Descriptor:user_data_segment() does not have Flags::LONG_MODE set

[leecannon]

The doc-comment of the function states Creates a segment descriptor for a long mode ring 3 data segment.

I have not made a pull request to change this as i'm unsure if Flags::LONG_MODE is deliberately not set. My knowledge of the intricacies of x86_64 is sorely lacking.

[josephlr]

This is intentional. For more information see section 4.8 of the AMD64 Architecture Programmer’s Manual, Volume 2: System Programming.

Basically, in 64-bit mode, data segments are almost completely ignored. From the manual:

Segment-limit checking is not performed on any data segments in 64-bit mode, and both the segment-limit field and granularity (G) bit are ignored. The D/B bit is unused in 64-bit mode.

The expand-down (E), writable (W), and accessed (A) type-field attributes are ignored.

A data-segment-descriptor DPL field is ignored in 64-bit mode, and segment-privilege checks are not
performed on data segments. System software can use the page-protection mechanisms to isolate and
protect data from unauthorized access.

In practice, this means that a Data Segment (for either the user or the kernel) only needs to set Flags::USER_SEGMENT and Flags::PRESENT, and not set Flags::EXECUTABLE, every other flag is ignored.

We actually set Flags::WRITABLE and Flags::DPL_RING_3 unnecessarily which probably confuses things.

[josephlr]

#181 clarifies some of the documentation here, but I think this can be closed.

[phil-opp]

Thanks for looking into this @josephlr!