linux-kernel - Re: [PATCH bpf-next v5 2/5] selftests/bpf: Add tests for open-coded style process file iterator

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Message-ID: <CAADnVQKK3vmfPmRxLuh6ad94FeioN2JV=v+L-93ZvwdYqR_Kcg@mail.gmail.com>
Date: Tue, 10 Dec 2024 10:51:17 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Juntong Deng <juntong.deng@...look.com>
Cc: Christian Brauner <brauner@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Eddy Z <eddyz87@...il.com>, 
	Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>, snorcht@...il.com, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 2/5] selftests/bpf: Add tests for open-coded
 style process file iterator

On Tue, Dec 10, 2024 at 8:23 AM Juntong Deng <juntong.deng@...look.com> wrote:
>
> >> +SEC("fentry/" SYS_PREFIX "sys_nanosleep")
> >> +int test_bpf_iter_task_file(void *ctx)
> >> +{
> >> +    struct bpf_iter_task_file task_file_it;
> >> +    struct bpf_iter_task_file_item *item;
> >> +    struct task_struct *task;
> >> +
> >> +    task = bpf_get_current_task_btf();
> >> +    if (task->parent->pid != parent_pid)
> >> +            return 0;
> >> +
> >> +    count++;
> >> +
> >> +    bpf_rcu_read_lock();
> >
> > What does the RCU read lock do here exactly?
> >
>
> Thanks for your reply.
>
> This is used to solve the problem previously discussed in v3 [0].
>
> Task ref may be released during iteration.
>
> [0]:
> https://lore.kernel.org/bpf/CAADnVQ+0LUXxmfm1YgyGDz=cciy3+dGGM-Zysq84fpAdaB74Qw@mail.gmail.com/

I think you misunderstood my comment.

"If this object _was_ RCU protected ..."

Adding rcu_read_lock doesn't make 'task' pointer RCU protected.
That's not how RCU works.

So patch 1 doing:

item->task = task;

is not correct.

See bpf_iter_task_vma_new(). It's doing:
kit->data->task = get_task_struct(task);
to make sure task stays valid while iterating.

pw-bot: cr