Public bug reported:
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$((end - start))
** Affects: initramfs-tools (Ubuntu)
Importance: Undecided
Status: New
** Patch added: "dd-count.patch"
https://bugs.launchpad.net/bugs/2055055/+attachment/5749511/+files/dd-count.patch
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https://bugs.launchpad.net/bugs/2055055
Title:
unmkinitramfs: wrong and unneeded count= in a dd call
Status in initramfs-tools package in Ubuntu:
New
Bug description:
Speaking about this line in unmkinitramfs:
113: dd < "$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))
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/initramfs-tools/+bug/2055055/+subscriptions
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