[boost] [context] Windows x64 fiber local storage GS offset

Subject: [boost] [context] Windows x64 fiber local storage GS offset
From: Elbert Mai (edmai_at_[hidden])
Date: 2017-04-24 05:18:39

The Windows x64 implementation of Boost.Context saves fiber local storage
from GS:[0x18]. For example, in jump_x86_64_ms_pe_masm.asm:

; load NT_TIB
mov  r10,  gs:[030h]
; save fiber local storage
mov  rax, [r10+018h]
mov  [rsp+0b0h], rax

However, this is the NT_TIB struct as defined in winnt.h

typedef struct _NT_TIB {
    struct _EXCEPTION_REGISTRATION_RECORD *ExceptionList;
    PVOID StackBase;
    PVOID StackLimit;
    PVOID SubSystemTib;
    union {
        PVOID FiberData;
        DWORD Version;
    };
    PVOID ArbitraryUserPointer;
    struct _NT_TIB *Self;
} NT_TIB;

This implies that fiber local storage is found in GS:[0x20], not GS:[0x18].

The Windows x86 implementation uses FS:[0x10], which is consistent with the
NT_TIB struct.

Am I missing something, or does Boost.Context save/restore fiber local
storage at the wrong address in Windows x64?