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

[Stephen Hemminger <shemminger@osdl.org>]

On Wed, 21 Dec 2005 19:08:49 +0000
Al Viro <viro@ftp.linux.org.uk> wrote:

> 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.
> 

Yeah, that is better.

-- 
Stephen Hemminger <shemminger@osdl.org>
OSDL http://developer.osdl.org/~shemminger