Re: Re: [PATCH] fanotify: allow freeze on suspend when waiting for response from userspace

[Jan Kara]

On Thu 22-02-18 15:14:54, Kunal Shubham wrote:
> >> On Fri 16-02-18 15:14:40, t.vivek@xxxxxxxxxxx wrote:
> >> From: Vivek Trivedi <t.vivek@xxxxxxxxxxx>
> >> 
> >> If fanotify userspace response server thread is frozen first,
> >> it may fail to send response from userspace to kernel space listener.
> >> In this scenario, fanotify response listener will never get response
> >> from userepace and fail to suspend.
> >> 
> >> Use freeze-friendly wait API to handle this issue.
> >> 
> >> Same problem was reported here:
> >> https://bbs.archlinux.org/viewtopic.php?id=232270
> >> 
> >> Freezing of tasks failed after 20.005 seconds
> >> (1 tasks refusing to freeze, wq_busy=0)
> >> 
> >> Backtrace:
> >> [<c0582f80>] (__schedule) from [<c05835d0>] (schedule+0x4c/0xa4)
> >> [<c0583584>] (schedule) from [<c01cb648>] (fanotify_handle_event+0x1c8/0x218)
> >> [<c01cb480>] (fanotify_handle_event) from [<c01c8238>] (fsnotify+0x17c/0x38c)
> >> [<c01c80bc>] (fsnotify) from [<c02676dc>] (security_file_open+0x88/0x8c)
> >> [<c0267654>] (security_file_open) from [<c01854b0>] (do_dentry_open+0xc0/0x338)
> >> [<c01853f0>] (do_dentry_open) from [<c0185a38>] (vfs_open+0x54/0x58)
> >> [<c01859e4>] (vfs_open) from [<c0195480>] (do_last.isra.10+0x45c/0xcf8)
> >> [<c0195024>] (do_last.isra.10) from [<c0196140>] (path_openat+0x424/0x600)
> >> [<c0195d1c>] (path_openat) from [<c0197498>] (do_filp_open+0x3c/0x98)
> >> [<c019745c>] (do_filp_open) from [<c0186b44>] (do_sys_open+0x120/0x1e4)
> >> [<c0186a24>] (do_sys_open) from [<c0186c30>] (SyS_open+0x28/0x2c)
> >> [<c0186c08>] (SyS_open) from [<c0010200>] (__sys_trace_return+0x0/0x20)
> >
> > Yeah, good catch.
> >
> >> @@ -63,7 +64,9 @@ static int fanotify_get_response(struct fsnotify_group *group,
> >>  
> >>  	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
> >>  
> >> -	wait_event(group->fanotify_data.access_waitq, event->response);
> >> +	while (!event->response)
> >> +		wait_event_freezable(group->fanotify_data.access_waitq,
> >> +				     event->response);
> >
> > But if the process gets a signal while waiting, we will just livelock the
> > kernel in this loop as wait_event_freezable() will keep returning
> > ERESTARTSYS. So you need to be a bit more clever here...
> 
> Hi Jack,
> Thanks for the quick review.
> To avoid livelock issue, is it fine to use below change? 
> If agree, I will send v2 patch.
> 
> @@ -63,7 +64,11 @@ static int fanotify_get_response(struct fsnotify_group *group,
> 
>         pr_debug("%s: group=%p event=%p\n", __func__, group, event);
> 
> -       wait_event(group->fanotify_data.access_waitq, event->response);
> +       while (!event->response) {
> +               if (wait_event_freezable(group->fanotify_data.access_waitq,
> +                                       event->response))
> +                       flush_signals(current);
> +       }

Hum, I don't think this is correct either as this way if any signal was
delivered while waiting for fanotify response, we'd just lose it while
previously it has been properly handled. So what I think needs to be done
is that we just use wait_event_freezable() and propagate non-zero return
value (-ERESTARTSYS) up to the caller to handle the signal and restart the
syscall as necessary.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR