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

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Date:   Tue, 6 Jun 2023 10:04:33 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Feng Zhou <zhoufeng.zf@...edance.com>
Cc:     Martin KaFai Lau <martin.lau@...ux.dev>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, yangzhenze@...edance.com,
        wangdongdong.6@...edance.com, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, haoluo@...gle.com,
        jolsa@...nel.org
Subject: Re: Re: [PATCH bpf-next] bpf: getsockopt hook to get optval without
 checking kernel retval

On Mon, Jun 5, 2023 at 8:20 PM Feng Zhou <zhoufeng.zf@...edance.com> wrote:
>
> 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@...edance.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@...edance.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)

I'm also failing to see what you're trying to do here. You can already
implement custom optnames via getsockopt, so what's missing?
If you need to pass some data from the userspace to the hook, then
setsockopt hook will serve you better.
getsockopt is about reading something from the kernel/bpf; ignoring
initial user buffer value is somewhat implied here.