Re: [PATCH v3 1/2] uio_hv_generic: Fix sysfs creation path for ring buffer

[Naman Jain <namjain@linux.microsoft.com>]

On 3/30/2025 9:05 PM, Michael Kelley wrote:
> From: Naman Jain <namjain@linux.microsoft.com> Sent: Thursday, March 27, 2025 10:28 PM
>>
>> On regular bootup, devices get registered to VMBus first, so when
>> uio_hv_generic driver for a particular device type is probed,
>> the device is already initialized and added, so sysfs creation in
>> uio_hv_generic probe works fine. However, when device is removed
>> and brought back, the channel rescinds and device again gets
>> registered to VMBus. However this time, the uio_hv_generic driver is
>> already registered to probe for that device and in this case sysfs
>> creation is tried before the device's kobject gets initialized
>> completely.
>>
>> Fix this by moving the core logic of sysfs creation for ring buffer,
>> from uio_hv_generic to HyperV's VMBus driver, where rest of the sysfs
>> attributes for the channels are defined. While doing that, make use
>> of attribute groups and macros, instead of creating sysfs directly,
>> to ensure better error handling and code flow.
>>
>> Problem path:
>> vmbus_process_offer (new offer comes for the VMBus device)
>>    vmbus_add_channel_work
>>      vmbus_device_register
>>        |-> device_register
>>        |     |...
>>        |     |-> hv_uio_probe
>>        |           |...
>>        |           |-> sysfs_create_bin_file (leads to a warning as
>>        |                 primary channel's kobject, which is used to
>>        |                 create sysfs is not yet initialized)
>>        |-> kset_create_and_add
>>        |-> vmbus_add_channel_kobj (initialization of primary channel's
>>                                    kobject happens later)
>>
>> Above code flow is sequential and the warning is always reproducible in
>> this path.
>>
>> Fixes: 9ab877a6ccf8 ("uio_hv_generic: make ring buffer attribute for primary channel")
>> Cc: stable@kernel.org
>> Suggested-by: Saurabh Sengar <ssengar@linux.microsoft.com>
>> Suggested-by: Michael Kelley <mhklinux@outlook.com>
>> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
>> ---
>>   drivers/hv/hyperv_vmbus.h    |   6 ++
>>   drivers/hv/vmbus_drv.c       | 110 ++++++++++++++++++++++++++++++++++-
>>   drivers/uio/uio_hv_generic.c |  33 +++++------
>>   include/linux/hyperv.h       |   6 ++
>>   4 files changed, 134 insertions(+), 21 deletions(-)
>>
> 
> [snip]
> 
>> +/**
>> + * hv_create_ring_sysfs() - create "ring" sysfs entry corresponding to ring buffers for a channel.
>> + * @channel: Pointer to vmbus_channel structure
>> + * @hv_mmap_ring_buffer: function pointer for initializing the function to be called on mmap of
>> + *                       channel's "ring" sysfs node, which is for the ring buffer of that channel.
>> + *                       Function pointer is of below type:
>> + *                       int (*hv_mmap_ring_buffer)(struct vmbus_channel *channel,
>> + *                                                  struct vm_area_struct *vma))
>> + *                       This has a pointer to the channel and a pointer to vm_area_struct,
>> + *                       used for mmap, as arguments.
>> + *
>> + * Sysfs node for ring buffer of a channel is created along with other fields, however its
>> + * visibility is disabled by default. Sysfs creation needs to be controlled when the use-case
>> + * is running.
>> + * For example, HV_NIC device is used either by uio_hv_generic or hv_netvsc at any given point of
>> + * time, and "ring" sysfs is needed only when uio_hv_generic is bound to that device. To avoid
>> + * exposing the ring buffer by default, this function is reponsible to enable visibility of
>> + * ring for userspace to use.
>> + * Note: Race conditions can happen with userspace and it is not encouraged to create new
>> + * use-cases for this. This was added to maintain backward compatibility, while solving
>> + * one of the race conditions in uio_hv_generic while creating sysfs.
>> + *
>> + * Returns 0 on success or error code on failure.
>> + */
>> +int hv_create_ring_sysfs(struct vmbus_channel *channel,
>> +			 int (*hv_mmap_ring_buffer)(struct vmbus_channel *channel,
>> +						    struct vm_area_struct *vma))
>> +{
>> +	struct kobject *kobj = &channel->kobj;
>> +	struct vmbus_channel *primary_channel = channel->primary_channel ?
>> +		channel->primary_channel : channel;
>> +
>> +	channel->mmap_ring_buffer = hv_mmap_ring_buffer;
>> +	channel->ring_sysfs_visible = true;
>> +
>> +	/*
>> +	 * Skip updating the sysfs group if the primary channel is not yet initialized and sysfs
>> +	 * group is not yet created. In those cases, the 'ring' will be created later in
>> +	 * vmbus_device_register() -> vmbus_add_channel_kobj().
>> +	 */
>> +	if  (!primary_channel->device_obj->channels_kset)
>> +		return 0;
> 
> This test doesn't accomplish what you want. It tests if the "channels" directory
> has been created, but not if the numbered subdirectory for this channel has been
> created. sysfs_update_group() operates on the numbered subdirectory and
> could still fail because it hasn't been created yet.
> 
> My recommendation is to not try to do a test, and just let sysfs_update_group()
> fail in that case (and ignore the error).
> 
> Michael
> 

Thanks Michael. Will remove it.

Regards,
Naman

>> +
>> +	return sysfs_update_group(kobj, &vmbus_chan_group);
>> +}
>> +EXPORT_SYMBOL_GPL(hv_create_ring_sysfs);