gcc栈溢出保护机制：stack-protector
引用：https://www.cnblogs.com/arnoldlu/p/11630979.html
1. gcc栈保护机制stack-protector简介
gcc提供了栈保护机制stack-protector。关于stack-protector包含三个选项，分别是stack-protector、stack-protector-all、stack-protector-strong、stack-protector-explicit四种。
关于stack-protector原理、实现、效果、局限参考《GCC 中的编译器堆栈保护技术》。
gcc中对这几种选项的介绍如下：
-Wstack-protector
This option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects.
This includes functions that call "alloca", and functions with buffers larger than 8 bytes.
The guards are initialized when a function is entered and then checked when the function exits.
If a guard check fails, an error message is printed and the program exits.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-fstack-protector-strong
Like -fstack-protector but includes additional functions to be protected --- those that have local array definitions, or have references to local frame addresses.
-fstack-protector-explicit
Like -fstack-protector but only protects those functions which have the "stack_protect" attribute.
stack-protector：保护函数中通过alloca()分配缓存以及存在大于8字节的缓存。缺点是保护能力有限。
stack-protector-all：保护所有函数的栈。缺点是增加很多额外栈空间，增加程序体积。
stack-protector-strong：在stack-protector基础上，增加本地数组、指向本地帧栈地址空间保护。
stack-protector-explicit：在stack-protector基础上，增加程序中显式属性"stack_protect"空间。
如果要停止使用stack-protector功能，需要加上-fno-stack-protector。
stack-protector性能：stack-protector > stack-protector-strong > stack-protector-all。
stack-protector覆盖范围：stack-protector < stack-protector-strong < stack-protector-all。
2. stack-protector测试
针对stack-protector的测试，主要对比stack-protector、stack-protector-strong、stack-protector-all三个选项的区别。
#include <string.h>
#include <stdio.h>
int main(void)
{
char array[2] = {0};
strcpy(array, "stackwilloverflow");
return 0;
}
分别使用如下编译选项：(1) gcc stack.c -o stack -ggdb -fstack-protector、(2)gcc stack.c -o stack -ggdb -fstack-protector-strong、(3)gcc stack.c -o stack -ggdb -fstack-protector-all。
(1)未能检查出栈溢出，(2)、(3)都检查出了stack smashing detected。结论：可以看出stack-protector-strong和stack-protector-all相对于stack-protector更多的检测了栈溢出。
*** stack smashing detected ***: ./stack terminated Aborted (core dumped)
查看core文件的bt full，可以勘测处发生栈溢出的点在array。
...
#3 0x000014653e07915c in __GI___fortify_fail (msg=<optimized out>, msg@entry=0x14653e0ef481 "stack smashing detected") at fortify_fail.c:37
do_abort = 1
#4 0x000014653e079100 in __stack_chk_fail () at stack_chk_fail.c:28
No locals.
#5 0x00000000004005a1 in main () at stack.c:11
array = "st"
修改array大小超过8字节之后，重新使用stack-protector进行测试。结论：当数组大小超过8字节过后，stack-protector才能检测出栈溢出。
#include <string.h>
#include <stdio.h>
int main(void)
{
char array[10] = {0};
strcpy(array, "stackwilloverflowoooooooooo");
return 0;
}
发现了stack smashing：
*** stack smashing detected ***: ./stack terminated Aborted (core dumped)
gdb查看backtrace如下：
...
#3 0x000015062304c15c in __GI___fortify_fail (msg=<optimized out>, msg@entry=0x1506230c2481 "stack smashing detected") at fortify_fail.c:37
do_abort = 1
#4 0x000015062304c100 in __stack_chk_fail () at stack_chk_fail.c:28
No locals.
#5 0x00000000004005b8 in main () at stack.c:11
array = "stackwillo"
3. 内核中使用stack-protector
首先需要定义HAVE_CC_STACKPROTECTOR，然后通过make menuconfig进行配置。
路径为General setup->Stack Protector buffer overflow detection。
参考文档：《stack-protector-strong》、《-fstack-protector-strong》、《"Strong" stack protection for GCC》。
