Just found out, aside for skip_bytes iflag, there is also count_bytes
one. So another fix count be:
113: dd < "$initramfs" skip=$start count=$((end - start))
iflag=skip_bytes,count_bytes 2> /dev/null |
** Description changed:
Speaking about this line in unmkinitramfs:
- 113: dd < "$initramfs" skip=$start count=$((end - start))
+ 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.
+ 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-$start)*512 bytes which is wrong.
Anyways, the script just works because the count is unneeded: dd's
output is piped into cpio which stops on the first end-of-archive
marker, no matter how much data it is fed with.
I think it is the best to just drop the count option (the patch is
attached).
+ If there is still a need to explicitly limit data fed to cpio, dd is
+ impractical in this case. The only way to count= in bytes is bs=1, which
+ makes dd extremely slow on lengthy data chunks. For example
+ ubuntu-23.10.1-desktop-amd64.iso contains initrd with embedded
+ uncompressed cpio archives of such sizes:
- If there is still a need to explicitly limit data fed to cpio, dd is
impractical in this case. The only way to count= in bytes is bs=1, which makes
dd extremely slow on lengthy data chunks. For example
ubuntu-23.10.1-desktop-amd64.iso contains initrd with embedded uncompressed
cpio archives of such sizes:
-
- 77312
- 7200768
- 78615040
+ 77312
+ 7200768
+ 78615040
A combo of dd+head could be used intead to skip and count respectively:
- 113: dd < "$initramfs" skip=$start iflag=skip_bytes 2> /dev/null |
- head -c$((end - start))
+ 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))
+ 113: tail -c+$((start+1)) "$initramfs" | head -c$((end - start)) |
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https://bugs.launchpad.net/bugs/2055055
Title:
unmkinitramfs: wrong and unneeded count= in a dd call
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