访问线程环境块时进入程序恐慌状态

Question

I'm developing on a Go library to access some internal Windows thread structures (Thread Environment Block) and this requires writing some assembly code. I've been trying to understand why this works on a Win32 C++ application but it doesn't on my Go library.

This snippet of Go assembly code accesses fs:[0x18] to return a pointer to the thread's associated TEB :

    // func ReadFsDword(offset uint32) (dword uint32)
    TEXT ·ReadFsDword(SB),$0-8
            MOVL offset+0(FP), AX
            // mov eax, dword ptr fs:[eax]
            BYTE $0x64; BYTE $0x8B; BYTE $0x00
            MOVL AX, ret+8(FP)
            RET

This is the equivalent MASM code, which compiles and runs just fine on MSVC:

void* readfsdword(unsigned offset_)
{
    unsigned dw;

    __asm {
        mov eax, offset_
        mov eax, fs:[eax]
        mov dw, eax
    }

    return (void*)dw;
}

The Go program panics when accessing the returned pointer to the TEB. Here's the message I get:

panic: runtime error: invalid memory address or nil pointer dereference [signal 0xc0000005 code=0x0 addr=0x0 pc=0x498d5b]

The Go assembly code seems right to me, but I can't understand how and why the program panics. Any help is much appreciated!

Here's an example to reproduce the issue:

intrinsics.s

#include "textflag.h"
#include "funcdata.h"

// func ReadFsDword(offset uint32) (ret uint32)
TEXT ·ReadFsDword(SB),$0-8
        MOVL offset+0(FP), AX
        // mov eax, dword ptr fs:[eax]
        BYTE $0x64; BYTE $0x8B; BYTE $0x00
        MOVL AX, ret+8(FP)
        RET

intrinsics.go

package nt

func ReadFsDword(offset uint32) (ret uint32)

test.go

package main

import "nt"

func main() {
    GetProcAddress("LoadLibraryExW")
}

func GetProcAddress(proc string) unsafe.Pointer {
    teb := nt.NtGetTeb()
    fmt.Printf("%p", teb)

    // todo: implement
    return nil
}

Answer 1

The problem was fixed. Apparently, Windows uses the gs register with an offset of 0x30 on x64 whereas fs and a 0x18 offset are used on x86/WoW64 mode. The solution is to use either fs or gs with the respective offsets depending on the value of GOARCH .