Re: [PATCH v3 0/2] usbip: usbip_host: remove legacy rebind_store in favor of drivers_probe

[Jeffin Philip <jeffinphilip14@gmail.com>]

On Tue, Aug 11 2026, at 16:53:23 -0600, Shuah Khan wrote:

>On 8/11/26 10:05, Jeffin Philip wrote:
>> do_rebind, which sleeps normally gets a mutex lock. However, it does not
>> or should I say, cannot check for null udev between spin lock dropped in
>> rebind_store and entering do_rebind. This is a potential race window
>> already. So, even if we check for null udev under spinlock, we cannot do
>> it outside. Regarding do_rebind, it is called during stub_device_rebind,
>> but that function is called during module exit when all files are removed.
>> So, do_rebind is not designed to work in a concurrent environment in the
>> first place.
>> 
>> We have a safer function that can already do what do_rebind does,
>> drivers_probe. So, we use drivers_probe to rebind the device rather than
>> use rebind_store.
>> 
>> usbip tool references this function immediately after the device is unbound,
>> which is safe for the tool itself but since we opted for drivers_probe, fix
>> it by using drivers_probe rather than rebind_store after unbinding device
>> which is more safer.
>> 
>> Tested and working in both userspace via the tool and manually echoing
>> the busid in the related nodes. rebind node is still left active with a
>> warning to use drivers_probe upon encountering rebind_store.
>> 
>> Thanks,
>> Jeffin.
>> 
>> Signed-off-by: Jeffin Philip <jeffinphilip14@gmail.com>
>> ---
>> Changes in v3:
>>   - Removed rebind_store in favor of drivers_probe to eliminate race
>>     condition
>
>How did you find this problem?

I found the problem on syzbot and had to reproduce it using the following
commands:
link to issue: https://syzkaller.appspot.com/bug?extid=af76b01c9a0f0ab60fb0
echo 'add 1-1' > /sys/bus/usb/drivers/ubsip-host/match_busid
echo '1-1' > /sys/bus/usb/drivers/usbip-host/rebind

>Is this generated code or did you write it?

No, I wrote the code myself.

>Also, the first patch removes code in rebind_store(), replacing it
>with a pr_warn()? The second patch points it driver_probe() - what
>happens with just the first patch?

It just prints out a warning. I did get a -Wunused function warning
while building the kernel for testing and considered removing it. I
ultimately didn't as scripts running on newer kernels(if this
was merged) would break as there is no rebind node. Should I remove it?
and is pr_warn not the right way to deal with this? If so, please advise.

>Did you run tests to see if you can bind and unbind devices - does the
>driver work correctly?

Yes the tool works correctly after switching to drivers_probe. Devices are
bound and unbound correctly.

The function also caused an invalid opcode while testing in an unpatched
kernel with a different sequence: write to match_busid, then write to bind 
and rebind, followed by writing to unbind.

Thanks,
Jeffin.