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SubjectRe: [RFC PATCH bpf-next 2/4] bpf: Introduce process open coded iterator kfuncs
Hello, Alexei.

在 2023/9/6 04:09, Alexei Starovoitov 写道:
> On Sun, Aug 27, 2023 at 12:21 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 | 31 +++++++++++++++++++++++++++++++
>> tools/include/uapi/linux/bpf.h | 4 ++++
>> tools/lib/bpf/bpf_helpers.h | 5 +++++
>> 5 files changed, 47 insertions(+)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 2a6e9b99564b..cfbd527e3733 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -7199,4 +7199,8 @@ 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 cf113ad24837..81a2005edc26 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2458,6 +2458,9 @@ 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 b1bdba40b684..a6717a76c1e0 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -862,6 +862,37 @@ __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)));
>> +
>> +__bpf_kfunc int bpf_iter_process_new(struct bpf_iter_process *it)
>> +{
>> + struct bpf_iter_process_kern *kit = (void *)it;
>> +
>> + BUILD_BUG_ON(sizeof(struct bpf_iter_process_kern) != sizeof(struct bpf_iter_process));
>> + BUILD_BUG_ON(__alignof__(struct bpf_iter_process_kern) !=
>> + __alignof__(struct bpf_iter_process));
>> +
>> + rcu_read_lock();
>> + kit->tsk = &init_task;
>> + return 0;
>> +}
>> +
>> +__bpf_kfunc struct task_struct *bpf_iter_process_next(struct bpf_iter_process *it)
>> +{
>> + struct bpf_iter_process_kern *kit = (void *)it;
>> +
>> + kit->tsk = next_task(kit->tsk);
>> +
>> + return kit->tsk == &init_task ? NULL : kit->tsk;
>> +}
>> +
>> +__bpf_kfunc void bpf_iter_process_destroy(struct bpf_iter_process *it)
>> +{
>> + rcu_read_unlock();
>> +}
>
> This iter can be used in all ctx-s which is nice, but let's
> make the verifier enforce rcu_read_lock/unlock done by bpf prog
> instead of doing in the ctor/dtor of iter, since
> in sleepable progs the verifier won't recognize that body is RCU CS.
> We'd need to teach the verifier to allow bpf_iter_process_new()
> inside in_rcu_cs() and make sure there is no rcu_read_unlock
> while BPF_ITER_STATE_ACTIVE.
> bpf_iter_process_destroy() would become a nop.

Thanks for your review!

I think bpf_iter_process_{new, next, destroy} should be protected by
bpf_rcu_read_lock/unlock explicitly whether the prog is sleepable or
not, right? I'm not very familiar with the BPF verifier, but I believe
there is still a risk in directly calling these kfuns even if
in_rcu_cs() is true.

Maby what we actually need here is to enforce BPF verifier to check
env->cur_state->active_rcu_lock is true when we want to call these kfuncs.

Thanks.


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