4.x mbuf binary compatibility; can it be broken?

[Mike Silbersack]

On Mon, 14 Jul 2003, Julian Elischer wrote:

> On Mon, 14 Jul 2003, Mike Silbersack wrote:
>
> >
> > In the process of hunting down reported panics in xl_newbuf, I've come to
> > the conclusion that the panics are a result of mbuf cluster refcounts
> > overflowing.  This is not too surprising, as we use an array of chars to
> > store the refcounts.  (-current uses ints, and doesn't have this problem.)
> >
> > It's easy enough to switch from a char to an int array in 4.x to fix the
> > problem there, but there is a problem:  Our friendly mbuf macros (MCLALLOC
> > and MCLFREE) manipulate the refcount.  This means that 3rd party modules
> > which use the macros will no longer work properly.
> >
>
> As the user of a 3rd party driver (binary only)
> PLEASE don't do this..
>
> How does it get 255+ references?

I don't know exactly at this point.  I can reproduce the situation at will
with (in kernel) test code, but I don't know what exactly causes it in the
wild.

Given that increasing the ref count limit is so easy, I was hoping to
avoid spending time tracking down one degenerate case. :)

Mike "Silby" Silbersack