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

Feng Zhou
2023-06-01T06:06:08Z

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.

>
> 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);
>