Re: [RFC PATCH 1/7] block: Support creating a struct file from a block device

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jan 31, 2023 at 11:27:59AM -0500, Demi Marie Obenour wrote:
> While it is easy to provide userspace with an FD to any struct file, it
> is *not* easy to obtain a struct file for a given struct block_device.
> I could have had device-mapper implement everything itself, but that
> would have duplicated a large amount of code already in the block layer.
> Instead, I decided to refactor the block layer to provide a function
> that does exactly what was needed.  The result was this patch.  In the
> future, I would like to add an ioctl for /dev/loop-control that creates
> a loop device and returns a file descriptor to the loop device.  I could
> also see iSCSI supporting this, with the socket file descriptor being
> passed in from userspace.

And it is somewhat intentional that you can't.  Block device inodes
have interesting life times and are never directly exposed to userspace
at all.  They are internal, and only f_mapping of a file system inode
delegates to them or I/O.  Your patch now magically exposes them to
userspace.  And it then bypasses all pathname and inode permission
based access checks and auditing.  So we can't just do it.

> blkdev_do_open() does not solve any problem for me at this time.
> Instead, it represents the code shared by blkdev_get_by_dev() and
> blkdev_get_file().  I decided to export it because it could be of
> independent use to others.  In particular, it could potentially
> simplify disk_scan_partitions() in block/genhd.c, pkt_new_dev() in
> pktcdvd, backing_dev_store() in zram, and f2fs_scan_devices() in f2fs.

All thse need to actually open the underlying device as they do I/O.
Doing I/O without opening the device is a no-go.