An example from an OpenBSD 4.3 machine:
Code:
Terry@vectra-$ dd if=/dev/wd0d of=~/image bs=512
dd: /dev/wd0d: Permission denied
Terry@vectra-$
I would test on FreeBSD, but I'm not using that system atm.
Simple fact:
If they can gain access to the machine, they can probably steal your data -- who needs to use your systems dd, when they can use a live cd, external usb drive, and go to town with their own tools. You should also always think twice, what you allow people without uid 0 (root) to read.
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Thou shalt check the array bounds of all strings (indeed, all arrays), for surely where thou typest ``foo'' someone someday shall type ``supercalifragilisticexpialidocious''.