Re: [PATCH v3 2/2] rpmsg: glink: Update cdev add/del API in rpmsg_ctrldev_release_device()

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Tue 02 Nov 18:51 CDT 2021, Sujit Kautkar wrote:

I like Stephen's suggestion about modifying the $subject.
Also note that the change isn't in the glink driver, so prefix should
reflect that:

$ git log --oneline --no-decorate -- drivers/rpmsg/rpmsg_char.c
f998d48f9b3c rpmsg: glink: Update cdev add/del API in rpmsg_ctrldev_release_device()
bc774a3887cb rpmsg: char: Remove useless include
964e8bedd5a1 rpmsg: char: Return an error if device already open
...

> Replace cdev add/del APIs with cdev_device_add/cdev_device_del to avoid
> below kernel warning. This correctly takes a reference to the parent
> device so the parent will not get released until all references to the
> cdev are released.
> 
> | ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x7c
> | WARNING: CPU: 7 PID: 19892 at lib/debugobjects.c:488 debug_print_object+0x13c/0x1b0
> | CPU: 7 PID: 19892 Comm: kworker/7:4 Tainted: G        W         5.4.147-lockdep #1
> | ==================================================================
> | Hardware name: Google CoachZ (rev1 - 2) with LTE (DT)
> | Workqueue: events kobject_delayed_cleanup
> | pstate: 60c00009 (nZCv daif +PAN +UAO)
> | pc : debug_print_object+0x13c/0x1b0
> | lr : debug_print_object+0x13c/0x1b0
> | sp : ffffff83b2ec7970
> | x29: ffffff83b2ec7970 x28: dfffffd000000000
> | x27: ffffff83d674f000 x26: dfffffd000000000
> | x25: ffffffd06b8fa660 x24: dfffffd000000000
> | x23: 0000000000000000 x22: ffffffd06b7c5108
> | x21: ffffffd06d597860 x20: ffffffd06e2c21c0
> | x19: ffffffd06d5974c0 x18: 000000000001dad8
> | x17: 0000000000000000 x16: dfffffd000000000
> | BUG: KASAN: use-after-free in qcom_glink_rpdev_release+0x54/0x70
> | x15: ffffffffffffffff x14: 79616c6564203a74
> | x13: 0000000000000000 x12: 0000000000000080
> | Write of size 8 at addr ffffff83d95768d0 by task kworker/3:1/150
> | x11: 0000000000000001 x10: 0000000000000000
> | x9 : fc9e8edec0ad0300 x8 : fc9e8edec0ad0300
> |
> | x7 : 0000000000000000 x6 : 0000000000000000
> | x5 : 0000000000000080 x4 : 0000000000000000
> | CPU: 3 PID: 150 Comm: kworker/3:1 Tainted: G        W         5.4.147-lockdep #1
> | x3 : ffffffd06c149574 x2 : ffffff83f77f7498
> | x1 : ffffffd06d596f60 x0 : 0000000000000061
> | Hardware name: Google CoachZ (rev1 - 2) with LTE (DT)
> | Call trace:
> |  debug_print_object+0x13c/0x1b0
> | Workqueue: events kobject_delayed_cleanup
> |  __debug_check_no_obj_freed+0x25c/0x3c0
> |  debug_check_no_obj_freed+0x18/0x20
> | Call trace:
> |  slab_free_freelist_hook+0xb4/0x1bc
> |  kfree+0xe8/0x2d8
> |  dump_backtrace+0x0/0x27c

Why is dump_backtrace in the callstack here inbetween
rpmsg_ctrldev_release_device() and kfree()? Isn't the error that we're
calling kfree() on an chunk of memory that contains a live object?

> |  rpmsg_ctrldev_release_device+0x78/0xb8
> |  device_release+0x68/0x14c
> |  show_stack+0x20/0x2c
> |  kobject_cleanup+0x12c/0x298
> |  kobject_delayed_cleanup+0x10/0x18
> |  dump_stack+0xe0/0x19c
> |  process_one_work+0x578/0x92c
> |  worker_thread+0x804/0xcf8
> |  print_address_description+0x3c/0x4a8
> |  kthread+0x2a8/0x314
> |  ret_from_fork+0x10/0x18
> |  __kasan_report+0x100/0x124
> 
> Signed-off-by: Sujit Kautkar <sujitka@chromium.org>
> ---
> Changes in v3:
> - Remove unecessary error check as per Matthias's comment
> 
> Changes in v2:
> - Fix typo in commit message
> 
>  drivers/rpmsg/rpmsg_char.c | 10 ++--------
>  1 file changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/rpmsg/rpmsg_char.c b/drivers/rpmsg/rpmsg_char.c
> index 876ce43df732b..a6a33155ca859 100644
> --- a/drivers/rpmsg/rpmsg_char.c
> +++ b/drivers/rpmsg/rpmsg_char.c
> @@ -458,7 +458,7 @@ static void rpmsg_ctrldev_release_device(struct device *dev)
>  
>  	ida_simple_remove(&rpmsg_ctrl_ida, dev->id);
>  	ida_simple_remove(&rpmsg_minor_ida, MINOR(dev->devt));
> -	cdev_del(&ctrldev->cdev);
> +	cdev_device_del(&ctrldev->cdev, &ctrldev->dev);

I am not able to find any other instance where cdev_device_del() is
called from the device's release function itself, which tells me that
this probably is not the right thing to do. Instead the appropriate way
seem to put the cdev_device_del() in rpmsg_chrdev_remove().


That said, we already do device_del() in rpmsg_chrdev_remove() so if the
warning is trying to tell us that ctrldev->dev is not deleted I think we
have an unbalanced put_device()?

Regards,
Bjorn

>  	kfree(ctrldev);
>  }
>  
> @@ -493,19 +493,13 @@ static int rpmsg_chrdev_probe(struct rpmsg_device *rpdev)
>  	dev->id = ret;
>  	dev_set_name(&ctrldev->dev, "rpmsg_ctrl%d", ret);
>  
> -	ret = cdev_add(&ctrldev->cdev, dev->devt, 1);
> +	ret = cdev_device_add(&ctrldev->cdev, &ctrldev->dev);
>  	if (ret)
>  		goto free_ctrl_ida;
>  
>  	/* We can now rely on the release function for cleanup */
>  	dev->release = rpmsg_ctrldev_release_device;
>  
> -	ret = device_add(dev);
> -	if (ret) {
> -		dev_err(&rpdev->dev, "device_add failed: %d\n", ret);
> -		put_device(dev);
> -	}
> -
>  	dev_set_drvdata(&rpdev->dev, ctrldev);
>  
>  	return ret;
> -- 
> 2.31.0
>