On Tue, Aug 9, 2022 at 3:35 PM Kui-Feng Lee <kuifeng@xxxxxx> wrote:
>
> On Tue, 2022-08-09 at 15:12 -0700, Alexei Starovoitov wrote:
> > On Tue, Aug 9, 2022 at 12:54 PM Kui-Feng Lee <kuifeng@xxxxxx> 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@xxxxxx>
> > > ---
> > > include/linux/bpf.h | 8 ++
> > > include/uapi/linux/bpf.h | 36 +++++++++
> > > kernel/bpf/task_iter.c | 134 +++++++++++++++++++++++++++--
> > > ----
> > > tools/include/uapi/linux/bpf.h | 36 +++++++++
> > > 4 files changed, 190 insertions(+), 24 deletions(-)
> > >
> > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > index 11950029284f..bef81324e5f1 100644
> > > --- a/include/linux/bpf.h
> > > +++ b/include/linux/bpf.h
> > > @@ -1718,6 +1718,14 @@ int bpf_obj_get_user(const char __user
> > > *pathname, int flags);
> > >
> > > struct bpf_iter_aux_info {
> > > struct bpf_map *map;
> > > + struct {
> > > + enum bpf_iter_task_type type;
> > > + union {
> > > + u32 tid;
> > > + u32 tgid;
> > > + u32 pid_fd;
> > > + };
> > > + } 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 ffcbf79a556b..3d0b9e34089f 100644
> > > --- a/include/uapi/linux/bpf.h
> > > +++ b/include/uapi/linux/bpf.h
> > > @@ -87,10 +87,46 @@ struct bpf_cgroup_storage_key {
> > > __u32 attach_type; /* program attach type
> > > (enum bpf_attach_type) */
> > > };
> > >
> > > +/*
> > > + * 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.
> > > + *
> > > + * BPF_TASK_ITER_PIDFD
> > > + * Iterate over resources of every task of a process /task
> > > group specified by a pidfd.
> > > + */
> > > +enum bpf_iter_task_type {
> > > + BPF_TASK_ITER_ALL = 0,
> > > + BPF_TASK_ITER_TID,
> > > + BPF_TASK_ITER_TGID,
> > > + BPF_TASK_ITER_PIDFD,
> > > +};
> > > +
> > > union bpf_iter_link_info {
> > > struct {
> > > __u32 map_fd;
> > > } map;
> > > + /*
> > > + * Parameters of task iterators.
> > > + */
> > > + struct {
> > > + enum bpf_iter_task_type type;
> > > + union {
> > > + __u32 tid;
> > > + __u32 tgid;
> > > + __u32 pid_fd;
> > > + };
> >
> > Sorry I'm late to this discussion, but
> > with enum and with union we kinda tell
> > the kernel the same information twice.
> > Here is how the selftest looks:
> > + linfo.task.tid = getpid();
> > + linfo.task.type = BPF_TASK_ITER_TID;
> >
> > first line -> use tid.
> > second line -> yeah. I really meant the tid.
> >
> > Instead of union and type can we do:
> > > + __u32 tid;
> > > + __u32 tgid;
> > > + __u32 pid_fd;
> >
> > as 3 separate fields?
> > The kernel would have to check that only one
> > of them is set.
> >
> > I could have missed an earlier discussion on this subj.
>
> We may have other parameter types later, for example, cgroups.
> Unfortunately, we don't have tagged enum or tagged union, like what
> Rust or Haskell has, in C. A separated 'type' field would be easier
> and clear to distinguish them. With 3 separated fields, people may
> wonder if they can be set them all, or just one of them, in my opinion.
> With an union, people should know only one of them should be set.
What stops us adding new fields to the end in such a case?
Some combinations will not be meaningful and the kernel
would have to check and error regardless.
Imagine extending union:
struct {
enum bpf_iter_task_type type;
union {
struct {
__u32 tid;
__u64 something_else;
};
__u32 tgid;
__u32 pid_fd;
};
};
and now we're suddenly hitting the same issue we discussed
with struct bpf_link_info in the other thread due to alignment
increasing from 4 to 8 bytes.
We might even need bpf_iter_link_info _v2.
If 'something_else' is u32 the kernel still needs to check
that it's zero in the tgid and pid_fd cases.
If we're extending fields we can add a comment:
struct {
__u32 tid;
__u32 tgid;
__u32 pid_fd;
__u32 something_else; /* useful in combination with tid */
};
and it's obvious what is used with what.
It still feels that 3 different fields are easier to use.
