Re: [PATCH bpf-next 1/3] bpf: Parameterize task iterators.

[Kui-Feng Lee <kuifeng@fb.com>]

On Thu, 2022-07-28 at 10:47 +0200, Kumar Kartikeya Dwivedi wrote:
> On Thu, 28 Jul 2022 at 07:25, Kui-Feng Lee <kuifeng@fb.com> wrote:
> > 
> > On Wed, 2022-07-27 at 10:19 +0200, Kumar Kartikeya Dwivedi wrote:
> > > On Wed, 27 Jul 2022 at 09:01, Kui-Feng Lee <kuifeng@fb.com>
> > > wrote:
> > > > 
> > > > On Tue, 2022-07-26 at 14:13 +0200, Jiri Olsa wrote:
> > > > > On Mon, Jul 25, 2022 at 10:17:11PM -0700, Kui-Feng Lee wrote:
> > > > > > Allow creating an iterator that loops through resources of
> > > > > > one
> > > > > > task/thread.
> > > > > > 
> > > > > > People could only create iterators to loop through all
> > > > > > resources of
> > > > > > files, vma, and tasks in the system, even though they were
> > > > > > interested
> > > > > > in only the resources of a specific task or process. 
> > > > > > Passing
> > > > > > the
> > > > > > additional parameters, people can now create an iterator to
> > > > > > go
> > > > > > through all resources or only the resources of a task.
> > > > > > 
> > > > > > Signed-off-by: Kui-Feng Lee <kuifeng@fb.com>
> > > > > > ---
> > > > > >  include/linux/bpf.h            |  4 ++
> > > > > >  include/uapi/linux/bpf.h       | 23 ++++++++++
> > > > > >  kernel/bpf/task_iter.c         | 81
> > > > > > +++++++++++++++++++++++++-
> > > > > > ----
> > > > > > ----
> > > > > >  tools/include/uapi/linux/bpf.h | 23 ++++++++++
> > > > > >  4 files changed, 109 insertions(+), 22 deletions(-)
> > > > > > 
> > > > > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > > > > index 11950029284f..c8d164404e20 100644
> > > > > > --- a/include/linux/bpf.h
> > > > > > +++ b/include/linux/bpf.h
> > > > > > @@ -1718,6 +1718,10 @@ int bpf_obj_get_user(const char
> > > > > > __user
> > > > > > *pathname, int flags);
> > > > > > 
> > > > > >  struct bpf_iter_aux_info {
> > > > > >         struct bpf_map *map;
> > > > > > +       struct {
> > > > > > +               __u32   tid;
> > > > > 
> > > > > should be just u32 ?
> > > > 
> > > > Or, should change the following 'type' to __u8?
> > > 
> > > Would it be better to use a pidfd instead of a tid here? Unset
> > > pidfd
> > > would mean going over all tasks, and any fd > 0 implies attaching
> > > to
> > > a
> > > specific task (as is the convention in BPF land). Most of the new
> > > UAPIs working on processes are using pidfds (to work with a
> > > stable
> > > handle instead of a reusable ID).
> > > The iterator taking an fd also gives an opportunity to BPF LSMs
> > > to
> > > attach permissions/policies to it (once we have a file local
> > > storage
> > > map) e.g. whether creating a task iterator for that specific
> > > pidfd
> > > instance (backed by the struct file) would be allowed or not.
> > > You are using getpid in the selftest and keeping track of
> > > last_tgid
> > > in
> > > the iterator, so I guess you don't even need to extend pidfd_open
> > > to
> > > work on thread IDs right now for your use case (and fdtable and
> > > mm
> > > are
> > > shared for POSIX threads anyway, so for those two it won't make a
> > > difference).
> > > 
> > > What is your opinion?
> > 
> > Do you mean removed both tid and type, and replace them with a
> > pidfd?
> > We can do that in uapi, struct bpf_link_info.  But, the interal
> > types,
> > ex. bpf_iter_aux_info, still need to use tid or struct file to
> > avoid
> > getting file from the per-process fdtable.  Is that what you mean?
> > 
> 
> Yes, just for the UAPI, it is similar to taking map_fd for map iter.
> In bpf_link_info we should report just the tid, just like map iter
> reports map_id.

It sounds good to me.

One thing I need a clarification. You mentioned that a fd > 0 implies
attaching to a specific task, however fd can be 0. So, it should be fd
>= 0. So, it forces the user to initialize the value of pidfd to -1. 
So, for convenience, we still need a field like 'type' to make it easy
to create iterators without a filter.