Subject: Re: [Patch bpf-next v3 05/10] af_unix: implement unix_dgram_bpf_recvmsg()

From: Jakub Sitnicki <[email-masked]>
Date: Fri, 07 May 2021 15:29:43 +0200

On Mon, Apr 26, 2021 at 04:49 AM CEST, Cong Wang wrote:
> From: Cong Wang <[email-masked]>
>
> We have to implement unix_dgram_bpf_recvmsg() to replace the
> original ->recvmsg() to retrieve skmsg from ingress_msg.
>
> AF_UNIX is again special here because the lack of
> sk_prot->recvmsg(). I simply add a special case inside
> unix_dgram_recvmsg() to call sk->sk_prot->recvmsg() directly.
>
> Cc: John Fastabend <[email-masked]>
> Cc: Daniel Borkmann <[email-masked]>
> Cc: Jakub Sitnicki <[email-masked]>
> Cc: Lorenz Bauer <[email-masked]>
> Signed-off-by: Cong Wang <[email-masked]>
> ---
>  include/net/af_unix.h |  3 +++
>  net/unix/af_unix.c    | 21 ++++++++++++++++---
>  net/unix/unix_bpf.c   | 49 +++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 70 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/af_unix.h b/include/net/af_unix.h
> index cca645846af1..e524c82794c9 100644
> --- a/include/net/af_unix.h
> +++ b/include/net/af_unix.h
> @@ -82,6 +82,9 @@ static inline struct unix_sock *unix_sk(const struct sock *sk)
>  long unix_inq_len(struct sock *sk);
>  long unix_outq_len(struct sock *sk);
>  
> +int __unix_dgram_recvmsg(struct sock *sk, struct msghdr *msg, size_t size,
> +			 int nonblock, int flags, int *addr_len);
> +
>  #ifdef CONFIG_SYSCTL
>  int unix_sysctl_register(struct net *net);
>  void unix_sysctl_unregister(struct net *net);
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index c4afc5fbe137..08458fa9f48b 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -2088,11 +2088,11 @@ static void unix_copy_addr(struct msghdr *msg, struct sock *sk)
>  	}
>  }
>  
> -static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
> -			      size_t size, int flags)
> +int __unix_dgram_recvmsg(struct sock *sk, struct msghdr *msg, size_t size,
> +			 int nonblock, int flags, int *addr_len)
>  {
>  	struct scm_cookie scm;
> -	struct sock *sk = sock->sk;
> +	struct socket *sock = sk->sk_socket;
>  	struct unix_sock *u = unix_sk(sk);
>  	struct sk_buff *skb, *last;
>  	long timeo;
> @@ -2195,6 +2195,21 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
>  	return err;
>  }
>  
> +static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
> +			      int flags)
> +{
> +	struct sock *sk = sock->sk;
> +	int addr_len = 0;
> +
> +#ifdef CONFIG_BPF_SYSCALL
> +	if (sk->sk_prot != &unix_proto)
> +		return sk->sk_prot->recvmsg(sk, msg, size, flags & MSG_DONTWAIT,
> +					    flags & ~MSG_DONTWAIT, &addr_len);
> +#endif
> +	return __unix_dgram_recvmsg(sk, msg, size, flags & MSG_DONTWAIT,
> +				    flags, &addr_len);
> +}
> +

Nit: We can just pass NULL instead of &addr_len here it seems.

[...]