linux-kernel - Re: [PATCH bpf-next 1/2] bpf: Add bpf_get_task

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Message-ID: <CAEf4BzaXozVSTsC7XZ8Ojkju1szk65nAg8Zc5Y_2OVewKV4heA@mail.gmail.com>
Date: Tue, 25 Nov 2025 15:32:09 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Tao Chen <chen.dylane@...ux.dev>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf: Add bpf_get_task_cmdline kfunc

On Fri, Nov 21, 2025 at 5:17 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Nov 18, 2025 at 4:58 AM Tao Chen <chen.dylane@...ux.dev> wrote:
> >
> > Add the bpf_get_task_cmdline kfunc. One use case is as follows: In
> > production environments, there are often short-lived script tasks executed,
> > and sometimes these tasks may cause stability issues. It is desirable to
> > detect these script tasks via eBPF. The common approach is to check
> > the process name, but it can be difficult to distinguish specific
> > tasks in some cases. Take the shell as an example: some tasks are
> > started via bash xxx.sh – their process name is bash, but the script
> > name of the task can be obtained through the cmdline. Additionally,
> > myabe this is helpful for security auditing purposes.
>
> maybe
>
> >
> > Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
> > ---
> >  kernel/bpf/helpers.c | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> >
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 865b0dae38d..7cac17d58d5 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -2685,6 +2685,27 @@ __bpf_kfunc struct task_struct *bpf_task_from_pid(s32 pid)
> >         return p;
> >  }
> >
> > +/*
> > + * bpf_get_task_cmdline - Get the cmdline to a buffer
> > + *
> > + * @task: The task whose cmdline to get.
> > + * @buffer: The buffer to save cmdline info.
> > + * @len: The length of the buffer.
> > + *
> > + * Return: the size of the cmdline field copied. Note that the copy does
> > + * not guarantee an ending NULL byte. A negative error code on failure.
> > + */
> > +__bpf_kfunc int bpf_get_task_cmdline(struct task_struct *task, char *buffer, size_t len)
>
> 'size_t len' doesn't make the verifier track the size of the buffer.
> while 'char *buffer' tells the verifier to check that _one_ byte is available.
> So this is buggy.
>
> In general the kfunc seems useful, but selftest in patch 2 is just bad
>

Besides that mm->arg_lock spinlock (which I don't think matters all
that much for BPF programs), is there anything special in
get_cmdline() that BPF program cannot just implemented? Ultimately,
it's just copying mm->arg_start and mm->env_start zero-separated
strings, no? We have bpf_copy_from_user_task_str() and also
dynptr-based equivalent of it for even more variable-length
flexibility. That should be all one needs, no?

> + ret = bpf_get_task_cmdline(task, buf, sizeof(buf));
> + if (ret < 0)
> +    err = 1;
> +
> + return 0;
> +}
>
> it's not testing much.
>
> Also you must explain the true motivation for the kfunc.
> "maybe helpful for security" is too vague.
> Do you have a proprietary bpf-lsm that needs it?
> What is the exact use case?
>
> pw-bot: cr