Edit Question - KnowledgeHub

Title Body

How can an user-space program configure "GS:" under 64-bit Windows (currently XP-64)? (By configure, set GS:0 at an arbitrary 64-bit linear address). I am trying to port a "JIT" environment to X86-64 that was originally developed for Win32. One unfortunate design aspect is that identical code needs to run on multiple user-space threads (eg, "fibers"). The Win32 version of the code uses the GS selector for this, and generates the proper prefix to access the local data - "mov eax,GS:[offset]" points to the correct data for the current task. The code from the Win32 version would load a value into GS, if only it had a value that would work. So far I've been able to find that 64-bit windows doesn't support the LDT, so the method used under Win32 won't work. However, the X86-64 instruction set includes "SWAPGS", as well as a method to load GS without using the legacy segmentation - but that only works in kernel space. According to X64 manuals, even if Win64 allowed access to descriptors -- which it doesn't -- there's no way to set the high 32-bits of the segment base. The only way to set these is through GS_BASE_MSR (and corresponding FS_BASE_MSR - the other segment bases are ignored in 64-bit mode). The WRMSR instruction is Ring0, so I can't use it directly. I am hoping for a Zw* function that allows me to change "GS:" in user space, or some other dark corner of the Windows API. I believe Windows still uses FS: for its own TLS, so some mechanism must be available? This sample code illustrates the problem. I apologize in advance for using byte code - VS won't do inline assembly for the 64-bit compile, and I was trying to keep this as one file for illustrative purposes. The program displays "PASS" on XP-32, and doesn't on XP-x64. #include <windows.h> #include <string.h> #include <stdio.h> unsigned char GetDS32[] =

Tags (comma-separated) Cancel