[Bug 1469143] Re: kpartx -d fails with image paths longer than 63 characters

[Simo Punnonen]

is the lo_name which gets stored in the loop_info structure actually
required to be an actual file path or is it enough to be an arbitrary
string identifier?

If it doesn't have to be a file path, could it be for instance a sha1
hash of the provided path?  That fits into 40 chars and is relatively
fast to calculate. A sha1 identifier would make it really hard to
collide with other loop device identifiers _unless_ you pass and use the
same relative image path in two separate instances (which is also an
existing problem).

Alternatively it would be nice if the lo_name identifier wasn't actually
based on path but some unique property of the mounted file, that way you
would only need to point to an image, regardless of the path and kpartx
could find if that specific image is mounted or not.

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Title:
  kpartx -d fails with image paths longer than 63 characters

Status in multipath-tools package in Ubuntu:
  In Progress
Status in multipath-tools source package in Precise:
  New
Status in multipath-tools source package in Trusty:
  New
Status in multipath-tools source package in Utopic:
  New
Status in multipath-tools source package in Vivid:
  New

Bug description:
  $ apt-show-versions multipath-tools
  multipath-tools:amd64/vivid 0.4.9-3ubuntu12 uptodate

  Reproduce:
  Mount an image from a path longer than 63 chars succeeds:
  $ sudo kpartx -av asfd1asdf2asdf3asdf4asdf5asdf6asfd7asdf8asdf9asf10asdf11asdf12asdf13/disk.img
  add map loop0p1 (252:0): 0 409600 linear /dev/loop0 2048
  add map loop0p2 (252:1): 0 2 linear /dev/loop0 411648
  add map loop0p5 : 0 819200 linear /dev/loop0 413696
  add map loop0p6 : 0 819200 linear /dev/loop0 1234944
  add map loop0p7 : 0 819200 linear /dev/loop0 2056192
  add map loop0p8 : 0 1316864 linear /dev/loop0 2877440

  but dismounting fails:
  $ sudo kpartx -dv asfd1asdf2asdf3asdf4asdf5asdf6asfd7asdf8asdf9asf10asdf11asdf12asdf13/disk.img

  strace shows that the parameter on the dismount appears to get cut at 63 chars:
  ioctl(3, DM_LIST_VERSIONS, 0x15b89b0)   = 0
  stat("asfd1asdf2asdf3asdf4asdf5asdf6asfd7asdf8asdf9asf10asdf11asdf12asdf13/disk.img", {st_mode=S_IFREG|0644, st_size=2147483648, ...}) = 0
  stat("/dev/loop0", {st_mode=S_IFBLK|0660, st_rdev=makedev(7, 0), ...}) = 0
  open("/dev/loop0", O_RDONLY)            = 4
  ioctl(4, LOOP_GET_STATUS, {number=0, offset=0, flags=0, name="asfd1asdf2asdf3asdf4asdf5asdf6asfd7asdf8asdf9asf10asdf11asdf12a", ...}) = 0
  close(4)                                = 0
  stat("/dev/loop1", {st_mode=S_IFBLK|0660, st_rdev=makedev(7, 1), ...}) = 0
  open("/dev/loop1", O_RDONLY)            = 4
  ioctl(4, LOOP_GET_STATUS, {number=0, offset=0, flags=0, name="asfd1asdf2asdf3asdf4asdf5asdf6asfd7asdf8asdf9asf10asdf11asdf12a", ...}) = -1 ENXIO (No such device or address)

  if the path is 63 chars or less, the dismount also succeeds.

  This is quickly becomes an issue if you want to use full disk paths in
  your shell scripts.

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