linux-kernel - Re: [PATCH bpf-next v3 1/4] bpf/crib: Introduce task

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Message-ID: <CAADnVQ+0LUXxmfm1YgyGDz=cciy3+dGGM-Zysq84fpAdaB74Qw@mail.gmail.com>
Date: Wed, 6 Nov 2024 14:13:46 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Juntong Deng <juntong.deng@...look.com>, 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, 
	Christian Brauner <brauner@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 1/4] bpf/crib: Introduce task_file open-coded
 iterator kfuncs

On Wed, Nov 6, 2024 at 2:10 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
> > > +__bpf_kfunc int bpf_iter_task_file_new(struct bpf_iter_task_file *it,
> > > +               struct task_struct *task)
> > > +{
> > > +       struct bpf_iter_task_file_kern *kit = (void *)it;
> > > +
> > > +       BUILD_BUG_ON(sizeof(struct bpf_iter_task_file_kern) > sizeof(struct bpf_iter_task_file));
> > > +       BUILD_BUG_ON(__alignof__(struct bpf_iter_task_file_kern) !=
> > > +                    __alignof__(struct bpf_iter_task_file));
> > > +
> > > +       kit->task = task;
> >
> > This is broken, since task refcnt can drop while iter is running.
>
> I noticed this as well, but I thought that given KF_TRUSTED_ARGS we
> should have a guarantee that the task survives the iteration? Am I
> mistaken?

KF_TRUSTED_ARGS will only guarantee that the task is valid when it's
passed into this kfunc. Right after the prog can call
bpf_task_release() to release the ref and kit->task will become
dangling.
If this object was RCU protected we could have marked this iter
as KF_RCU_PROTECTED, then the verifier would make sure that
RCU unlock doesn't happen between iter_new and iter_destroy.