Re: [PATCH bpf-next v3 3/7] bpf: Introduce task open coded iterator kfuncs

[Chuyi Zhou <zhouchuyi@bytedance.com>]

Hello,

在 2023/9/28 07:20, Andrii Nakryiko 写道:
> On Mon, Sep 25, 2023 at 3:56 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>
>> This patch adds kfuncs bpf_iter_task_{new,next,destroy} which allow
>> creation and manipulation of struct bpf_iter_task in open-coded iterator
>> style. BPF programs can use these kfuncs or through bpf_for_each macro to
>> iterate all processes in the system.
>>
>> The API design keep consistent with SEC("iter/task"). bpf_iter_task_new()
>> accepts a specific task and iterating type which allows:
>> 1. iterating all process in the system
>>
>> 2. iterating all threads in the system
>>
>> 3. iterating all threads of a specific task
>> Here we also resuse enum bpf_iter_task_type and rename BPF_TASK_ITER_TID
>> to BPF_TASK_ITER_THREAD, rename BPF_TASK_ITER_TGID to BPF_TASK_ITER_PROC.
>>
>> The newly-added struct bpf_iter_task has a name collision with a selftest
>> for the seq_file task iter's bpf skel, so the selftests/bpf/progs file is
>> renamed in order to avoid the collision.
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>> ---
>>   include/linux/bpf.h                           |  8 +-
>>   kernel/bpf/helpers.c                          |  3 +
>>   kernel/bpf/task_iter.c                        | 96 ++++++++++++++++---
>>   .../testing/selftests/bpf/bpf_experimental.h  |  5 +
>>   .../selftests/bpf/prog_tests/bpf_iter.c       | 18 ++--
>>   .../{bpf_iter_task.c => bpf_iter_tasks.c}     |  0
>>   6 files changed, 106 insertions(+), 24 deletions(-)
>>   rename tools/testing/selftests/bpf/progs/{bpf_iter_task.c => bpf_iter_tasks.c} (100%)
>>
> 
> [...]
> 
>> @@ -692,9 +692,9 @@ static int bpf_iter_fill_link_info(const struct bpf_iter_aux_info *aux, struct b
>>   static void bpf_iter_task_show_fdinfo(const struct bpf_iter_aux_info *aux, struct seq_file *seq)
>>   {
>>          seq_printf(seq, "task_type:\t%s\n", iter_task_type_names[aux->task.type]);
>> -       if (aux->task.type == BPF_TASK_ITER_TID)
>> +       if (aux->task.type == BPF_TASK_ITER_THREAD)
>>                  seq_printf(seq, "tid:\t%u\n", aux->task.pid);
>> -       else if (aux->task.type == BPF_TASK_ITER_TGID)
>> +       else if (aux->task.type == BPF_TASK_ITER_PROC)
>>                  seq_printf(seq, "pid:\t%u\n", aux->task.pid);
>>   }
>>
>> @@ -856,6 +856,80 @@ __bpf_kfunc void bpf_iter_css_task_destroy(struct bpf_iter_css_task *it)
>>          bpf_mem_free(&bpf_global_ma, kit->css_it);
>>   }
>>
>> +struct bpf_iter_task {
>> +       __u64 __opaque[2];
>> +       __u32 __opaque_int[1];
> 
> this should be __u64 __opaque[3], because struct takes full 24 bytes
> 
>> +} __attribute__((aligned(8)));
>> +
>> +struct bpf_iter_task_kern {
>> +       struct task_struct *task;
>> +       struct task_struct *pos;
>> +       unsigned int type;
>> +} __attribute__((aligned(8)));
>> +
>> +__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task, unsigned int type)
> 
> nit: type -> flags, so we can add a bit more stuff, if necessary
> 
>> +{
>> +       struct bpf_iter_task_kern *kit = (void *)it;
> 
> empty line after variable declarations
> 
>> +       BUILD_BUG_ON(sizeof(struct bpf_iter_task_kern) != sizeof(struct bpf_iter_task));
>> +       BUILD_BUG_ON(__alignof__(struct bpf_iter_task_kern) !=
>> +                                       __alignof__(struct bpf_iter_task));
> 
> and I'd add empty line here to keep BUILD_BUG_ON block separate
> 
>> +       kit->task = kit->pos = NULL;
>> +       switch (type) {
>> +       case BPF_TASK_ITER_ALL:
>> +       case BPF_TASK_ITER_PROC:
>> +       case BPF_TASK_ITER_THREAD:
>> +               break;
>> +       default:
>> +               return -EINVAL;
>> +       }
>> +
>> +       if (type == BPF_TASK_ITER_THREAD)
>> +               kit->task = task;
>> +       else
>> +               kit->task = &init_task;
>> +       kit->pos = kit->task;
>> +       kit->type = type;
>> +       return 0;
>> +}
>> +
>> +__bpf_kfunc struct task_struct *bpf_iter_task_next(struct bpf_iter_task *it)
>> +{
>> +       struct bpf_iter_task_kern *kit = (void *)it;
>> +       struct task_struct *pos;
>> +       unsigned int type;
>> +
>> +       type = kit->type;
>> +       pos = kit->pos;
>> +
>> +       if (!pos)
>> +               goto out;
>> +
>> +       if (type == BPF_TASK_ITER_PROC)
>> +               goto get_next_task;
>> +
>> +       kit->pos = next_thread(kit->pos);
>> +       if (kit->pos == kit->task) {
>> +               if (type == BPF_TASK_ITER_THREAD) {
>> +                       kit->pos = NULL;
>> +                       goto out;
>> +               }
>> +       } else
>> +               goto out;
>> +
>> +get_next_task:
>> +       kit->pos = next_task(kit->pos);
>> +       kit->task = kit->pos;
>> +       if (kit->pos == &init_task)
>> +               kit->pos = NULL;
> 
> I can't say I completely follow the logic (e.g., for
> BPF_TASK_ITER_PROC, why do we do next_task() on first next() call)?
> Can you elabore the expected behavior for various combinations of
> types and starting task argument?
> 

Thanks for the review.

The expected behavior of current implementation is:

BPF_TASK_ITER_PROC:

init_task->first_process->second_process->...->last_process->init_task

We would exit before visiting init_task again.

BPF_TASK_ITER_THREAD:

group_task->first_thread->second_thread->...->last_thread->group_task

We would exit before visiting group_task again.

BPF_TASK_ITER_ALL:

init_task -> first_process -> second_process -> ...
                 |                    |
		-> first_thread..    |
				     -> first_thread

Actually, every next() call, we would return the "pos" which was 
prepared by previous next() call, and use next_task()/next_thread() to 
update kit->pos. Once we meet the exit condition (next_task() return 
init_task or next_thread() return group_task), we would update kit->pos 
to NULL. In this way, when next() is called again, we will terminate the 
iteration.

Here "kit->pos = NULL;" means we would return the last valid "pos" and 
will return NULL in next call to exit from the iteration.

Am I miss something important?

Thanks.