On Fri, Feb 05, 2010 at 08:51:25AM -0500, Andrew Gallatin wrote: > Kostik Belousov wrote: > >On Thu, Feb 04, 2010 at 03:40:28PM -0500, Andrew Gallatin wrote: > >>I've got a commercial driver that uses device cloning. > >>At unload time, the driver calls clone_cleanup(). When I unload > >>the driver when the kernel is built with INVARIANTS, I'll see a > >>panic in devfs_populate_loop(). This happens in 6-stable, > >>as well as 8-stable. > >> > >>From what I can see the clone has been freed, but it > >>remains on the devfs cdevp_list. Then the next time > >>devfs_populate_loop() is called, it trips over the bad > >>entry (cdp->cdp_dirents points to 0xdeadc0dedeadc0de) > >>See appended kgdb session. > >> > >>If I trace the code path, it looks like clone_cleanup() > >>calls destroy_devl(). And destroy_devl() will eventually > >>call devfs_free() if the si_refcnt is zero. But I don't > >>see anything which will get the cdev removed from > >>the cdevp_list prior to it being freed. > >> > >>The only code I see which will get the cdev removed from > >>the cdevp_list() seems to be the "GC any lingering devices" > >>block in devfs_populate_loop > >> > >>What am I missing? > > > >You did not mentioned it, but my guess is that you create clones from > >the dev_clone event handler. Please note that devfs_lookup() that fires > > Yes, I do. > > >dev_clone event, consumes a device reference. Thus clone handlers shall > >do dev_ref(). > > > >Due to races with cleanup, you should use MAKEDEV_REF flag for > >make_dev_credv(9) KPI instead of doing make_dev()/dev_ref() pair. > > I need to support FreeBSD going all the way back to 6, so that's not an > option in some versions. > > But, I'm talking about device removal time. If I call clone_cleanup() > where the clones have dev->si_refcount==1, then I get the use-after-free > panic. If I hack things to elevate the reference count (such that > dev->si_refcount==2 when clone_cleanup() is called), then I don't > get the panic. > > Are you saying I should have been taking the extra reference > via my dev_clone eventhandler? Won't having the extra reference > lead to a memory leak? Or am I just mis-reading the code, and > this will lead to things being freed normally? Yes, clone handler shall do dev_ref(). Either by doing race-free make_dev_credf(MAKEDEV_REF) call, or by using dev_ref() after make_dev().
> > >That said, do you really need clones at all ? > > I need to support FreeBSD back to 6.x, and I need to support the > linux-like model of opening the "same" /dev/node multiple times > and getting unique handles. So I think I need clones. Wouldn't it be cleaner to use cdevpriv for the 7/8/HEAD where it is present ? And have special #ifdef-ed code for 6, that could be eventually dropped.
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