udev: add /dev/loop/ symlinks

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.

diff --git a/rules.d/60-persistent-storage.rules.in b/rules.d/60-persistent-storage.rules.in
index 498b24c22d33e09f76a6598cecc7932827ca1ff3..43c2060d5955f36807a55c3f54f6c0569b137629 100644 (file)
--- a/rules.d/60-persistent-storage.rules.in
+++ b/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"