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

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Message-ID: <CAADnVQKuyqY7J4iJ=FZVNoon2y_v866H9hvjAn-06c8nq577Ng@mail.gmail.com>
Date: Wed, 6 Nov 2024 13:31:58 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Juntong Deng <juntong.deng@...look.com>
Cc: 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 11:39 AM Juntong Deng <juntong.deng@...look.com> wrote:
>
> This patch adds the open-coded iterator style process file iterator
> kfuncs bpf_iter_task_file_{new,next,destroy} that iterates over all
> files opened by the specified process.

This is ok.

> In addition, this patch adds bpf_iter_task_file_get_fd() getter to get
> the file descriptor corresponding to the file in the current iteration.

Unnecessary. Use CORE to read iter internal fields.

> The reference to struct file acquired by the previous
> bpf_iter_task_file_next() is released in the next
> bpf_iter_task_file_next(), and the last reference is released in the
> last bpf_iter_task_file_next() that returns NULL.
>
> In the bpf_iter_task_file_destroy(), if the iterator does not iterate to
> the end, then the last struct file reference is released at this time.
>
> Signed-off-by: Juntong Deng <juntong.deng@...look.com>
> ---
>  kernel/bpf/helpers.c   |  4 ++
>  kernel/bpf/task_iter.c | 96 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 100 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 395221e53832..1f0f7ca1c47a 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -3096,6 +3096,10 @@ BTF_ID_FLAGS(func, bpf_iter_css_destroy, KF_ITER_DESTROY)
>  BTF_ID_FLAGS(func, bpf_iter_task_new, KF_ITER_NEW | KF_TRUSTED_ARGS | KF_RCU_PROTECTED)
>  BTF_ID_FLAGS(func, bpf_iter_task_next, KF_ITER_NEXT | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_iter_task_destroy, KF_ITER_DESTROY)
> +BTF_ID_FLAGS(func, bpf_iter_task_file_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
> +BTF_ID_FLAGS(func, bpf_iter_task_file_next, KF_ITER_NEXT | KF_RET_NULL)
> +BTF_ID_FLAGS(func, bpf_iter_task_file_get_fd)
> +BTF_ID_FLAGS(func, bpf_iter_task_file_destroy, KF_ITER_DESTROY)
>  BTF_ID_FLAGS(func, bpf_dynptr_adjust)
>  BTF_ID_FLAGS(func, bpf_dynptr_is_null)
>  BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index 5af9e130e500..32e15403a5a6 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -1031,6 +1031,102 @@ __bpf_kfunc void bpf_iter_task_destroy(struct bpf_iter_task *it)
>  {
>  }
>
> +struct bpf_iter_task_file {
> +       __u64 __opaque[3];
> +} __aligned(8);
> +
> +struct bpf_iter_task_file_kern {
> +       struct task_struct *task;
> +       struct file *file;
> +       int fd;
> +} __aligned(8);
> +
> +/**
> + * bpf_iter_task_file_new() - Initialize a new task file iterator for a task,
> + * used to iterate over all files opened by a specified task
> + *
> + * @it: the new bpf_iter_task_file to be created
> + * @task: a pointer pointing to a task to be iterated over
> + */
> +__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.

Before doing any of that I'd like to see a long term path for crib.
All these small additions are ok if they're generic and useful elsewhere.
I'm afraid there is no path forward for crib itself though.

pw-bot: cr