Re: Re: [PATCH bpf-next] bpf: getsockopt hook to get optval without checking kernel retval

Feng Zhou
2023-06-06T03:20:52Z

在 2023/6/1 23:50, Martin KaFai Lau 写道:
> On 5/31/23 11:05 PM, Feng Zhou wrote:
>> 在 2023/6/1 13:37, Martin KaFai Lau 写道:
>>> On 5/31/23 7:49 PM, Feng zhou wrote:
>>>> From: Feng Zhou <zhoufeng.zf@bytedance.com>
>>>>
>>>> Remove the judgment on retval and pass bpf ctx by default. The
>>>> advantage of this is that it is more flexible. Bpf getsockopt can
>>>> support the new optname without using the module to call the
>>>> nf_register_sockopt to register.
>>>>
>>>> Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
>>>> ---
>>>>   kernel/bpf/cgroup.c | 35 +++++++++++++----------------------
>>>>   1 file changed, 13 insertions(+), 22 deletions(-)
>>>>
>>>> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
>>>> index 5b2741aa0d9b..ebad5442d8bb 100644
>>>> --- a/kernel/bpf/cgroup.c
>>>> +++ b/kernel/bpf/cgroup.c
>>>> @@ -1896,30 +1896,21 @@ int
>>>> __cgroup_bpf_run_filter_getsockopt(struct sock *sk, int level,
>>>>       if (max_optlen < 0)
>>>>           return max_optlen;
>>>> -    if (!retval) {
>>>> -        /* If kernel getsockopt finished successfully,
>>>> -         * copy whatever was returned to the user back
>>>> -         * into our temporary buffer. Set optlen to the
>>>> -         * one that kernel returned as well to let
>>>> -         * BPF programs inspect the value.
>>>> -         */
>>>> -
>>>> -        if (get_user(ctx.optlen, optlen)) {
>>>> -            ret = -EFAULT;
>>>> -            goto out;
>>>> -        }
>>>> +    if (get_user(ctx.optlen, optlen)) {
>>>> +        ret = -EFAULT;
>>>> +        goto out;
>>>> +    }
>>>> -        if (ctx.optlen < 0) {
>>>> -            ret = -EFAULT;
>>>> -            goto out;
>>>> -        }
>>>> -        orig_optlen = ctx.optlen;
>>>> +    if (ctx.optlen < 0) {
>>>> +        ret = -EFAULT;
>>>> +        goto out;
>>>> +    }
>>>> +    orig_optlen = ctx.optlen;
>>>> -        if (copy_from_user(ctx.optval, optval,
>>>> -                   min(ctx.optlen, max_optlen)) != 0) {
>>>> -            ret = -EFAULT;
>>>> -            goto out;
>>>> -        }
>>>> +    if (copy_from_user(ctx.optval, optval,
>>>> +                min(ctx.optlen, max_optlen)) != 0) {
>>> What is in optval that is useful to copy from if the kernel didn't
>>> handle the optname?
>>
>> For example, if the user customizes a new optname, it will not be
>> processed if the kernel does not support it. Then the data stored in
>> optval is the data put
>
>
>
>> by the user. If this part can be seen by bpf prog, the user can
>> implement processing logic of the custom optname through bpf prog.
>
> This part does not make sense. It is a (get)sockopt. Why the bpf prog
> should expect anything useful in the original __user optval? Other than
> unnecessary copy for other common cases, it looks like a bad api, so
> consider it a NAK.
>
>>
>>>
>>> and there is no selftest also.
>>>
>>
>> Yes, if remove this restriction, everyone thinks it's ok, I'll add it
>> in the next version.
>>
>>>> +        ret = -EFAULT;
>>>> +        goto out;
>>>>       }
>>>>       lock_sock(sk);
>>>
>>
>

According to my understanding, users will have such requirements,
customize an optname, which is not available in the kernel. All logic is
completed in bpf prog, and bpf prog needs to obtain the user data passed
in by the system call, and then return the data required by the user
according to this data.

For optname not in the kernel, the error code is
#define ENOPROTOOPT 92/* Protocol not available */
Whether to consider the way of judging with error codes,
If (! retval | | retval == -ENOPROTOOPT)