[Bug 2055055] Re: unmkinitramfs: wrong and unneeded count= in a dd call

[Ubuntu Foundations Team Bug Bot]

The attachment "dd-count.patch" seems to be a patch.  If it isn't,
please remove the "patch" flag from the attachment, remove the "patch"
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** Tags added: patch

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Title:
  unmkinitramfs: wrong and unneeded count= in a dd call

Status in initramfs-tools package in Ubuntu:
  New

Bug description:
  Speaking about this line in unmkinitramfs:

      113: dd < "$initramfs" skip=$start count=$((end - start))
  iflag=skip_bytes 2> /dev/null |

  dd's block size is 512 by default. iflag=skip_bytes does not change
  that. Both $end and $start are byte-offsets. Hence the count is
  ($end-$start) blocks or ($end-$start)*512 bytes which is wrong.

  Anyways, the script just works because the count is unneeded: dd's
  output is piped into cpio which stops on the first end-of-archive
  marker, no matter how much data it is fed with.

  I think it is the best to just drop the count option (the patch is
  attached).

  If there is still a need to explicitly limit data fed to cpio, dd is
  impractical in this case. The only way to count= in bytes is bs=1,
  which makes dd extremely slow on lengthy data chunks. For example
  ubuntu-23.10.1-desktop-amd64.iso contains initrd with embedded
  uncompressed cpio archives of such sizes:

      77312
      7200768
      78615040

  A combo of dd+head could be used intead to skip and count
  respectively:

      113: dd < "$initramfs" skip=$start iflag=skip_bytes 2> /dev/null |
  head -c$((end - start)) |

  Or even tail+head:

      113: tail -c+$((start+1)) "$initramfs" | head -c$((end - start)) |

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