Re: [PATCH bpf-next v3 1/4] bpf/crib: Introduce task_file open-coded iterator kfuncs

[Juntong Deng]

On 2024/11/6 21:31, Alexei Starovoitov wrote:
> On Wed, Nov 6, 2024 at 11:39 AM Juntong Deng <juntong.deng@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
> >   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

Thanks for your reply.

The long-term path of CRIB is consistent with the initial goal, adding
kfuncs to help the bpf program obtain process-related information.

I think most of the CRIB kfuncs are generic, such as process file
iterator, skb iterator, bpf_fget_task() that gets struct file based on
file descriptor, etc.

This is because obtaining process-related information is not a
requirement specific to checkpoint/restore scenarios, but is
required in other scenarios as well.

Here I would like to quote your vision on LPC 2022 [0] [1].

"Starovoitov concluded his presentation by sharing his vision for the
future of BPF: replacing kernel modules as the de-facto means of
extending the kernel."

"BPF programs are safe and portable kernel modules"

[0]: https://lwn.net/Articles/909095/
[1]: 
https://lpc.events/event/16/contributions/1346/attachments/1021/1966/bpf_LPC_2022.pdf

If the future of BPF is to become a better kernel module and BPF kfuncs
is the equivalent of a better EXPORT_SYMBOL_GPL.

Then all CRIB kfuncs are useful. CRIB essentially gives bpf programs the
ability to access process information.

Giving bpf the ability to access process information is part of making
bpf a generic and better "kernel module".

Therefore I believe that CRIB is consistent with the long-term vision of
BPF, or in other words CRIB is part of the long-term vision of BPF.

Many thanks.