In Ubuntu 14.04 amd64, I dump the GDT info in a kernel module:
0000: NULL desc
0008: 32-bit CODE desc, DPL 0
0010: 64-bit CODE desc, DPL 0
0018: DATA desc, DPL 0
0020: 32-bit CODE desc, DPL 3
0028: DATA desc, DPL 3
0030: 64-bit CODE desc, DPL 3
0038: NULL desc
0040: busy TSS desc
0050: NULL desc
0058: NULL desc
0060: NULL desc
0068: NULL desc
0070: NULL desc
0078: 16-bit DATA desc, DPL 3
and also dump the segment registers for the kernel module:
CS = 10H
DS = 00H
ES = 00H
SS = 00H
FS = 00H
GS = 00H
Q1: Does kernel module not use DS and SS?
On the other side, I wrote a ring 3 application, also dump the segment registers in main():
CS = 33H
DS = 00H
ES = 00H
SS = 2BH
FS = 00H
GS = 00H
Q2: The app's CS points to 64-bit code desc in GDT, SS points to DATA desc in GDT. That's no problem, but why the app's DS is also 00H?
I don't think gcc can compile all the code without using DS or SS.
64-bit mode (aka long mode) does not use segmentation, so any null selector (i.e. 00H) will work for data segments (all but CS). The code segment is still necessary for selecting 64-bit mode vs 32-bit (compatibility) mode, but the offset and length parts of the selector are not used.
So, in a sense, DS and SS are still being "used" (because it's more-or-less impossible to avoid them), but not from the GDT.