netdev - Re: [PATCH bpf-next v1 11/19] bpf: add task and task/file targets

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Date:   Wed, 29 Apr 2020 19:08:43 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next v1 11/19] bpf: add task and task/file targets

On Mon, Apr 27, 2020 at 1:17 PM Yonghong Song <yhs@...com> wrote:
>
> Only the tasks belonging to "current" pid namespace
> are enumerated.
>
> For task/file target, the bpf program will have access to
>   struct task_struct *task
>   u32 fd
>   struct file *file
> where fd/file is an open file for the task.
>
> Signed-off-by: Yonghong Song <yhs@...com>
> ---
>  kernel/bpf/Makefile    |   2 +-
>  kernel/bpf/task_iter.c | 319 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 320 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/bpf/task_iter.c
>

[...]

> +static void *task_seq_start(struct seq_file *seq, loff_t *pos)
> +{
> +       struct bpf_iter_seq_task_info *info = seq->private;
> +       struct task_struct *task;
> +       u32 id = info->id;
> +
> +       if (*pos == 0)
> +               info->ns = task_active_pid_ns(current);

I wonder why pid namespace is set in start() callback each time, while
net_ns was set once when seq_file is created. I think it should be
consistent, no? Either pid_ns is another feature and is set
consistently just once using the context of the process that creates
seq_file, or net_ns could be set using the same method without
bpf_iter infra knowing about this feature? Or there are some
non-obvious aspects which make pid_ns easier to work with?

Either way, process read()'ing seq_file might be different than
process open()'ing seq_file, so they might have different namespaces.
We need to decide explicitly which context should be used and do it
consistently.

> +
> +       task = task_seq_get_next(info->ns, &id);
> +       if (!task)
> +               return NULL;
> +
> +       ++*pos;
> +       info->task = task;
> +       info->id = id;
> +
> +       return task;
> +}
> +
> +static void *task_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> +{
> +       struct bpf_iter_seq_task_info *info = seq->private;
> +       struct task_struct *task;
> +
> +       ++*pos;
> +       ++info->id;

this would make iterator skip pid 0? Is that by design?

> +       task = task_seq_get_next(info->ns, &info->id);
> +       if (!task)
> +               return NULL;
> +
> +       put_task_struct(info->task);

on very first iteration info->task might be NULL, right?

> +       info->task = task;
> +       return task;
> +}
> +
> +struct bpf_iter__task {
> +       __bpf_md_ptr(struct bpf_iter_meta *, meta);
> +       __bpf_md_ptr(struct task_struct *, task);
> +};
> +
> +int __init __bpf_iter__task(struct bpf_iter_meta *meta, struct task_struct *task)
> +{
> +       return 0;
> +}
> +
> +static int task_seq_show(struct seq_file *seq, void *v)
> +{
> +       struct bpf_iter_meta meta;
> +       struct bpf_iter__task ctx;
> +       struct bpf_prog *prog;
> +       int ret = 0;
> +
> +       prog = bpf_iter_get_prog(seq, sizeof(struct bpf_iter_seq_task_info),
> +                                &meta.session_id, &meta.seq_num,
> +                                v == (void *)0);
> +       if (prog) {

can it happen that prog is NULL?


> +               meta.seq = seq;
> +               ctx.meta = &meta;
> +               ctx.task = v;
> +               ret = bpf_iter_run_prog(prog, &ctx);
> +       }
> +
> +       return ret == 0 ? 0 : -EINVAL;
> +}
> +
> +static void task_seq_stop(struct seq_file *seq, void *v)
> +{
> +       struct bpf_iter_seq_task_info *info = seq->private;
> +
> +       if (!v)
> +               task_seq_show(seq, v);

hmm... show() called from stop()? what's the case where this is necessary?
> +
> +       if (info->task) {
> +               put_task_struct(info->task);
> +               info->task = NULL;
> +       }
> +}
> +

[...]