Module ring3_diag 

Pre-IRETQ diagnostics for transitioning from Ring 0 to Ring 3

validate_ring3_state should be invoked immediately before the IRETQ trampoline. It checks the four prerequisites required for a safe switch to Ring 3:

1. GDT – CS/SS descriptors have DPL=3, P=1 and the code segment has L=1 (64‑bit).
2. Paging – every level (PML4 → PDPT → PD → PT) for target_rip and target_rsp has the USER_ACCESSIBLE flag.
3. Alignment – target_rsp is 16‑byte aligned (System V ABI requirement).
4. TSS – a TSS is loaded (TR ≠ 0) and rsp0 points into kernel space (≥ 0xffff_8000_0000_0000), ensuring the CPU can switch back on exception.

If any check fails the function panic!s with a detailed description.

Constants

DESC_DPL_MASK 🔒

DESC_DPL_SHIFT 🔒

Bits [46:45] : DPL.

DESC_EXEC_BIT 🔒

Bit 43 : Executable (type bit for code segments).

DESC_L_BIT 🔒

Bit 53 : L Long Mode code segment (64-bit).

DESC_PRESENT_BIT 🔒

Bit 47 : Present.

DESC_S_BIT 🔒

Bit 44 : S 1 = segment code/data, 0 = system descriptor.

PHYS_ADDR_MASK 🔒

Physical address mask inside a page-table entry (bits [51:12]).

Functions

check_user_mapping 🔒

Checks that vaddr is mapped and USER_ACCESSIBLE at every level of the page hierarchy (PML4 → PDPT → PD → PT).

desc_dpl 🔒

desc_is_code 🔒

desc_long_mode 🔒

desc_present 🔒

read_gdt_raw 🔒 ^⚠

Reads an 8-byte GDT descriptor at the given index (base 0).

validate_ring3_state

Validates all CPU preconditions for a safe Ring 3 transition via iretq.