Re: [PATCH bpf-next v8 1/5] bpf: Parameterize task iterators.

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

On Tue, 2022-08-30 at 16:54 -0700, Yonghong Song wrote:
> 
> 
> On 8/29/22 12:23 PM, Kui-Feng Lee wrote:
> > Allow creating an iterator that loops through resources of one
> > thread/process.
> > 
> > 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>
> > Acked-by: Yonghong Song <yhs@fb.com>
> > ---
> >   include/linux/bpf.h            |  25 +++++
> >   include/uapi/linux/bpf.h       |   6 ++
> >   kernel/bpf/task_iter.c         | 184
> > +++++++++++++++++++++++++++++----
> >   tools/include/uapi/linux/bpf.h |   6 ++
> >   4 files changed, 199 insertions(+), 22 deletions(-)
> > 
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 9c1674973e03..31ac2c1181f5 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1730,6 +1730,27 @@ 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 resources of every task of a process / task
> > group.
> > + */
> > +enum bpf_iter_task_type {
> > +       BPF_TASK_ITER_ALL = 0,
> > +       BPF_TASK_ITER_TID,
> > +       BPF_TASK_ITER_TGID,
> > +};
> > +
> >   struct bpf_iter_aux_info {
> >         /* for map_elem iter */
> >         struct bpf_map *map;
> > @@ -1739,6 +1760,10 @@ struct bpf_iter_aux_info {
> >                 struct cgroup *start; /* starting cgroup */
> >                 enum bpf_cgroup_iter_order order;
> >         } cgroup;
> > +       struct {
> > +               enum bpf_iter_task_type type;
> > +               u32 pid;
> > +       } task;
> >   };
> >   
> >   typedef int (*bpf_iter_attach_target_t)(struct bpf_prog *prog,
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 962960a98835..f212a19eda06 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -110,6 +110,12 @@ union bpf_iter_link_info {
> >                 __u32   cgroup_fd;
> >                 __u64   cgroup_id;
> >         } cgroup;
> > +       /* Parameters of task iterators. */
> > +       struct {
> > +               __u32   tid;
> > +               __u32   pid;
> > +               __u32   pid_fd;
> > +       } task;
> >   };
> >   
> >   /* BPF syscall commands, see bpf(2) man-page for more details. */
> > diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> > index 8c921799def4..0bc7277d1ee1 100644
> > --- a/kernel/bpf/task_iter.c
> > +++ b/kernel/bpf/task_iter.c
> > @@ -12,6 +12,9 @@
> >   
> >   struct bpf_iter_seq_task_common {
> >         struct pid_namespace *ns;
> > +       enum bpf_iter_task_type type;
> > +       u32 pid;
> > +       u32 pid_visiting;
> >   };
> >   
> >   struct bpf_iter_seq_task_info {
> > @@ -22,18 +25,107 @@ struct bpf_iter_seq_task_info {
> >         u32 tid;
> >   };
> >   
> > -static struct task_struct *task_seq_get_next(struct pid_namespace
> > *ns,
> > +static struct task_struct *task_group_seq_get_next(struct
> > bpf_iter_seq_task_common *common,
> > +                                                  u32 *tid,
> > +                                                  bool
> > skip_if_dup_files)
> > +{
> > +       struct task_struct *task, *next_task;
> > +       struct pid *pid;
> > +       u32 saved_tid;
> > +
> > +       if (!*tid) {
> 
> Add a comment in the above to say that this is for the *very first* 
> visit of tasks in the process.
> 
> > +               pid = find_pid_ns(common->pid, common->ns);
> > +               if (pid)
> > +                       task = get_pid_task(pid, PIDTYPE_TGID);
> 
> 'task' is not initialized, so it is possible task could hold a
> garbase value here if !pid, right?
> 
> Also if indeed task is NULL, here, should we return NULL here
> first?

yes, it should return earlier.

> 
> > +
> > +               *tid = common->pid;
> > +               common->pid_visiting = common->pid;
> > +
> > +               return task;
> > +       }
> > +
> > +       /* The callers increase *tid by 1 once they want next task.
> > +        * However, next_thread() doesn't return tasks in
> > incremental
> > +        * order of pids. We can not find next task by just finding
> > a
> > +        * task whose pid is greater or equal to *tid. 
> > pid_visiting
> > +        * remembers the pid value of the task returned last time.
> > By
> > +        * comparing pid_visiting and *tid, we known if the caller
> > +        * wants the next task.
> > +        */
> > +       if (*tid == common->pid_visiting) {
> > +               pid = find_pid_ns(common->pid_visiting, common-
> > >ns);
> > +               task = get_pid_task(pid, PIDTYPE_PID);
> > +
> > +               return task;
> > +       }
> 
> Do not understand the above code. Why we need it? Looks like
> the code below trying to get the *next_task* and will return NULL
> if wrap around happens(the tid again equals tgid), right?

The above code is to handle the case that the caller want to visit the
same task again.  For example, task_file_seq_get_next() will call this
function several time to return the same task, and move to next task by
increasing info->tid.  The above code checks the value of *tid to
return the same task if the value doesn't change.

> 
> > +
> > +retry:
> > +       pid = find_pid_ns(common->pid_visiting, common->ns);
> > +       if (!pid)
> > +               return NULL;
> > +
> > +       task = get_pid_task(pid, PIDTYPE_PID);
> > +       if (!task)
> > +               return NULL;
> > +
> > +       next_task = next_thread(task);
> > +       put_task_struct(task);
> > +       if (!next_task)
> > +               return NULL;
> > +
> > +       saved_tid = *tid;
> > +       *tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common-
> > >ns);
> > +       if (*tid == common->pid) {
> > +               /* Run out of tasks of a process.  The tasks of a
> > +                * thread_group are linked as circular linked list.
> > +                */
> > +               *tid = saved_tid;
> > +               return NULL;
> > +       }
> > +
> > +       get_task_struct(next_task);
> > +       common->pid_visiting = *tid;
> 
> We could do quite some redundant works here if the following
> condition is true. Basically, we get next_task and get a tid
> and release it, but in the next iteration, from tid, we try to get
> the task again.

Yes, I will move 'retry' and move next_task to task to avoid the
redundant work.

> 
> > +
> > +       if (skip_if_dup_files && task->files == task->group_leader-
> > >files)
> > +               goto retry;
> > +
> > +       return next_task;
> > +}
> > +
> > +static struct task_struct *task_seq_get_next(struct
> > bpf_iter_seq_task_common *common,
> >                                              u32 *tid,
> >                                              bool
> > skip_if_dup_files)
> >   {
> >         struct task_struct *task = NULL;
> >         struct pid *pid;
> >   
> > +       if (common->type == BPF_TASK_ITER_TID) {
> > +               if (*tid && *tid != common->pid)
> > +                       return NULL;
> > +               rcu_read_lock();
> > +               pid = find_pid_ns(common->pid, common->ns);
> > +               if (pid) {
> > +                       task = get_pid_task(pid, PIDTYPE_TGID);
> > +                       *tid = common->pid;
> > +               }
> > +               rcu_read_unlock();
> > +
> > +               return task;
> > +       }
> > +
> > +       if (common->type == BPF_TASK_ITER_TGID) {
> > +               rcu_read_lock();
> > +               task = task_group_seq_get_next(common, tid,
> > skip_if_dup_files);
> > +               rcu_read_unlock();
> > +
> > +               return task;
> > +       }
> > +
> >         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;
> > @@ -56,7 +148,7 @@ 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 +165,7 @@ 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 +209,45 @@ static void task_seq_stop(struct seq_file
> > *seq, void *v)
> >                 put_task_struct((struct task_struct *)v);
> >   }
> >   
> [...]