内存问题的排查工具和方法– Clang的AddressSanitizer
1 概述
Valgrind可以有效地监测处大多数内存问题，你肯定忍不住会想，既然c/c++的内存问题这么常见，为什么不在编译器中加入内存问题检测的功能呢？ 很可惜，GCC中还目前还不支持内存检测，可喜的是，clang支持。这里我们看看如何用clang发现内存问题
2 clang
clang 是一个C、C++、Objective-C编程语言的编译器前端。它采用了底层虚拟机作为其后端。它的目标是提供一个GNU编译器套装（GCC）的替代品， 作者是克里斯·拉特纳，在苹果公司的赞助下进行开发。
3 内存泄漏监测
AddressSanitizer是clang中的一个内存错误检测器，它可以检测到以下问题：
- Out-of-bounds accesses to heap, stack and globals
- Use-after-free
- Use-after-return (to some extent)
- Double-free, invalid free
- Memory leaks (experimental)
使用clang编译代码时用-fsanitize=address就能打开AddressSanitizer工具，为了在检测到内存错误时打印出您的程序调用栈，需要在编译时加上选项 -fno-omit-frame-pointer选项，同时为了得出更清晰的调用栈信息，请用-O1选项编译程序。
4 示例代码
下面我用clang3.4做一个示例
1: int main()
2: {
3: char *p = malloc(sizeof(char) * 10);
4: if (p == NULL) {
5: return 0;
6: }
7:
8: struct elem *e = malloc(sizeof(struct elem));
9: if (e == NULL) {
10: free(p);
11: return 0;
12: }
13:
14: e->a = 10;
15: e->b = 10.10;
16: e->c = p;
17:
18: double *xx = &e->b;
19:
20: printf("%f\n", *xx);
21:
22: free(e);
23:
24: printf("%f\n", *xx);
25:
26: return 0;
27: }
上面的代码中有两处问题，一是p未被释放，导致了内存泄漏；二是xx指向了一块被释放了的内存。我们看看怎么用clang检测这两个问题
4.1 编译它
1: clang -O1 -g -fsanitize=address -fno-omit-frame-pointer -o core core.c
4.2 用AddressSanitizer监测进程的内存泄漏
直接运行core文件，它就会自动打印出检测到的内存错误
1: [cobbliu@kftest25 test]$ ./core
2: 10.100000
3: =================================================================
4: ==11254==ERROR: AddressSanitizer: heap-use-after-free on address 0x60300000efe8 at pc 0x48211a bp 0x7fff2c776450 sp 0x7fff2c776448
5: READ of size 8 at 0x60300000efe8 thread T0
6: #0 0x482119 in main /home/cobbliu/test/core.c:35
7: #1 0x36a101ecdc in __libc_start_main (/lib64/libc.so.6+0x36a101ecdc)
8: #2 0x481f3c in _start (/home/cobbliu/test/core+0x481f3c)
9:
10: 0x60300000efe8 is located 8 bytes inside of 24-byte region [0x60300000efe0,0x60300000eff8)
11: freed by thread T0 here:
12: #0 0x46bca9 in __interceptor_free /home/ads/build23_6u0_x64/workspace/t-coresystem-clang/label/build23_6u0_x64/t-coresystem-clang/llvm-3.4/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:64
13: #1 0x4820c0 in main /home/cobbliu/test/core.c:32
14: #2 0x36a101ecdc in __libc_start_main (/lib64/libc.so.6+0x36a101ecdc)
15:
16: previously allocated by thread T0 here:
17: #0 0x46be29 in malloc /home/ads/build23_6u0_x64/workspace/t-coresystem-clang/label/build23_6u0_x64/t-coresystem-clang/llvm-3.4/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:74
18: #1 0x48202a in main /home/cobbliu/test/core.c:18
19: #2 0x36a101ecdc in __libc_start_main (/lib64/libc.so.6+0x36a101ecdc)
20:
21: SUMMARY: AddressSanitizer: heap-use-after-free /home/cobbliu/test/core.c:35 main
22: Shadow bytes around the buggy address:
23: 0x0c067fff9da0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
24: 0x0c067fff9db0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
25: 0x0c067fff9dc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
26: 0x0c067fff9dd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
27: 0x0c067fff9de0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
28: =>0x0c067fff9df0: fa fa fa fa fa fa fa fa fa fa fa fa fd[fd]fd fa
29: 0x0c067fff9e00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
30: 0x0c067fff9e10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
31: 0x0c067fff9e20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
32: 0x0c067fff9e30: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
33: 0x0c067fff9e40: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
34: Shadow byte legend (one shadow byte represents 8 application bytes):
35: Addressable: 00
36: Partially addressable: 01 02 03 04 05 06 07
37: Heap left redzone: fa
38: Heap right redzone: fb
39: Freed heap region: fd
40: Stack left redzone: f1
41: Stack mid redzone: f2
42: Stack right redzone: f3
43: Stack partial redzone: f4
44: Stack after return: f5
45: Stack use after scope: f8
46: Global redzone: f9
47: Global init order: f6
48: Poisoned by user: f7
49: ASan internal: fe
50: ==11254==ABORTING
可以看到，程序在提示core.c的第35行有个heap-use-after-free的错误，而且在最后还有个summary，把出错的代码位置和相应的栈信息打了出来。
5 示例代码2
上面的代码做一些小修改，我们看看它对double-free问题的检测
1: /...
2: struct elem *e2 = e;
3: free(e);
4: free(e2);
5: /...
6: }
按照上面相同的方法编译并运行后，提示信息如下：
1: [cobbliu@kftest25 test]$ ./core
2: 10.100000
3: =================================================================
4: ==11952==ERROR: AddressSanitizer: attempting double-free on 0x60300000efe0 in thread T0:
5: #0 0x46bca9 in __interceptor_free /home/ads/build23_6u0_x64/workspace/t-coresystem-clang/label/build23_6u0_x64/t-coresystem-clang/llvm-3.4/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:64
6: #1 0x4820bd in main /home/cobbliu/test/core.c:34
7: #2 0x36a101ecdc in __libc_start_main (/lib64/libc.so.6+0x36a101ecdc)
8: #3 0x481f3c in _start (/home/cobbliu/test/core+0x481f3c)
9:
10: 0x60300000efe0 is located 0 bytes inside of 24-byte region [0x60300000efe0,0x60300000eff8)
11: freed by thread T0 here:
12: #0 0x46bca9 in __interceptor_free /home/ads/build23_6u0_x64/workspace/t-coresystem-clang/label/build23_6u0_x64/t-coresystem-clang/llvm-3.4/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:64
13: #1 0x4820b0 in main /home/cobbliu/test/core.c:33
14: #2 0x36a101ecdc in __libc_start_main (/lib64/libc.so.6+0x36a101ecdc)
15:
16: previously allocated by thread T0 here:
17: #0 0x46be29 in malloc /home/ads/build23_6u0_x64/workspace/t-coresystem-clang/label/build23_6u0_x64/t-coresystem-clang/llvm-3.4/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:74
18: #1 0x482026 in main /home/cobbliu/test/core.c:18
19: #2 0x36a101ecdc in __libc_start_main (/lib64/libc.so.6+0x36a101ecdc)
20:
21: SUMMARY: AddressSanitizer: double-free /home/ads/build23_6u0_x64/workspace/t-coresystem-clang/label/build23_6u0_x64/t-coresystem-clang/llvm-3.4/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:64 __interceptor_free
22: ==11952==ABORTING
可以看到，AddressSanitizer报错，说core.c的34行有一个double-free的错误
6 示例代码3
上面的代码做一些小修改，把释放e的代码注释掉，看看它对内存泄漏的检测
1: /...
2: //free(e);
3: /...
4: }
按照上面相同的方法编译并运行后，提示信息如下：
1: [cobbliu@kftest25 test]$ ./core
2: 10.100000
可以看到，对内存泄漏，AddressSanitizer无法检测出来 clang中有一个工具叫LeakSanitizer，它的设计目标是用来检测内存泄漏。直到3.7版，LeakSanitizer也是在实验阶段。
7 AddressSanitizer的缺陷
- AddressSanitizer工具编译的程序的堆栈和栈占用比原生程序的大。
- AddressSanitizer不支持静态编译
更新：gcc4.8版本之后，有了对AddressSanitizer的支持！
