Re: [PATCH net v3] af_unix: Read with MSG_PEEK loops if the first unread byte is OOB

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Rao Shoaib <Rao.Shoaib@oracle.com>
Date: Mon, 22 Apr 2024 02:25:03 -0700
> Read with MSG_PEEK flag loops if the first byte to read is an OOB byte.
> commit 22dd70eb2c3d ("af_unix: Don't peek OOB data without MSG_OOB.")
> addresses the loop issue but does not address the issue that no data
> beyond OOB byte can be read.
> 
> >>> from socket import *
> >>> c1, c2 = socketpair(AF_UNIX, SOCK_STREAM)
> >>> c1.send(b'a', MSG_OOB)
> 1
> >>> c1.send(b'b')
> 1
> >>> c2.recv(1, MSG_PEEK | MSG_DONTWAIT)
> b'b'
> 
> Fixes: 314001f0bf92 ("af_unix: Add OOB support")
> Signed-off-by: Rao Shoaib <Rao.Shoaib@oracle.com>
> ---
>  net/unix/af_unix.c | 26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 9a6ad5974dff..ed5f70735435 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -2658,19 +2658,19 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
>  		if (skb == u->oob_skb) {
>  			if (copied) {
>  				skb = NULL;
> -			} else if (sock_flag(sk, SOCK_URGINLINE)) {
> -				if (!(flags & MSG_PEEK)) {
> +			} else if (!(flags & MSG_PEEK)) {
> +				if (sock_flag(sk, SOCK_URGINLINE)) {
>  					WRITE_ONCE(u->oob_skb, NULL);
>  					consume_skb(skb);
> +				} else {
> +					skb_unlink(skb, &sk->sk_receive_queue);
> +					WRITE_ONCE(u->oob_skb, NULL);
> +					if (!WARN_ON_ONCE(skb_unref(skb)))
> +						kfree_skb(skb);
> +					skb = skb_peek(&sk->sk_receive_queue);

I added a comment about this case.


>  				}
> -			} else if (flags & MSG_PEEK) {
> -				skb = NULL;
> -			} else {
> -				skb_unlink(skb, &sk->sk_receive_queue);
> -				WRITE_ONCE(u->oob_skb, NULL);
> -				if (!WARN_ON_ONCE(skb_unref(skb)))
> -					kfree_skb(skb);
> -				skb = skb_peek(&sk->sk_receive_queue);
> +			} else if (!sock_flag(sk, SOCK_URGINLINE)) {
> +				skb = skb_peek_next(skb, &sk->sk_receive_queue);
>  			}
>  		}
>  	}
> @@ -2747,9 +2747,11 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
>  #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
>  		if (skb) {
>  			skb = manage_oob(skb, sk, flags, copied);
> -			if (!skb && copied) {
> +			if (!skb) {
>  				unix_state_unlock(sk);
> -				break;
> +				if (copied || (flags & MSG_PEEK))
> +					break;
> +				goto redo;

Here, copied == 0 && !(flags & MSG_PEEK) && skb == NULL, so it means
skb_peek(&sk->sk_receive_queue) above returned NULL.  Then, we need
not jump to the redo label, where we call the same skb_peek().

Instead, we can just fall through the if (!skb) clause below.

Thanks!

>  			}
>  		}
>  #endif
> -- 
> 2.39.3