[PATCH v4 0/1] nvme : Add ioctl to query nvme device attributes

[Joel Granados]

What?
In this patch I add an ioctl that provides nvme controller attributes to
the user that can open the char device file. We also provide them for the
block devices for convenience.

Why?
Applications with write permissions should not need to be privileged to
write to the device. With Kanchan's latest patch
(https://lore.kernel.org/linux-nvme/20221020070205.57366-1-joshi.k@samsung.com/)
the nvme IO and identify commands in passthru now follow device
permissions; however there are still some controller attributes like
minimal data transfer size (MDTS) which need a privileged user to be
queried.

How?
Added an ioctl (NVME_IOCTL_GET_ATTR) that fetches existing and new
attributes from nvme_ctrl struct. The new attributes are dmsl, vsl and wusl
from the I/O command set specific identify controller data structure in the
nvme spec.

I have made the call extensible by embedding the struct size as the first
member and using it as a version. I call copy_struct_from_user to make sure
that struct members that are unique are zeroed out.  For this I followed
in the steps of the openat2 system call [1] and the extensible ioctl for
vfio [2].  Another interesting reference for this is here
https://lwn.net/Articles/830666/

Feedback is greatly appreciated :)

[1] https://github.com/torvalds/linux/commit/fddb5d430ad9
[2] https://github.com/torvalds/linux/blob/master/include/uapi/linux/vfio.h#L56

V4:
* Fixed an error where the ioctl number would change if new members were
  added. Now I use _IO instead of _IOWR to avoid leave the ioctl number
  static. This is very similar to the implementation contained in
  https://github.com/torvalds/linux/blob/master/include/uapi/linux/vfio.h

V3:
* Removed unneeded comments in nvme_ioctl.h
* Added a comment to the nvme_sectors_to_mps function
* Moved size checks to nvme_check_size in core.h
* Changed struct initialization to match what we use in nvme driver {} vs
  {0}

V2:
* Changed comment from // to /**/
* Took a call out from an if condition and assigned it to ret var.

Joel Granados (1):
  nvme : Add ioctl to query nvme attributes

 drivers/nvme/host/core.c        |  8 ++++-
 drivers/nvme/host/ioctl.c       | 61 +++++++++++++++++++++++++++++++++
 drivers/nvme/host/nvme.h        | 11 ++++++
 include/uapi/linux/nvme_ioctl.h | 23 +++++++++++++
 4 files changed, 102 insertions(+), 1 deletion(-)

-- 
2.30.2