At Sat, 16 Aug 2014 04:50:07 +0200,
Luis R. Rodriguez wrote:
>
> On Fri, Aug 15, 2014 at 04:39:02PM +0200, Oleg Nesterov wrote:
> > On 08/15, Luis R. Rodriguez wrote:
> > >
> > > On Wed, Aug 13, 2014 at 07:51:01PM +0200, Oleg Nesterov wrote:
> > > > On 08/12, Luis R. Rodriguez wrote:
> > > > >
> > > > > +/* To be used by modules which can take over 30 seconds at probe */
> > > >
> > > > Probably the comment should explain that this hack should only be
> > > > used if the driver is buggy and is wating for "real fix".
> > > >
> > > > > +#define module_long_probe_init(initfn)
> > > > > \
> > > > > + static struct task_struct *__init_thread; \
> > > > > + static int _long_probe_##initfn(void *arg) \
> > > > > + { \
> > > > > + return initfn(); \
> > > > > + } \
> > > > > + static inline __init int __long_probe_##initfn(void) \
> > > > > + { \
> > > > > + __init_thread = kthread_run(_long_probe_##initfn,\
> > > > > + NULL, \
> > > > > + #initfn); \
> > > > > + if (IS_ERR(__init_thread)) \
> > > > > + return PTR_ERR(__init_thread); \
> > > > > + return 0; \
> > > > > + } \
> > > > > + module_init(__long_probe_##initfn);
> > > > > +/* To be used by modules that require module_long_probe_init() */
> > > > > +#define module_long_probe_exit(exitfn)
> > > > > \
> > > > > + static inline void __long_probe_##exitfn(void) \
> > > > > + { \
> > > > > + exitfn(); \
> > > > > + if (__init_thread) \
> > > > > + kthread_stop(__init_thread); \
> > > > > + } \
> > > >
> > > > exitfn() should be called after kthread_stop(), and only if initfn()
> > > > returns 0. So it should probably do
> > > >
> > > > int err = kthread_stop(__init_thread);
> > > > if (!err)
> > > > exitfn();
> > >
> > > Thanks! With the check for __init_thread as well as it can be
> > > ERR_PTR(-ENOMEM), ERR_PTR(-EINTR), or NULL (for whatever other
> > > reason).
> >
> > Do you mean __long_probe_##exitfn() should also check
> > ERR_PTR(__init_thread)?
> > I don't think so. If kthread_run() above fails, module_init() should return
> > the error (it does), so module_exit() won't be called.
>
> Good point.
>
> > > > But there is an additional complication, you can't use __init_thread
> > > > without get_task_struct(),
> > >
> > > Can you elaborate why ? kthread_stop() uses get_task_struct(),
> >
> > This is too late. This task_struct can be already freed/reused. See below.
> >
> > > wake_up_process() and finally put_task_struct(), and we're the
> > > only user of this thread. Also kthread_run() ensures wake_up_process()
> > > gets called on startup, so not sure where the race would be provided
> > > all users here and with the respective helpers on buggy drivers.
> > >
> > > > so __long_probe_##initfn() can't use
> > > > kthread_run(). It needs kthread_create() + get_task_struct() + wakeup.
> > >
> > > I fail to see why we'd need to add get_task_struct() on
> > > module_long_probe_init(), can you clarify?
> >
> > kthread_stop(kthread_run(callback)) is only safe if callback() can not exit
> > on its own, without checking kthread_should_stop(). And btw that is why
> > kthread_stop() does get_task_struct()).
> >
> > If callback() can exit (if it calls do_exit() or simply returns), then
> > nothing
> > protects this task_struct, it will be freed.
>
> OK thanks, yeah I see the issue now, and I was able to create a null
> pointer dereference by simply calling schedule() quite a bit, will
> roll in the required fixes, but come to think of it if there are
> other uses (I haven't SmPLd grep'd for grammar uses yet) perhaps
> generic helpers would be good? kthread_run_alloc() kthread_run_free().
How about just increasing/decreasing the module count for blocking the
exit call? For example:
#define module_long_probe_init(initfn) \
static int _long_probe_##initfn(void *arg) \
{ \
int ret = initfn(); \
module_put(THIS_MODULE); \
return ret; \
} \
static inline __init int __long_probe_##initfn(void) \
{ \
struct task_struct *__init_thread; \
__module_get(THIS_MODULE); \
__init_thread = kthread_run(_long_probe_##initfn,\
NULL, \
#initfn); \
if (IS_ERR(__init_thread)) { \
module_put(THIS_MODULE); \
return PTR_ERR(__init_thread); \
} \
return 0; \
} \
module_init(__long_probe_##initfn);
/* To be used by modules that require module_long_probe_init() */
#define module_long_probe_exit(exitfn) \
module_exit(exitfn);
Takashi
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