Re: [PATCH net-next 2/4] net: core: add getsockopt SO_PEERCGROUPID

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Date: Sun,  9 Mar 2025 14:28:13 +0100
> Add SO_PEERCGROUPID which allows to get cgroup_id
> for a socket.

This looks useless for SOCK_DGRAM as the server can't have a
peer for clients to send() or connect() (see unix_may_send()).

Is there any reason to support only the connect()ed peer ?
It seems the uAPI group expects to retrieve data per message
as SCM_CGROUPID.


> 
> We already have analogical interfaces to retrieve this
> information:
> - inet_diag: INET_DIAG_CGROUP_ID
> - eBPF: bpf_sk_cgroup_id
> 
> Having getsockopt() interface makes sense for many
> applications, because using eBPF is not always an option,
> while inet_diag has obvious complexety and performance drawbacks
> if we only want to get this specific info for one specific socket.
[...]
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 2b2c0036efc9..3455f38f033d 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -901,6 +901,66 @@ static void unix_show_fdinfo(struct seq_file *m, struct socket *sock)
>  #define unix_show_fdinfo NULL
>  #endif
>  
> +static int unix_getsockopt(struct socket *sock, int level, int optname,
> +			   char __user *optval, int __user *optlen)
> +{
> +	struct sock *sk = sock->sk;
> +
> +	union {
> +		int val;
> +		u64 val64;

As Willem pointed out, simply use val64 only.


> +	} v;
> +
> +	int lv = sizeof(int);
> +	int len;
> +
> +	if (level != SOL_SOCKET)
> +		return -ENOPROTOOPT;
> +
> +	if (get_user(len, optlen))
> +		return -EFAULT;
> +
> +	if (len < 0)
> +		return -EINVAL;
> +
> +	memset(&v, 0, sizeof(v));
> +
> +	switch (optname) {
> +#ifdef CONFIG_SOCK_CGROUP_DATA
> +	case SO_PEERCGROUPID:
> +	{
> +		struct sock *peer;
> +		u64 peer_cgroup_id = 0;

Same remarks in patch 1, reverse xmas tree order, and no
initialisation needed.


> +
> +		lv = sizeof(u64);
> +		if (len < lv)
> +			return -EINVAL;
> +
> +		peer = unix_peer_get(sk);
> +		if (!peer)
> +			return -ENODATA;
> +
> +		peer_cgroup_id = cgroup_id(sock_cgroup_ptr(&peer->sk_cgrp_data));
> +		sock_put(peer);
> +
> +		v.val64 = peer_cgroup_id;
> +		break;
> +	}
> +#endif
> +	default:
> +		return -ENOPROTOOPT;
> +	}
> +
> +	if (len > lv)
> +		len = lv;
> +	if (copy_to_user(optval, &v, len))
> +		return -EFAULT;
> +	if (put_user(len, optlen))
> +		return -EFAULT;
> +
> +	return 0;
> +}
> +