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

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Date:   Fri, 15 Sep 2023 12:48:24 +0800
From:   Chuyi Zhou <zhouchuyi@...edance.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...nel.org, tj@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 3/6] bpf: Introduce process open coded
 iterator kfuncs

Hello.

在 2023/9/15 07:26, Andrii Nakryiko 写道:
> On Tue, Sep 12, 2023 at 12:02 AM Chuyi Zhou <zhouchuyi@...edance.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@...edance.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