Re: [PATCH v5 net-next 07/15] tcp: allow embedding leftover into option padding

[Paolo Abeni <pabeni@redhat.com>]

On 4/22/25 5:35 PM, chia-yu.chang@nokia-bell-labs.com wrote:
> @@ -709,6 +709,8 @@ static __be32 *process_tcp_ao_options(struct tcp_sock *tp,
>  	return ptr;
>  }
>  
> +#define NOP_LEFTOVER	((TCPOPT_NOP << 8) | TCPOPT_NOP)
> +
>  /* Write previously computed TCP options to the packet.
>   *
>   * Beware: Something in the Internet is very sensitive to the ordering of
> @@ -727,8 +729,10 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
>  			      struct tcp_out_options *opts,
>  			      struct tcp_key *key)
>  {
> +	u16 leftover_bytes = NOP_LEFTOVER;      /* replace next NOPs if avail */
>  	__be32 *ptr = (__be32 *)(th + 1);
>  	u16 options = opts->options;	/* mungable copy */
> +	int leftover_size = 2;
>  
>  	if (tcp_key_is_md5(key)) {
>  		*ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
> @@ -763,17 +767,22 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
>  	}
>  
>  	if (unlikely(OPTION_SACK_ADVERTISE & options)) {
> -		*ptr++ = htonl((TCPOPT_NOP << 24) |
> -			       (TCPOPT_NOP << 16) |
> +		*ptr++ = htonl((leftover_bytes << 16) |
>  			       (TCPOPT_SACK_PERM << 8) |
>  			       TCPOLEN_SACK_PERM);
> +		leftover_bytes = NOP_LEFTOVER;

Why? isn't leftover_bytes already == NOP_LEFTOVER?

>  	}
>  
>  	if (unlikely(OPTION_WSCALE & options)) {
> -		*ptr++ = htonl((TCPOPT_NOP << 24) |
> +		u8 highbyte = TCPOPT_NOP;
> +
> +		if (unlikely(leftover_size == 1))

How can the above conditional be true?

> +			highbyte = leftover_bytes >> 8;
> +		*ptr++ = htonl((highbyte << 24) |
>  			       (TCPOPT_WINDOW << 16) |
>  			       (TCPOLEN_WINDOW << 8) |
>  			       opts->ws);
> +		leftover_bytes = NOP_LEFTOVER;

Why? isn't leftover_bytes already == NOP_LEFTOVER?

>  	}
>  
>  	if (unlikely(opts->num_sack_blocks)) {
> @@ -781,8 +790,7 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
>  			tp->duplicate_sack : tp->selective_acks;
>  		int this_sack;
>  
> -		*ptr++ = htonl((TCPOPT_NOP  << 24) |
> -			       (TCPOPT_NOP  << 16) |
> +		*ptr++ = htonl((leftover_bytes << 16) |
>  			       (TCPOPT_SACK <<  8) |
>  			       (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
>  						     TCPOLEN_SACK_PERBLOCK)));
> @@ -794,6 +802,10 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
>  		}
>  
>  		tp->rx_opt.dsack = 0;
> +	} else if (unlikely(leftover_bytes != NOP_LEFTOVER)) {

I really feel like I'm missing some code chunk, but I don't see any
possible value for leftover_bytes other than NOP_LEFTOVER

/P