Re: [PATCH bpf-next v6 1/4] bpf: Parameterize task iterators.

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

On Wed, 2022-08-24 at 15:20 -0700, Andrii Nakryiko wrote:
> On Fri, Aug 19, 2022 at 3:09 PM Kui-Feng Lee <kuifeng@fb.com> 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            |  25 +++++++
> >  include/uapi/linux/bpf.h       |   6 ++
> >  kernel/bpf/task_iter.c         | 116 ++++++++++++++++++++++++++---
> > ----
> >  tools/include/uapi/linux/bpf.h |   6 ++
> >  4 files changed, 129 insertions(+), 24 deletions(-)
> > 
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 39bd36359c1e..59712dd917d8 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1729,8 +1729,33 @@ int bpf_obj_get_user(const char __user
> > *pathname, int flags);
> >         extern int bpf_iter_ ## target(args);                   \
> >         int __init bpf_iter_ ## target(args) { return 0; }
> > 
> > +/*
> > + * The task type of iterators.
> > + *
> > + * For BPF task iterators, they can be parameterized with various
> > + * parameters to visit only some of tasks.
> > + *
> > + * BPF_TASK_ITER_ALL (default)
> > + *     Iterate over resources of every task.
> > + *
> > + * BPF_TASK_ITER_TID
> > + *     Iterate over resources of a task/tid.
> > + *
> > + * BPF_TASK_ITER_TGID
> > + *     Iterate over reosurces of evevry task of a process / task
> > group.
> 
> typos: resources, every
> 
> > + */
> > +enum bpf_iter_task_type {
> > +       BPF_TASK_ITER_ALL = 0,
> > +       BPF_TASK_ITER_TID,
> > +       BPF_TASK_ITER_TGID,
> > +};
> > +
> 
> [...]
> 
> >         rcu_read_lock();
> >  retry:
> > -       pid = find_ge_pid(*tid, ns);
> > +       pid = find_ge_pid(*tid, common->ns);
> >         if (pid) {
> > -               *tid = pid_nr_ns(pid, ns);
> > +               *tid = pid_nr_ns(pid, common->ns);
> >                 task = get_pid_task(pid, PIDTYPE_PID);
> >                 if (!task) {
> >                         ++*tid;
> >                         goto retry;
> > -               } else if (skip_if_dup_files &&
> > !thread_group_leader(task) &&
> > -                          task->files == task->group_leader-
> > >files) {
> > +               } else if ((skip_if_dup_files &&
> > !thread_group_leader(task) &&
> > +                           task->files == task->group_leader-
> > >files) ||
> > +                          (common->type == BPF_TASK_ITER_TGID &&
> > +                           __task_pid_nr_ns(task, PIDTYPE_TGID,
> > common->ns) != common->pid)) {
> 
> it gets super hard to follow this logic, would a simple helper
> function to calculate this condition (and maybe some comments to
> explain the logic behind these checks?) make it a bit more readable?

!matched_task(task, common, skip_if_dup_file)?

bool matched_task(struct task_struct *task, 
                  struct bpf_iter_seq_task_common *common,
                  bool skip_if_dup_file) {
        /* Should not have the same 'files' if skip_if_dup_file is true
*/
        bool diff_files_if =
                !skip_if_dup_file ||
                (thread_group_leader(task) &&
                task->file != task->gorup_leader->fies);
        /* Should have the given tgid if the type is BPF_TASK_ITER_TGI
*/
        bool have_tgid_if =
                common->type != BPF_TASK_ITER_TGID ||
                __task_pid_nr_ns(task, PIDTYPE_TGID,
                common->ns) == common->pid;
        return diff_files_if && have_tgid_if;
}

How about this?

> 
> >                         put_task_struct(task);
> >                         task = NULL;
> >                         ++*tid;
> > @@ -56,7 +73,7 @@ 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;
> > 
> > -       task = task_seq_get_next(info->common.ns, &info->tid,
> > false);
> > +       task = task_seq_get_next(&info->common, &info->tid, false);
> >         if (!task)
> >                 return NULL;
> > 
> > @@ -73,7 +90,7 @@ static void *task_seq_next(struct seq_file *seq,
> > void *v, loff_t *pos)
> >         ++*pos;
> >         ++info->tid;
> >         put_task_struct((struct task_struct *)v);
> > -       task = task_seq_get_next(info->common.ns, &info->tid,
> > false);
> > +       task = task_seq_get_next(&info->common, &info->tid, false);
> >         if (!task)
> >                 return NULL;
> > 
> > @@ -117,6 +134,48 @@ static void task_seq_stop(struct seq_file
> > *seq, void *v)
> >                 put_task_struct((struct task_struct *)v);
> >  }
> > 
> > +static int bpf_iter_attach_task(struct bpf_prog *prog,
> > +                               union bpf_iter_link_info *linfo,
> > +                               struct bpf_iter_aux_info *aux)
> > +{
> > +       unsigned int flags;
> > +       struct pid_namespace *ns;
> > +       struct pid *pid;
> > +       pid_t tgid;
> 
> it seems it would be simpler to first check that at most one of
> tid/pid/pid_fd is set instead of making sure that aux->task.type
> wasn't already set.
> 
> How about
> 
> if (!!linfo->task.tid + !!linfo->task.pid + !!linfo->task.pid_fd > 1)
>     return -EINVAL;
> 
> ?

Agree

> 
> > +
> > +       aux->task.type = BPF_TASK_ITER_ALL;
> > +       if (linfo->task.tid != 0) {
> > +               aux->task.type = BPF_TASK_ITER_TID;
> > +               aux->task.pid = linfo->task.tid;
> > +       }
> > +       if (linfo->task.pid != 0) {
> > +               if (aux->task.type != BPF_TASK_ITER_ALL)
> > +                       return -EINVAL;
> > +
> > +               aux->task.type = BPF_TASK_ITER_TGID;
> > +               aux->task.pid = linfo->task.pid;
> > +       }
> > +       if (linfo->task.pid_fd != 0) {
> > +               if (aux->task.type != BPF_TASK_ITER_ALL)
> > +                       return -EINVAL;
> > +
> > +               aux->task.type = BPF_TASK_ITER_TGID;
> > +               ns = task_active_pid_ns(current);
> > +               if (IS_ERR(ns))
> > +                       return PTR_ERR(ns);
> > +
> > +               pid = pidfd_get_pid(linfo->task.pid_fd, &flags);
> > +               if (IS_ERR(pid))
> > +                       return PTR_ERR(pid);
> > +
> > +               tgid = pid_nr_ns(pid, ns);
> > +               aux->task.pid = tgid;
> > +               put_pid(pid);
> > +       }
> > +
> > +       return 0;
> > +}
> > +
> 
> [...]