Messages in this thread Patch in this message | | | Date | Mon, 9 Feb 2009 16:18:31 +0100 | From | Michal Hocko <> | Subject | Re: [RFC] Suspicious bug in module refcounting |
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Hi David,
On Wed 04-02-09 14:18:08, Rusty Russell wrote: > On Wednesday 04 February 2009 00:17:21 Karsten Keil wrote: > > The refcount is a per CPU atomic variable, module_refcount() simple add > > in a fully unprotected loop (not disabled irqs, not protected against > > scheduling) all per cpu values. > > Hi Karsten, > > Yes, the BUG_ON() is overly aggressive. And I really hate __module_get, > and it looks like most of the callers are completely bogus. The watchdog > drivers use it to nail themselves in place in their open routines: this is > OK, if a bit weird. > > We should only use __module_get() when you *can't handle* failure; > otherwise you should accept that the admin did rmmod --wait and don't use the > module any further. > > dmaengine.c seems to be taking liberties like this. AFAICT it can error > out, so why not just try_module_get() always? > > gameport.c, serio.c and input.c increment their own refcount, but to get > into those init functions someone must be holding a refcount already (ie. a > module depends on this module). Ditto cyber2000fb.c, and MTD. > > mdio-bitbang.c should definitely use try_module_get. > > loop.c bumping its own refcount, Al might know why, but definitely can be > try_module_get() if it's valid at all. > > net/socket.c can also handle failure, so that's another try_module_get. > > etc. > > > I think we should replace all unprotected __module_get() calls with > > try_module_get(), or remove __module_get() completely. > > Agreed. We will need a "nail_module()" call for those legitimate uses (which > should clear mod->exit, rather than manipulating the refcount at all). > > Meanwhile, I'll remove the BUG_ON for 2.6.29. > > Thanks, > Rusty. > > module: remove over-zealous check in __module_get() > > module_refcount() isn't reliable outside stop_machine(), as demonstrated > by Karsten Keil <kkeil@suse.de>, networking can trigger it under load > (an inc on one cpu and dec on another while module_refcount() is tallying > can give false results, for example). > > Almost noone should be using __module_get, but that's another issue. > > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> > > diff --git a/include/linux/module.h b/include/linux/module.h > --- a/include/linux/module.h > +++ b/include/linux/module.h > @@ -407,7 +407,6 @@ static inline void __module_get(struct m > static inline void __module_get(struct module *module) > { > if (module) { > - BUG_ON(module_refcount(module) == 0); > local_inc(__module_ref_addr(module, get_cpu())); > put_cpu(); > }
Based on this change, would it make sense to update sys_accept to change __module_get to try_module_get like in the following patch?
From 368c52b25414d1ccd0851d77fa5f20431487c172 Mon Sep 17 00:00:00 2001 From: Michal Hocko <mhocko@suse.cz> Date: Mon, 9 Feb 2009 16:06:15 +0100 Subject: [PATCH] [NET] replace __module_get by try_module_get in accept4
After 7f9a50a5b89b87f8e754f59ae9968da28be618a5 we are not checking for potential BUG in module reference counting. Therefore we should replace __module_get by try_module_get and BUG if the module is being unloaded.
Signed-off-by: Michal Hocko <mhocko@suse.cz> --- net/socket.c | 7 ++++--- 1 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/net/socket.c b/net/socket.c index 35dd737..d0d4c92 100644 --- a/net/socket.c +++ b/net/socket.c @@ -1444,10 +1444,11 @@ SYSCALL_DEFINE4(accept4, int, fd, struct sockaddr __user *, upeer_sockaddr, newsock->ops = sock->ops; /* - * We don't need try_module_get here, as the listening socket (sock) - * has the protocol module (sock->ops->owner) held. + * Socket's owner cannot be in unloading path because there + * must be at least one listening reference */ - __module_get(newsock->ops->owner); + if (unlikely(!try_module_get(newsock->ops->owner))) + BUG(); newfd = sock_alloc_fd(&newfile, flags & O_CLOEXEC); if (unlikely(newfd < 0)) { -- 1.5.6.5
-- Michal Hocko L3 team SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic
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