rwd0a and label sector marked read-only

[Jean-Yves Migeon <jean-yves.migeon%espci.fr@localhost>]

Hi list,

A few days back I tried to restore partition images through dd(1).

After setting up the labels, next step was to restore the partitions 
directly through their rwd* devices.

Everything went fine, except for the "a" label, due to the sector that 
contains the labels' description. By default, it is marked as read-only, 
hence returning EROFS when using dd directly:

dd if=/mnt/rwd0a.img of=/dev/rwd0a
dd: /dev/rwd0a: Read-only file system
2+0 records in
1+0 records out
512 bytes transferred in 0.001 secs (512000 bytes/sec)

Of course, playing with skip/seek works around the issue. Marking the 
labels as writable (through disklabel -W) is another solution.

However, I have a few questions:

- is such behaviour expected when manipulating the char device directly?
- why EROFS and not EPERM?
- why is it possible to override the aforementioned sector when using 
label d (under x86) directly, and not when using other labels? Should 
the checks be any different?

Many thanks in advance for your answers!

--
Jean-Yves Migeon
jean-yves.migeon%espci.fr@localhost