netdev - Re: [net-next-2.6 PATCH v4 3/3] TCPCT part 1c: initial SYN exchange with SYNACK data

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Date:	Mon, 02 Nov 2009 07:25:12 -0500
From:	William Allen Simpson <william.allen.simpson@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [net-next-2.6 PATCH v4 3/3] TCPCT part 1c: initial SYN exchange
 with SYNACK data

Eric Dumazet wrote:
> This part is really hard to review, and might be splitted ?
> 
> cleanups could be done in a cleanup patch only
> 
> Examples:
> 
> -	tmp_opt.mss_clamp = 536;
> -	tmp_opt.user_mss  = tcp_sk(sk)->rx_opt.user_mss;
> +	tmp_opt.mss_clamp = TCP_MIN_RCVMSS;
> +	tmp_opt.user_mss  = tp->rx_opt.user_mss;
> 
> 
> -	tp->mss_cache = 536;
> +	tp->mss_cache = TCP_MIN_RCVMSS;
> 
Often hard to decide what's "cleanup" and what's essential.  I'll look at
that again for the next round, but I've already split the original single
patch into multiple parts.


> Also your tests are reversed, if you look at the existing coding style.
> 
I checked Documentation/CodingStyle, and that's not specified.  I've seen
plenty of examples of modern security coding style around here.

As a long-time (25+ years) consultant and 30 years C programmer, I'm
heedful of the project coding style, and had to endure many variants.

Where I'm working with others' code, you'll note that I keep the same
style, no matter how ugly, as that makes patches easier to read.


> Example :
> 
> +	/* TCP Cookie Transactions */
> +	if (0 < sysctl_tcp_cookie_size) {
> +		/* Default, cookies without s_data. */
> +		tp->cookie_values =
> +			kzalloc(sizeof(*tp->cookie_values),
> +				sk->sk_allocation);
> +		if (NULL != tp->cookie_values)
> +			kref_init(&tp->cookie_values->kref);
> +	}
> 
> should be ->
> 
> +	/* TCP Cookie Transactions */
> +	if (sysctl_tcp_cookie_size > 0) {
> +		/* Default, cookies without s_data. */
> +		tp->cookie_values =
> +			kzalloc(sizeof(*tp->cookie_values),
> +				sk->sk_allocation);
> +		if (tp->cookie_values != NULL)
> +			kref_init(&tp->cookie_values->kref);
> +	}
> 
And "tp->cookie_values != NULL" is egregiously poor C practice.  It's very
hard for code review to ensure that didn't get truncated to "= NULL".  The
important visual element is the NULL, not the variable name.

Also, avoid "!tp->cookie_values", as this is *not* a boolean.

When I'm adding new code, I use constant-to-the-left security coding style,
as they teach in modern universities (lately also for PHP).  And this is a
security extension, so a security style is particularly appropriate.

As in switch statements, constant-to-the-left makes the value obvious,
especially in a series (and assists transforming if series into a switch).

For complex tests, this makes the code much more readable and easier to
visually verify on code walk-through:

+	if (0 < tmp_opt.cookie_plus
+	 && tmp_opt.saw_tstamp
+	 && !tp->cookie_out_never
+	 && (0 < sysctl_tcp_cookie_size
+	  || (NULL != tp->cookie_values
+	   && 0 < tp->cookie_values->cookie_desired))) {

Consistent use of security style would have obviated a lot of foolish >= 0
tests that seem to be constantly in need of fixing.  It's a bad idea to
depend on the compiler to catch non-executable code.
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