[OpenWrt-Devel] [PATCH v2] package/base-files: caldata: work around dd's limitation

[Thibaut]

Hi Étienne,

> Le 17 mai 2020 à 22:24, Etienne Champetier <champetier.etienne at gmail.com> a écrit :
> 
> Hi Thibaut,
> 
> Le dim. 17 mai 2020 à 15:46, Thibaut VARÈNE <hacks at slashdirt.org <mailto:hacks at slashdirt.org>> a écrit :
>> 
>> -       dd if=$source of=$target iflag=skip_bytes bs=$count skip=$offset count=1 2>/dev/null || \
>> +       # dd doesn't handle partial reads from special files: use cat
>> +       cat $source | dd of=$target iflag=skip_bytes bs=$count skip=$offset count=1 2>/dev/null || \
> 
> Not way more elegant, but you could use something like
> tail -c+$start $source | head -c$count > $target
> with $start == $offset+1 I think
> 
> # head -c10 /dev/zero | wc
>        0         0        10
> # head -c10 /dev/zero | tail -c+3 | wc
>        0         0         8

I’m not sure there’s a good reason to do it that way: my change barely touches the current logic: it is trivial to confirm that it doesn’t break anything that was previously working. On the other hand, doing away with dd entirely will need more proofing, and if some extra dd flag is needed in the future it will have to be reinstated.

Your approach also puts more burden on the shell (it uses redirection to output the final file).
All in all I’m afraid it doesn’t look more elegant to me, and it seems less efficient too.

The really clean fix which isn’t available to us as busybox dd doesn’t support the ‘count_bytes’ iflag would be something along those lines:

dd if=$source of=$target iflag=skip_bytes,count_bytes bs=$PAGE_SIZE skip=$offset count=$count

And then everything would work correctly. This is a different approach which I tested successfully with the « real » dd.

Cheers,
Thibaut

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