qemu 在 user mode 下是用 static code gen buffer，size為32mb
in ./exec.c line 408 it mentioned:

Currently it is not recommended to allocate big chunks of data in
user mode. It will change when a dedicated libc will be used

but don't know why. Why not use mmap as system mode use?

Segment selector Format:

15 ...... 3	2 1	0
index	TI	RPL

TI Table indicator:

0 means selector indexes into GDT
1 means selector indexes into LDT

RPL Privelege level. Linux uses only two privelege levels.

0 means kernel
3 means user

Examples:

Kernel code segment

TI=0, index=1, RPL=0, therefore selector = 0x08 (GDT[1])

User data segment

TI=1, index=2, RPL=3, therefore selector = 0x17 (LDT[2])

stack 弄壞了！

先弄一個 exit basic block，然後再將 pass 給去掉。

現在要focus 在 indirect branch 上，要知道的是第一個參數的 address 是什麼？
FP->getPassName()
lib/CodeGen/StackProtector.cpp

DefaultJITMemoryManager

lib/CodeGen/LLVMTargetMachine.cpp:
LLVMTargetMachine::addPassesToEmitMachineCode
LLVMTargetMachine::addCommonCodeGenPasses

=============================
Prevent spill register code gen.
enable to emit epilog for branch and indirect brance
=============================
錯在這：0x40095872: mov (%edx),%eax
%edx 為 init_stack+c 的位置，其中的值
在 loader_exec 中就放了！
在 create_elf_tables() 之後！
在 loader_build_argptr() 之後！
在 put_user_ual(stringp, envp) 之後！