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usb: gadget: f_ncm: Fix net_device lifecycle with device_move

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commit ec35c19696 upstream.

The network device outlived its parent gadget device during
disconnection, resulting in dangling sysfs links and null pointer
dereference problems.

A prior attempt to solve this by removing SET_NETDEV_DEV entirely [1]
was reverted due to power management ordering concerns and a NO-CARRIER
regression.

A subsequent attempt to defer net_device allocation to bind [2] broke
1:1 mapping between function instance and network device, making it
impossible for configfs to report the resolved interface name. This
results in a regression where the DHCP server fails on pmOS.

Use device_move to reparent the net_device between the gadget device and
/sys/devices/virtual/ across bind/unbind cycles. This preserves the
network interface across USB reconnection, allowing the DHCP server to
retain their binding.

Introduce gether_attach_gadget()/gether_detach_gadget() helpers and use
__free(detach_gadget) macro to undo attachment on bind failure. The
bind_count ensures device_move executes only on the first bind.

[1] https://lore.kernel.org/lkml/f2a4f9847617a0929d62025748384092e5f35cce.camel@crapouillou.net/
[2] https://lore.kernel.org/linux-usb/795ea759-7eaf-4f78-81f4-01ffbf2d7961@ixit.cz/

Fixes: 40d133d7f5 ("usb: gadget: f_ncm: convert to new function interface with backward compatibility")
Cc: stable <stable@kernel.org>
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://patch.msgid.link/20260309-f-ncm-revert-v2-7-ea2afbc7d9b2@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Kuen-Han Tsai 2026-03-09 20:04:52 +08:00 • committed by Greg Kroah-Hartman
commit e584cb58a2
4 changed files with 74 additions and 14 deletions
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