sysctl: Respect max length when handle a variable string

[zlei]

Summary

sysctl_handle_string() may microoptimise and treat a variable string as a readonly one, but we should still respect the max length ( arg2 ) so that we will not read / write out of bounds. This is the API contract.

Fixes: 210176ad76ee sysctl(9): add CTLFLAG_NEEDGIANT flag

[zlei]

@markj
You may be interested with this. Spotted this while hacking 284073 , in case the buf overflows ...

[markj]

I think this change is right, but it took me a while to understand it: the point is to harden sysctl_handle_string() against unterminated strings, so I'd suggest explaining a bit further in the commit log message.

sys/kern/kern_sysctl.c

1914

These lines should be wrapped.

[kib]

sys/kern/kern_sysctl.c

1914

In fact, nested assignment inside the ?: operator whines for the logic to be split. And since the same expression is repeated below, it might be reasonable to spent an effort to add a helper function there.