A PCI endpoint function driver can define function specific attributes
under its function configfs directory using the add_cfs() endpoint driver
operation. This is done by tying up the mkdir operation for the function
configfs directory to a call to the add_cfs() operation.  However, there
are no checks preventing the user from repeatedly creating function
specific attribute directories with different names, resulting in the same
endpoint specific attributes group being added multiple times, which also
result in an invalid reference counting for the attribute groups. E.g.,
using the pci-epf-ntb function driver as an example, the user creates the
function as follows:

  $ modprobe pci-epf-ntb
  $ cd /sys/kernel/config/pci_ep/functions/pci_epf_ntb
  $ mkdir func0
  $ tree func0
  func0/
  |-- baseclass_code
  |-- cache_line_size
  |-- ...
  `-- vendorid

  $ mkdir func0/attrs
  $ tree func0
  func0/
  |-- attrs
  |   |-- db_count
  |   |-- mw1
  |   |-- mw2
  |   |-- mw3
  |   |-- mw4
  |   |-- num_mws
  |   `-- spad_count
  |-- baseclass_code
  |-- cache_line_size
  |-- ...
  `-- vendorid

At this point, the function can be started by linking the EP controller.
However, if the user mistakenly creates again a directory:

  $ mkdir func0/attrs2
  $ tree func0
  func0/
  |-- attrs
  |   |-- db_count
  |   |-- mw1
  |   |-- mw2
  |   |-- mw3
  |   |-- mw4
  |   |-- num_mws
  |   `-- spad_count
  |-- attrs2
  |   |-- db_count
  |   |-- mw1
  |   |-- mw2
  |   |-- mw3
  |   |-- mw4
  |   |-- num_mws
  |   `-- spad_count
  |-- baseclass_code
  |-- cache_line_size
  |-- ...
  `-- vendorid

The endpoint function specific attributes are duplicated and cause a crash
when the endpoint function device is torn down:

  refcount_t: addition on 0; use-after-free.
  WARNING: CPU: 2 PID: 834 at lib/refcount.c:25 refcount_warn_saturate+0xc8/0x144
  CPU: 2 PID: 834 Comm: rmdir Not tainted 6.3.0-rc1 #1
  Hardware name: Pine64 RockPro64 v2.1 (DT)
  pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
  ...
  Call trace:
  refcount_warn_saturate+0xc8/0x144
  config_item_get+0x7c/0x80
  configfs_rmdir+0x17c/0x30c
  vfs_rmdir+0x8c/0x204
  do_rmdir+0x158/0x184
  __arm64_sys_unlinkat+0x64/0x80
  invoke_syscall+0x48/0x114
  ...

Fix this by modifying pci_epf_cfs_work() to execute the new function
pci_ep_cfs_add_type_group() which itself calls pci_epf_type_add_cfs() to
obtain the function specific attribute group and the group name (directory
name) from the endpoint function driver. If the function driver defines an
attribute group, pci_ep_cfs_add_type_group() then proceeds to register this
group using configfs_register_group(), thus automatically exposing the
function type specific configfs attributes to the user. E.g.:

  $ modprobe pci-epf-ntb
  $ cd /sys/kernel/config/pci_ep/functions/pci_epf_ntb
  $ mkdir func0
  $ tree func0
  func0/
  |-- baseclass_code
  |-- cache_line_size
  |-- ...
  |-- pci_epf_ntb.0
  |   |-- db_count
  |   |-- mw1
  |   |-- mw2
  |   |-- mw3
  |   |-- mw4
  |   |-- num_mws
  |   `-- spad_count
  |-- primary
  |-- ...
  `-- vendorid

With this change, there is no need for the user to create or delete
directories in the endpoint function attributes directory. The
pci_epf_type_group_ops group operations are thus removed.

Also update the documentation for the pci-epf-ntb and pci-epf-vntb function
drivers to reflect this change, removing the explanations showing the need
to manually create the sub-directory for the function specific attributes.

Link: https://lore.kernel.org/r/20230415023542.77601-2-dlemoal@kernel.orgSigned-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>