85 lines
3 KiB
C++
85 lines
3 KiB
C++
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/* Descriptors.cc
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* vim: set tw=80:
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* Eryn Wells <eryn@erynwells.me>
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*/
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/**
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* Oh god oh god. Descriptor tables.
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*/
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#include <stdint.h>
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namespace kernel {
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/** Descriptor privilege level. */
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enum class DPL {
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Ring0 = 0x0,
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Ring1 = 0x1,
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Ring2 = 0x2,
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Ring3 = 0x3
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};
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/** A four bit value describing the type of the segment. */
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enum class Type {
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// Data segment types
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DataRO = 0x0, // Read-only
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DataROA = 0x1, // Read-only, accessed
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DataRW = 0x2, // Read/write
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DataRWA = 0x3, // Read/write, accessed
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DataROEX = 0x4, // Read-only, expand-down
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DataROEXA = 0x5, // Read-only, expand-down, accessed
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DataRWEX = 0x6, // Read/write, expand-down
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DataRWEXA = 0x7, // Read/write, expand-down, accessed
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// Code segment types
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CodeEX = 0x8, // Execute-only
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CodeEXA = 0x9, // Execute-only, accessed
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CodeEXR = 0xa, // Execute/read
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CodeEXRA = 0xb, // Execute/read, accessed
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CodeEXC = 0xc, // Execute-only, conforming
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CodeEXCA = 0xd, // Execute-only, conforming, accessed
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CodeEXRC = 0xe, // Execute/read, conforming
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CodeEXRCA = 0xf // Execute/read, conforming, accessed
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};
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/**
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* SegmentDescriptors are entries in the GDT and LDT that describe memory
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* segments. Each descriptor is two double-words (8 bytes, 64 bits) long.
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*/
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typedef uint64_t SegmentDescriptor;
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SegmentDescriptor sGDT[4];
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static inline SegmentDescriptor
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createSegmentDescriptor(uint32_t base,
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uint32_t limit,
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Type type,
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DPL dpl)
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{
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SegmentDescriptor descriptor = 0;
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uint8_t t = static_cast<uint8_t>(type);
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uint8_t d = static_cast<uint8_t>(dpl);
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descriptor = base & 0xFF000000; // Bits 31:24 of the base address.
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descriptor |= (0x1 << 23); // Granularity field: segment limit is interpreted in 4KB units.
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descriptor |= (0x1 << 22); // D/B field: default operation/stack size flag, 1 for 32-bit.
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descriptor |= (0x0 << 21); // L field: 64-bit code segment, 0 for 32-bit.
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descriptor |= (0x0 << 20); // AVL field: system determined, unused here.
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descriptor |= limit & 0x000F0000; // Bits 19:16 of the segment limit.
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descriptor |= (0x1 << 15); // P field: segment is present.
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descriptor |= (d << 13) & 0x00006000; // DPL field: privilege level of the segment.
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descriptor |= (0x1 << 12); // S field: 0 for system, 1 for code/data.
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descriptor |= (t << 8) & 0x00000F00; // Type field: see Type
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descriptor |= (base >> 16) & 0x000000FF; // Bits 23:16 of the base address.
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// Shift everything up by 32 to make room for the lower 4 bytes
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descriptor <<= 32;
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descriptor |= base << 16; // Bits 15:00 of the base address.
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descriptor |= limit & 0x0000FFFF; // Bits 15:00 of the segment limit.
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return descriptor;
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}
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} /* namespace kernel */
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