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

[Adam Vodopjan]

> The count_bytes solution looks better to me

The updated patch is attached.

I've just checked: all the *_bytes flags were introduced to dd in the
same commit
https://github.com/coreutils/coreutils/commit/140eca15c4a3d3213629a048cc307fde0d094738,
so it is safe to throw in count_bytes in the mix.

> I see four dd calls in unmkinitramfs. Can you check those as well?

The other dd calls are good:
- 2x of bs=1 kind
- the last one is "dump from X till the end" utilizing skip_bytes for X, which is correct

> Debian is affected as well. If you can spare the time, please submit
your patch

Will do when they activate my just created account on salsa

** Patch added: "Add count_bytes iflag"
   https://bugs.launchpad.net/ubuntu/+source/initramfs-tools/+bug/2055055/+attachment/5749755/+files/dd-count-2.patch

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to initramfs-tools in Ubuntu.
https://bugs.launchpad.net/bugs/2055055

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

Status in initramfs-tools package in Ubuntu:
  Triaged

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)) |

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/initramfs-tools/+bug/2055055/+subscriptions