I would prefer not changing this code, unless you prove there is a real problem.

(sysctl_max_syn_backlog defauts to 4096, and syncookies are enabled by
default, for a good reason)

Basically, sysctl_max_syn_backlog is not used today (because
syncookies are enabled...)

Your change might break users that suddenly will get behavior changes
if their sysctl_max_syn_backlog was set to a small value.
Unfortunately some sysctl values are often copied/pasted from various
web pages claiming how to get best TCP performance.

It would be quite silly to change the kernel to adapt a change
(sysctl_max_syn_backlog set to 200 ... ) done by one of these admins.

Thanks.

Of course, the sysctl_max_syn_backlog is set to 200 just for the sake of example, not the actual configuration.

I found this bug when summarizing how the kernel handles syn attacks. I'm reading the kernel source code and not really encountering errors.

I also thought of another scenario where the above BUG might cause problems.

Imagine a machine with low performance and small memory. Set sysctl_max_syn_backlog to a small value to save memory (304 bytes for a connection request), and enable syn cookies to handle excessive requests.

Because the sysctl_max_syn_backlog is invalid after syn cookies are enabled, the entire backlog is consumed and too much memory is consumed.

Of course, the above scenario is rarely encountered in general.

I fixed this bug mainly because I thought its logic was indeed wrong, not because it caused some serious problems, I was a bit obsessive about the correctness of the code.

So if, as you said, it can cause backward compatibility issues, just leave it as it is.