Race-free block device opening

[Demi Marie Obenour <demi@invisiblethingslab.com>]

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Right now, opening block devices in a race-free way is incredibly hard.
The only reasonable approach I know of is sd_device_new_from_path() +
sd_device_open(), and is only available in systemd git main.  It also
requires waiting on systemd-udev to have processed udev rules, which can
be a bottleneck.  There are better approaches in various special cases,
such as using device-mapper ioctls to check that the device one has
opened still has the name and/or UUID one expects.  However, none of
them works for a plain call to open(2).

A much better approach would be for udev to point its symlinks at
"/dev/disk/by-diskseq/$DISKSEQ" for non-partition disk devices, or at
"/dev/disk/by-diskseq/${DISKSEQ}p${PARTITION}" for partitions.  A
filesystem would then be mounted at "/dev/disk/by-diskseq" that provides
for race-free opening of these paths.  This could be implemented in
userspace using FUSE, either with difficulty using the current kernel
API, or easily and efficiently using a new kernel API for opening a
block device by diskseq + partition.  However, I think this should be
handled by the Linux kernel itself.

What would be necessary to get this into the kernel?  I would like to
implement this, but I don’t have the time to do so anytime soon.  Is
anyone else interested in taking this on?  I suspect the kernel code
needed to implement this would be quite a bit smaller than the FUSE
implementation.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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