Re: [PATCH] mfd: core: Increment of_node's refcount before linking it to the platform device

[Lee Jones <lee@kernel.org>]

On Wed, 20 Aug 2025, Bastien Curutchet wrote:

> When an MFD device is added, a platform_device is allocated. If this
> device is linked to a DT description, the corresponding OF node is linked
> to the new platform device but the OF node's refcount isn't incremented.
> As of_node_put() is called during the platform device release, it leads
> to a refcount underflow.
> 
> Call of_node_get() to increment the OF node's refcount when the node is
> linked to the newly created platform device.
> 
> Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
> ---
> Hi all,
> 
> I'm currently working on a new MFD driver and I encountered some
> underflow errors with the of_node refcount. As you can see in the logs
> below, I reproduced the issue on a mainline driver (atmel-hlcdc):
> 
> > # modprobe atmel-hlcdc
> > # modprobe -r atmel-hlcdc
> > # modprobe atmel-hlcdc
> > # modprobe -r atmel-hlcdc
> > [   22.932128] OF: ERROR: of_node_release() detected bad of_node_put() on /amba_pl/atmel_sama5@43a00000/dc
> > [   22.941586] CPU: 1 UID: 0 PID: 103 Comm: modprobe Not tainted 6.17.0-rc2-00053-gb19a97d57c15-dirty #81 NONE
> > [   22.941608] Hardware name: Xilinx Zynq Platform
> > [   22.941615] Call trace:
> > [   22.941626]  unwind_backtrace from show_stack+0x10/0x14
> > [   22.941660]  show_stack from dump_stack_lvl+0x54/0x68
> > [   22.941680]  dump_stack_lvl from of_node_release+0x140/0x16c
> > [   22.941707]  of_node_release from kobject_put+0x110/0x130
> > [   22.941745]  kobject_put from platform_device_release+0x10/0x3c
> > [   22.941782]  platform_device_release from device_release+0x30/0xa0
> > [   22.941814]  device_release from kobject_put+0x88/0x130
> > [   22.941845]  kobject_put from klist_prev+0xd4/0x16c
> > [   22.941879]  klist_prev from device_for_each_child_reverse+0x88/0xc8
> > [   22.941911]  device_for_each_child_reverse from devm_mfd_dev_release+0x30/0x54
> > [   22.941941]  devm_mfd_dev_release from devres_release_all+0xb0/0x114
> > [   22.941974]  devres_release_all from device_unbind_cleanup+0xc/0x58
> > [   22.942003]  device_unbind_cleanup from device_release_driver_internal+0x190/0x1c4
> > [   22.942025]  device_release_driver_internal from driver_detach+0x54/0xa0
> > [   22.942046]  driver_detach from bus_remove_driver+0x58/0xa4
> > [   22.942066]  bus_remove_driver from sys_delete_module+0x178/0x25c
> > [   22.942094]  sys_delete_module from ret_fast_syscall+0x0/0x54
> > [   22.942116] Exception stack(0xf0a1dfa8 to 0xf0a1dff0)
> > [   22.942130] dfa0:                   004ec438 005a0870 005a0d40 00000080 00000000 005a0d18
> > [   22.942144] dfc0: 004ec438 005a0870 005a0190 00000081 005a0d60 005a0870 00000001 0059f6bc
> > [   22.942155] dfe0: beccdb20 beccdb10 004ed1f4 b6e9eb40
> > [   22.942163] OF: ERROR: next of_node_put() on this node will result in a kobject warning 'refcount_t: underflow; use-after-free.'
> > [   23.098617] OF: ERROR: of_node_release() detected bad of_node_put() on /amba_pl/atmel_sama5@43a00000/pwm
> > [   23.108137] CPU: 1 UID: 0 PID: 103 Comm: modprobe Not tainted 6.17.0-rc2-00053-gb19a97d57c15-dirty #81 NONE
> > [   23.108159] Hardware name: Xilinx Zynq Platform
> > [   23.108166] Call trace:
> > [   23.108173]  unwind_backtrace from show_stack+0x10/0x14
> > [   23.108206]  show_stack from dump_stack_lvl+0x54/0x68
> > [   23.108227]  dump_stack_lvl from of_node_release+0x140/0x16c
> > [   23.108252]  of_node_release from kobject_put+0x110/0x130
> > [   23.108288]  kobject_put from platform_device_release+0x10/0x3c
> > [   23.108324]  platform_device_release from device_release+0x30/0xa0
> > [   23.108354]  device_release from kobject_put+0x88/0x130
> > [   23.108384]  kobject_put from klist_prev+0xd4/0x16c
> > [   23.108418]  klist_prev from device_for_each_child_reverse+0x88/0xc8
> > [   23.108450]  device_for_each_child_reverse from devm_mfd_dev_release+0x30/0x54
> > [   23.108479]  devm_mfd_dev_release from devres_release_all+0xb0/0x114
> > [   23.108513]  devres_release_all from device_unbind_cleanup+0xc/0x58
> > [   23.108541]  device_unbind_cleanup from device_release_driver_internal+0x190/0x1c4
> > [   23.108563]  device_release_driver_internal from driver_detach+0x54/0xa0
> > [   23.108585]  driver_detach from bus_remove_driver+0x58/0xa4
> > [   23.108605]  bus_remove_driver from sys_delete_module+0x178/0x25c
> > [   23.108631]  sys_delete_module from ret_fast_syscall+0x0/0x54
> > [   23.108653] Exception stack(0xf0a1dfa8 to 0xf0a1dff0)
> > [   23.108667] dfa0:                   004ec438 005a0870 005a0d40 00000080 00000000 005a0d18
> > [   23.108681] dfc0: 004ec438 005a0870 005a0190 00000081 005a0d60 005a0870 00000001 0059f6bc
> > [   23.108691] dfe0: beccdb20 beccdb10 004ed1f4 b6e9eb40
> > [   23.108698] OF: ERROR: next of_node_put() on this node will result in a kobject warning 'refcount_t: underflow; use-after-free.'
> ---
>  drivers/mfd/mfd-core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/mfd/mfd-core.c b/drivers/mfd/mfd-core.c
> index 76bd316a50afc5c07ff2a3303c4363b16d0bc023..7d14a1e7631ee8d5e91b228a07b2d05695e41b6e 100644
> --- a/drivers/mfd/mfd-core.c
> +++ b/drivers/mfd/mfd-core.c
> @@ -131,6 +131,7 @@ static int mfd_match_of_node_to_dev(struct platform_device *pdev,
>  	of_entry->np = np;
>  	list_add_tail(&of_entry->list, &mfd_of_node_list);
>  
> +	of_node_get(np);

Looks okay at first blush.

My question would be, why isn't this required for all calls to device_set_node()?

>  	device_set_node(&pdev->dev, of_fwnode_handle(np));
>  #endif
>  	return 0;
> 
> ---
> base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
> change-id: 20250820-mfd-refcount-0d561c25b10b
> 
> Best regards,
> -- 
> Bastien Curutchet <bastien.curutchet@bootlin.com>
> 

-- 
Lee Jones [李琼斯]