linux-kernel - Re: [PATCH bpf-next v2 2/6] bpf: Introduce css

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Date:   Wed, 13 Sep 2023 22:02:15 +0800
From:   Chuyi Zhou <zhouchuyi@...edance.com>
To:     Alexei Starovoitov <alexei.starovoitov@...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 2/6] bpf: Introduce css_task open-coded
 iterator kfuncs

Hello, Alexei.

在 2023/9/13 01:13, Alexei Starovoitov 写道:
> On Tue, Sep 12, 2023 at 03:01:45PM +0800, Chuyi Zhou wrote:
>> This patch adds kfuncs bpf_iter_css_task_{new,next,destroy} which allow
>> creation and manipulation of struct bpf_iter_css_task in open-coded
>> iterator style. These kfuncs actually wrapps css_task_iter_{start,next,
>> end}. BPF programs can use these kfuncs through bpf_for_each macro for
>> iteration of all tasks under a css.
>>
>> css_task_iter_*() would try to get the global spin-lock *css_set_lock*, so
>> the bpf side has to be careful in where it allows to use this iter.
>> Currently we only allow it in bpf_lsm and bpf iter-s.
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
>> ---
>>   include/uapi/linux/bpf.h       |  4 +++
>>   kernel/bpf/helpers.c           |  3 +++
>>   kernel/bpf/task_iter.c         | 48 ++++++++++++++++++++++++++++++++++
>>   kernel/bpf/verifier.c          | 23 ++++++++++++++++
>>   tools/include/uapi/linux/bpf.h |  4 +++
>>   tools/lib/bpf/bpf_helpers.h    |  7 +++++
>>   6 files changed, 89 insertions(+)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 73b155e52204..de02c0971428 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -7318,4 +7318,8 @@ struct bpf_iter_num {
>>   	__u64 __opaque[1];
>>   } __attribute__((aligned(8)));
>>   
>> +struct bpf_iter_css_task {
>> +	__u64 __opaque[1];
>> +} __attribute__((aligned(8)));
>> +
>>   #endif /* _UAPI__LINUX_BPF_H__ */
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index b0a9834f1051..d6a16becfbb9 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2504,6 +2504,9 @@ BTF_ID_FLAGS(func, bpf_dynptr_slice_rdwr, KF_RET_NULL)
>>   BTF_ID_FLAGS(func, bpf_iter_num_new, KF_ITER_NEW)
>>   BTF_ID_FLAGS(func, bpf_iter_num_next, KF_ITER_NEXT | KF_RET_NULL)
>>   BTF_ID_FLAGS(func, bpf_iter_num_destroy, KF_ITER_DESTROY)
>> +BTF_ID_FLAGS(func, bpf_iter_css_task_new, KF_ITER_NEW)
> 
> Looking at selftest:
> struct cgroup_subsys_state *css = &cgrp->self;
> 
> realized that we're missing KF_TRUSTED_ARGS here.
> 
>> +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_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 7473068ed313..d8539cc05ffd 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -803,6 +803,54 @@ const struct bpf_func_proto bpf_find_vma_proto = {
>>   	.arg5_type	= ARG_ANYTHING,
>>   };
>>   
>> +struct bpf_iter_css_task_kern {
>> +	struct css_task_iter *css_it;
>> +} __attribute__((aligned(8)));
>> +
>> +__bpf_kfunc int bpf_iter_css_task_new(struct bpf_iter_css_task *it,
>> +		struct cgroup_subsys_state *css, unsigned int flags)
> 
> the verifier does a type check, but it's not strong enough.
> We need KF_TRUSTED_ARGS to make sure the pointer is valid.
> The BTF_TYPE_SAFE_RCU(struct cgroup) {
> probably doesn't need to change, since '&cgrp->self' is not a pointer deref.
> The verifier should understand that cgroup_subsys_state is also PTR_TRUSTED
> just like 'cgrp' pointer.

Got it. It seems we should also apply this to bpf_iter_css_{pre,post}_new.

> 
> Also please add negative tests in patch 6.
> Like doing bpf_rcu_read_unlock() in the middle and check that the verifier
> catches such mistake.

I will do it in next version.

Thanks.