vernet <void@...ifault.com>
> ---
> [...]
> +
> +SEC("tp_btf/task_newtask")
> +int BPF_PROG(task_kfunc_acquire_trusted_nested, struct task_struct *task, u64 clone_flags)
> +{
> +       struct task_struct *acquired;
> +
> +       if (!is_test_kfunc_task())
> +               return 0;
> +
> +       /* Can't invoke bpf_task_acquire() on a trusted pointer at a nonzero offset. */
> +       acquired = bpf_task_acquire(task->last_wakee);

The comment is incorrect, that would be &task->last_wakee instead,
this is PTR_TO_BTF_ID | PTR_NESTED.

> +       if (!acquired)
> +               return 0;
> +       bpf_task_release(acquired);
> +
> +       return 0;
> +}
> +
> [...]
> +
> +static int test_acquire_release(struct task_struct *task)
> +{
> +       struct task_struct *acquired;
> +
> +       acquired = bpf_task_acquire(task);

Unfortunately a side effect of this change is that now since
PTR_TO_BTF_ID without ref_obj_id is considered trusted, the bpf_ct_*
functions would begin working with tp_btf args. That probably needs to
be fixed so that they reject them (ideally with a failing test case to
make sure it doesn't resurface), probably with a new suffix __ref/or
__owned as added here [0].

Alexei, since you've suggested avoiding adding that suffix, do you see
any other way out here?
It's questionable whether bpf_ct_set_timeout/status should work for CT
not owned by the BPF program.

  [0]: https://lore.kernel.org/bpf/dfb859a6b76a9234baa194e795ae89cb7ca5694b.1662383493.git.lorenzo@kernel.org

> +       if (!acquired) {
> +               err = 1;
> +               return 0;
> +       }
> +
> +       bpf_task_release(acquired);
> +
> +       return 0;
> +}
> +
> [...]