linux-kernel - Re: [PATCH bpf-next] bpf, sockops: Enhance the return capability of sockops

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Message-ID: <ZKb9ObvvhOaKXJcp@google.com>
Date:   Thu, 6 Jul 2023 10:43:21 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Xin Liu <liuxin350@...wei.com>
Cc:     daniel@...earbox.net, andrii@...nel.org, ast@...nel.org,
        bpf@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        hsinweih@....edu, jakub@...udflare.com, john.fastabend@...il.com,
        kuba@...nel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, pabeni@...hat.com,
        syzbot+49f6cef45247ff249498@...kaller.appspotmail.com,
        syzkaller-bugs@...glegroups.com, yanan@...wei.com,
        wuchangye@...wei.com, xiesongyang@...wei.com,
        kongweibin2@...wei.com, zhangmingyi5@...wei.com
Subject: Re: [PATCH bpf-next] bpf, sockops: Enhance the return capability of sockops

On 07/06, Xin Liu wrote:
> Since commit 2585cd62f098 ("bpf: Only reply field should be writeable"),
> sockops is not allowd to modify the replylong field except replylong[0].
> The reason is that the replylong[1] to replylong[3] field is not used
> at that time.
> 
> But in actual use, we can call `BPF_CGROUP_RUN_PROG_SOCK_OPS` in the
> kernel modules and expect sockops to return some useful data.
> 
> The design comment about bpf_sock_ops::replylong in 
> include/uapi/linux/bpf.h is described as follows:
> 
> ```
>   struct bpf_sock_ops {
> 	__u32 op;
> 	union {
> 		__u32 args[4];		/* Optionally passed to bpf program */
> 		__u32 reply;		/* Returned by bpf program	    */
> 		__u32 replylong[4];	/* Optioznally returned by bpf prog  */
> 	};
>   ...
> ```
> 
> It seems to contradict the purpose for which the field was originally
> designed. Let's remove this restriction.
> 
> Fixes: 2585cd62f098 ("bpf: Only reply field should be writeable")

The commit you reference explicitly says that there is no reason to allow
replylong[1..3] because there is no use for them. Has something changed
since it was added? Any reason to expose those fields?