Re: [PATCH] handle module ref count on sysctl tables.

[Al Viro <viro@ftp.linux.org.uk>]

On Wed, Dec 21, 2005 at 10:35:19AM -0800, Stephen Hemminger wrote:
> Right now there is a hole in the module ref counting system because
> there is no proper ref counting for sysctl tables used by modules.
> This means that if an application is holding /proc/sys/foo open and
> module that created it is unloaded, then the application touches the
> file the kernel will oops.
> 
> This patch fixes that by maintaining source compatibility via macro.
> I am sure someone already thought of this, it just doesn't appear to
> have made it in yet.

NAK.
	a) holding the file open will *NOT* pin any module structures down.
IO in progress will, but it unregistering sysctl table will block until it's
over.  The same goes for sysctl(2) in progress.  See use_table() and
friends in kernel/sysctl.c
	b) you are not protecting any code in module; what needs protection
(and gets it) is a pile of data structures.  With lifetimes that don't have
to be related to module lifetimes.  IOW, use of reference to module is 100%
wrong here - it wouldn't fix anything.

As a general rule, when you pin something down, think what you are trying
to protect; if it's not just a bunch of function references - module is
the wrong thing to hold.

In particular, sysctl tables are dynamically created and removed in a
kernel that is not modular at all.  Which kills any hope to get a solution
based on preventing rmmod.

Solution is fairly simple:
	* put use counter into sysctl table head (i.e. object allocated by
kernel/sysctl.c)
	* bump use counter when examining table in sysctl(2) and around the
actual IO in procfs access; put reference to table into proc_dir_entry to
be able to do the latter.  Decrement when done with the table; if it had
hit zero _and_ there's unregistration waiting for completion - kick it.
	* have unregistration kill all reference to table head and if use
counter is positive - wait for completion.  Once we get it, we know that
we can safely proceed.