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udev: add /dev/loop/ symlinks

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This adds symlinks that allow accessing loopback block devices via stable
names that reference their backing block devices, make the unpredictable
naming of loopback devices less of an issue.

Example:

1. Create a loopback block device for a file $F

   losetup --find $F

2. Reference the backing block device via its inode:

   L="$(stat -c '/dev/loop/by-inode/%Hd:%Ld-%i' $F)"
   fdisk $L

In the above the loop device name (which might be /dev/loop47 or any
other name) is not used at all.
This commit is contained in:
Lennart Poettering 2023-03-06 12:55:59 +01:00
parent 6f5ef9e4c7
commit 5ac52d1f7b
1 changed files with 11 additions and 0 deletions

11
rules.d/60-persistent-storage.rules.in

@ -141,4 +141,15 @@ ENV{ID_PART_ENTRY_SCHEME}=="gpt", ENV{ID_PART_ENTRY_NAME}=="?*", SYMLINK+="disk/
ENV{DISKSEQ}=="?*", ENV{DEVTYPE}!="partition", ENV{ID_IGNORE_DISKSEQ}!="1", SYMLINK+="disk/by-diskseq/$env{DISKSEQ}"
ENV{DISKSEQ}=="?*", ENV{DEVTYPE}=="partition", ENV{ID_IGNORE_DISKSEQ}!="1", SYMLINK+="disk/by-diskseq/$env{DISKSEQ}-part%n"
# Create symlinks that allow referencing loopback devices by their backing file's inode number
ENV{DEVTYPE}!="partition", ENV{ID_LOOP_BACKING_DEVICE}!="", ENV{ID_LOOP_BACKING_INODE}!="", SYMLINK+="loop/by-inode/$env{ID_LOOP_BACKING_DEVICE}-$env{ID_LOOP_BACKING_INODE}"
ENV{DEVTYPE}=="partition", ENV{ID_LOOP_BACKING_DEVICE}!="", ENV{ID_LOOP_BACKING_INODE}!="", SYMLINK+="loop/by-inode/$env{ID_LOOP_BACKING_DEVICE}-$env{ID_LOOP_BACKING_INODE}-part%n"
# Similar, but uses the .lo_file_name field of the loopback device (note that
# this is basically just a free-form string passed from userspace to the kernel
# when the device is created, it is not necessarily a file system path like the
# "loop/backing_file" sysfs attribute, which is always an absolute path)
ENV{DEVTYPE}!="partition", ENV{ID_LOOP_BACKING_FILENAME_ENC}!="", SYMLINK+="loop/by-ref/$env{ID_LOOP_BACKING_FILENAME_ENC}"
ENV{DEVTYPE}=="partition", ENV{ID_LOOP_BACKING_FILENAME_ENC}!="", SYMLINK+="loop/by-ref/$env{ID_LOOP_BACKING_FILENAME_ENC}-part%n"
LABEL="persistent_storage_end"