cvs commit: src/sys/net if_vlan.c

[Yar Tikhiy]

On Thu, Aug 03, 2006 at 10:11:11AM -0700, Sam Leffler wrote:
> Yar Tikhiy wrote:
> > yar         2006-08-03 09:59:09 UTC
> > 
> >   FreeBSD src repository
> > 
> >   Modified files:
> >     sys/net              if_vlan.c 
> >   Log:
> >   Should vlan_input() ever be called with ifp pointing to a non-Ethernet
> >   interface, do not just assign -1 to tag because it breaks the logic of
> >   the code to follow.  The better way is to handle this case as an unsupported
> >   protocol and return unless INVARIANTS is in effect and we can panic.
> >   Panic is good there because the scenario can happen only because of a
> >   coding error elsewhere.
> >   
> >   We also should show the interface name in the panic message for easier
> >   debugging of the problem, should it ever emerge.
> 
> Introducing a panic in a place where you can trivially recover is bad
> regardless of why you got there.  Many people run production systems
> with INVARIANTS turned on.  Is it now possible to send a "packet of
> death" by exploiting this code path?

No nastygram can ever achieve this; only FreeBSD commiters possess
the ability to :-)

The panic can never be reached unless one manages to attach a vlan
interface to a non-Ethernet physical interface in advance, which
is totally prohibited by the code at the beginning of vlan_config();
and vlan_config() is the only way to attach a vlan interface to a
physical interface.

I.e., it will take a developer breaking the logic in /sys/net to
make the code path expoloitable.

OTOH, you are right that we can at least attempt to recover from
the situation.  Perhaps it's time to introduce a common macro or
function that emits a message on the console and then just calls
kdb_backtrace() instead of dumping core and halting the system?
So users will be able to post the stack traces to the lists and
thus help to spot the possible bugs w/o having to go through panics.
I'm unsure if sticking raw kdb_backtrace() calls in such places
is a good idea, so I'm suggesting a wrapper function or macro.
It is to be used in "can absolutely never happen" cases that are
not fatal, like the one under discussion.

-- 
Yar