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ASan(address sanitizer)初体验
背景
在一个C++代码中，出现了内存问题。专家建议我用工具address sanitizer(ASan)检查一下。
产品
ASan可以发现下面的问题。
- Use after free (dangling pointer dereference)
- Heap buffer overflow
- Stack buffer overflow
- Global buffer overflow
- Use after return
- Use after scope
- Initialization order bugs
- Memory leaks
初体验
环境
在LLVM 3.1和GCC 4.8中，已经包含了ASan。
示例代码
% cat tests/use-after-free.c
#include <stdlib.h>
int main() {
char *x = (char*)malloc(10 * sizeof(char*));
free(x);
return x[5];
}
编译
g++ -fsanitize=address -O1 -fno-omit-frame-pointer -g use-after-free.c
执行
=================================================================
==2388220==ERROR: AddressSanitizer: heap-use-after-free on address 0x607000000105 at pc 0x55d3273db21b bp 0x7ffdf5dce230 sp 0x7ffdf5dce220
READ of size 1 at 0x607000000105 thread T0
#0 0x55d3273db21a in main /home/abeffect/cppProjects/asan/use-after-free.c:5
#1 0x7fa73d9da082 in __libc_start_main ../csu/libc-start.c:308
#2 0x55d3273db10d in _start (/home/xuqian/cppProjects/asan/a.out+0x110d)
0x607000000105 is located 5 bytes inside of 80-byte region [0x607000000100,0x607000000150)
freed by thread T0 here:
#0 0x7fa73e00140f in __interceptor_free ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:122
#1 0x55d3273db1ea in main /home/abeffect/cppProjects/asan/use-after-free.c:4
#2 0x7fa73d9da082 in __libc_start_main ../csu/libc-start.c:308
previously allocated by thread T0 here:
#0 0x7fa73e001808 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:144
#1 0x55d3273db1df in main /home/abeffect/cppProjects/asan/use-after-free.c:3
#2 0x7fa73d9da082 in __libc_start_main ../csu/libc-start.c:308
SUMMARY: AddressSanitizer: heap-use-after-free /home/abeffect/cppProjects/asan/use-after-free.c:5 in main
Shadow bytes around the buggy address:
0x0c0e7fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c0e7fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c0e7fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c0e7fff8000: fa fa fa fa fd fd fd fd fd fd fd fd fd fa fa fa
0x0c0e7fff8010: fa fa 00 00 00 00 00 00 00 00 05 fa fa fa fa fa
=>0x0c0e7fff8020:[fd]fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa
0x0c0e7fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c0e7fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c0e7fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c0e7fff8060: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c0e7fff8070: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
Shadow gap: cc
==2388220==ABORTING
原理
[2]
程序申请的每 8bytes 内存映射到 1byte 的 shadown 内存上。
因为 malloc 返回的地址都是基于8字节对齐的，所以每8个字节实际可能有以下几个状态：
case 1：8 个字节全部可以访问，例如char* p = new char[8]; 将0写入到这8个字节对应的1个字节的shadow内存。
case 2：前 1<=n<8 个字节可以访问, 例如char* p = new char[n], 将数值n写入到相应的1字节的shadow内存，尽管这个对象实际只占用5bytes，malloc的实现里[p+5, p+7]这尾部的3个字节的内存也不会再用于分配其他对象，所以通过指针p来越界访问最后3个字节的内存也是被允许的。
asan还会在程序申请的内存的前后，各增加一个redzone区域（n * 8bytes），用来解决overflow/underflow类问题。
free对象时，asan不会立即把这个对象的内存释放掉，而是写入1个负数到该对象的shadown内存中，即将该对象成不可读写的状态， 并将它记录放到一个隔离区(book keeping)中, 这样当有野指针或use-after-free的情况时，就能跟进shadow内存的状态，发现程序的异常；一段时间后如果程序没有异常，就会再释放隔离区中的对象。
编译器在对每个变量的load/store操作指令前都插入检查代码，确认是否有overflow、underflow、use-after-free等问题。