Re: [RFC bpf-next v2 4/6] bpf: Provide bpf_copy_from_user() and bpf_copy_to_user().

[Kui-Feng Lee <sinquersw@gmail.com>]

On 8/11/23 16:05, Stanislav Fomichev wrote:
> On 08/10, thinker.li@gmail.com wrote:
>> From: Kui-Feng Lee <kuifeng@meta.com>
>>
>> Provide bpf_copy_from_user() and bpf_copy_to_user() to the BPF programs
>> attached to cgroup/{set,get}sockopt. bpf_copy_to_user() is a new kfunc to
>> copy data from an kernel space buffer to a user space buffer. They are only
>> available for sleepable BPF programs. bpf_copy_to_user() is only available
>> to the BPF programs attached to cgroup/getsockopt.
>>
>> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
>> ---
>>   kernel/bpf/cgroup.c  |  6 ++++++
>>   kernel/bpf/helpers.c | 31 +++++++++++++++++++++++++++++++
>>   2 files changed, 37 insertions(+)
>>
>> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
>> index 5bf3115b265c..c15a72860d2a 100644
>> --- a/kernel/bpf/cgroup.c
>> +++ b/kernel/bpf/cgroup.c
>> @@ -2461,6 +2461,12 @@ cg_sockopt_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>>   #endif
>>   	case BPF_FUNC_perf_event_output:
>>   		return &bpf_event_output_data_proto;
>> +
>> +	case BPF_FUNC_copy_from_user:
>> +		if (prog->aux->sleepable)
>> +			return &bpf_copy_from_user_proto;
>> +		return NULL;
> 
> If we just allow copy to/from, I'm not sure I understand how the buffer
> sharing between sleepable/non-sleepable works.
> 
> Let's assume I have two progs in the chain:
> 1. non-sleepable - copies the buffer, does some modifications; since
>     we don't copy the buffer back after every prog run, the modifications
>     stay in the kernel buffer
> 2. sleepable - runs and just gets the user pointer? does it mean this
>    sleepable program doesn't see the changes from (1)?
> 
> IOW, do we need some custom sockopt copy_to/from that handle this
> potential buffer location transparently or am I missing something?
> 
> Assuming we want to support this at all. If we do, might deserve a
> selftest.

It is why BPF_SOCKOPT_FLAG_OPTVAL_USER is there.
It helps programs to make a right decision.
However, I am going to remove bpf_copy_from_user()
since we have bpf_so_optval_copy_to() and bpf_so_optval_copy_to_r().
Does it make sense to you?