linux-kernel - Re: [PATCH bpf-next 1/2] bpf/docs: Document struct task

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Date:   Mon, 5 Dec 2022 14:58:22 -0600
From:   David Vernet <void@...ifault.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>, F@...iforge.lan
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...a.com, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH bpf-next 1/2] bpf/docs: Document struct task_struct *
 kfuncs

On Mon, Dec 05, 2022 at 10:10:14AM -0600, David Vernet wrote:
> > > +.. code-block:: c
> > > +
> > > +	SEC("tp_btf/task_newtask")
> > > +	int BPF_PROG(task_get_pid_example, struct task_struct *task, u64 clone_flags)
> > > +	{
> > > +		struct task_struct *lookup;
> > > +
> > > +		lookup = bpf_task_from_pid(task->pid);
> > > +		if (!lookup)
> > > +			/* A task should always be found, as %task is a tracepoint arg. */
> > > +			return -ENOENT;
> > > +
> > > +		if (lookup->pid != task->pid) {
> > > +			/* The pid of the lookup task should be the same as the input task. */
> > 
> > I suspect both "errors" are actually possible in practice,
> > since bpf_task_from_pid is using init_pid_ns.
> > But this taskd might be in different pid_ns. See task_active_pid_ns.
> > Probably worth mentioning this aspect of bpf_task_from_pid.
> 
> Yep, agreed. Will add

Actually, I don't think either error can ever happen. p->pid is globally
unique, and always uses the init_pid_ns. See [0] where p->pid is set,
and [1] for the implementation of pid_nr(). So I think the existing
example is actually correct, though I'll still add some comments to
explain that the kfunc only works for p->pid / the init_pid_ns.

[0]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/fork.c#n2326
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/pid.h#n181