Re: [PATCH bpf-next v2 3/6] bpf: Introduce process open coded iterator kfuncs

Chuyi Zhou
2023-09-15T04:48:33Z

Hello.

在 2023/9/15 07:26, Andrii Nakryiko 写道:
> On Tue, Sep 12, 2023 at 12:02 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>
>> This patch adds kfuncs bpf_iter_process_{new,next,destroy} which allow
>> creation and manipulation of struct bpf_iter_process in open-coded iterator
>> style. BPF programs can use these kfuncs or through bpf_for_each macro to
>> iterate all processes in the system.
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>> ---
>> include/uapi/linux/bpf.h | 4 ++++
>> kernel/bpf/helpers.c | 3 +++
>> kernel/bpf/task_iter.c | 29 +++++++++++++++++++++++++++++
>> tools/include/uapi/linux/bpf.h | 4 ++++
>> tools/lib/bpf/bpf_helpers.h | 5 +++++
>> 5 files changed, 45 insertions(+)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index de02c0971428..befa55b52e29 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -7322,4 +7322,8 @@ struct bpf_iter_css_task {
>> __u64 __opaque[1];
>> } __attribute__((aligned(8)));
>>
>> +struct bpf_iter_process {
>> + __u64 __opaque[1];
>> +} __attribute__((aligned(8)));
>> +
>> #endif /* _UAPI__LINUX_BPF_H__ */
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index d6a16becfbb9..9b7d2c6f99d1 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2507,6 +2507,9 @@ BTF_ID_FLAGS(func, bpf_iter_num_destroy, KF_ITER_DESTROY)
>> BTF_ID_FLAGS(func, bpf_iter_css_task_new, KF_ITER_NEW)
>> BTF_ID_FLAGS(func, bpf_iter_css_task_next, KF_ITER_NEXT | KF_RET_NULL)
>> BTF_ID_FLAGS(func, bpf_iter_css_task_destroy, KF_ITER_DESTROY)
>> +BTF_ID_FLAGS(func, bpf_iter_process_new, KF_ITER_NEW)
>> +BTF_ID_FLAGS(func, bpf_iter_process_next, KF_ITER_NEXT | KF_RET_NULL)
>> +BTF_ID_FLAGS(func, bpf_iter_process_destroy, KF_ITER_DESTROY)
>> BTF_ID_FLAGS(func, bpf_dynptr_adjust)
>> BTF_ID_FLAGS(func, bpf_dynptr_is_null)
>> BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>> index d8539cc05ffd..9d1927dc3a06 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -851,6 +851,35 @@ __bpf_kfunc void bpf_iter_css_task_destroy(struct bpf_iter_css_task *it)
>> kfree(kit->css_it);
>> }
>>
>> +struct bpf_iter_process_kern {
>> + struct task_struct *tsk;
>> +} __attribute__((aligned(8)));
>> +
>
> Few high level thoughts. I think it would be good to follow
> SEC("iter/task") naming and approach. Open-coded iterators in many
> ways are in-kernel counterpart to iterator programs, so keeping them
> close enough within reason is useful for knowledge transfer.
>
> SEC("iter/task") allows to:
> a) iterate all threads in the system
> b) iterate all threads for a given TGID
> c) it also allows to "iterate" a single thread or process, but that's
> a bit less relevant for in-kernel iterator, but we can still support
> them, why not?
>
> I'm not sure if it supports iterating all processes (as in group
> leaders of each task group) in the system, but if it's possible I
> think we should support it at least for open-coded iterator, seems
> like a very useful functionality.
>
> So to that end, let's design a small set of input arguments for
> bpf_iter_process_new() that would allow to specify this as flags +
> either (optional) struct task_struct * pointer to represent
> task/process or PID/TGID.
>

IIUC, we should define the following task_new kfunc

struct bpf_iter_task_kern {
struct task_struct *start;
struct task_struct *cur;
unsigned int flag;
} __attribute__((aligned(8)));

__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_css_task *it,
struct task_struct *start, unsigned int flags)

If we want to iterate all threads of a task, just pass it to *start*,
and if we want to iterating all process in the system, users may need to
pass a nullptr to the *start*. But it seems current BPF verifier will
reject the nullptr to task_struct. The error message meybe:
"Possibly NULL pointer passed to trusted argx"

I noticed the __opt annotation in kfunc document. It seems with
following we can pass the nullptr to *start*

__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_css_task *it,
void *start__opt, unsigned int flags__szk)

However, in this way, user can pass any invalid pointer to the kfunc
without verifying. Besides, it seems __opt is only allowed to use with
__szk together and flags__szk is ambiguous in semantics.

Do you have better ideas? Or I missing something ?

Thanks