Subject: Re: [net-next v2] tcp: Explicitly mark reserved field in tcp_zerocopy_receive args.

From: David Ahern <[email-masked]>
Date: Sun, 7 Feb 2021 10:49:41 -0700

On 2/6/21 1:36 PM, Arjun Roy wrote:
> From: Arjun Roy <[email-masked]>
> 
> Explicitly define reserved field and require it to be 0-valued.
> 
> Fixes: 7eeba1706eba ("tcp: Add receive timestamp support for receive zerocopy.")
> Signed-off-by: Arjun Roy <[email-masked]>
> Signed-off-by: Eric Dumazet <[email-masked]>
> Signed-off-by: Soheil Hassas Yeganeh <[email-masked]>
> Suggested-by: David Ahern <[email-masked]>
> Suggested-by: Leon Romanovsky <[email-masked]>
> Suggested-by: Jakub Kicinski <[email-masked]>
> ---
>  include/uapi/linux/tcp.h | 2 +-
>  net/ipv4/tcp.c           | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> index 42fc5a640df4..8fc09e8638b3 100644
> --- a/include/uapi/linux/tcp.h
> +++ b/include/uapi/linux/tcp.h
> @@ -357,6 +357,6 @@ struct tcp_zerocopy_receive {
>  	__u64 msg_control; /* ancillary data */
>  	__u64 msg_controllen;
>  	__u32 msg_flags;
> -	/* __u32 hole;  Next we must add >1 u32 otherwise length checks fail. */
> +	__u32 reserved; /* set to 0 for now */
>  };
>  #endif /* _UAPI_LINUX_TCP_H */
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index e1a17c6b473c..c8469c579ed8 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -4159,6 +4159,8 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
>  		}
>  		if (copy_from_user(&zc, optval, len))
>  			return -EFAULT;
> +		if (zc.reserved)
> +			return -EINVAL;
>  		lock_sock(sk);
>  		err = tcp_zerocopy_receive(sk, &zc, &tss);
>  		release_sock(sk);
> 

The 'switch (len)' statement needs to be updated now that 'len' is not
going to end on the 'msg_flags' boundary? But then, how did that work
before if there was 4 byte padding?

Maybe I am missing something here. You currently have:

	switch (len) {
	case offsetofend(struct tcp_zerocopy_receive, msg_flags):

which should == 60, yet the struct size is 64 with 4-bytes of padding. A
user doing

	int optlen = sizeof(struct tcp_zerocopy_receive);

	getsockopt(...., &optlen)

would pass in a value of 64, right?