Re: [RFC] Suspicious bug in module refcounting

[Michal Hocko <mhocko@suse.cz>]

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.
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Czech Republic