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

[Naman Jain]

On 3/30/2025 9:05 PM, Michael Kelley wrote:
> From: Naman Jain <namjain@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx
> > Suggested-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
> > Suggested-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> > Signed-off-by: Naman Jain <namjain@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   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);