Linux 下的 AddressSanitizer

AddressSanitizer 是一个性能非常好的 C/C++ 内存错误探测工具。它由编译器的插桩模块（目前，LLVM 通过）和替换了 malloc
函数的运行时库组成。这个工具可以探测如下这些类型的错误：

ASAN_OPTIONS=detect_stack_use_after_return=1

这个工具非常快。通常情况下，内存问题探测这类调试工具的引入，会导致原有应用程序运行性能的大幅下降，比如大名鼎鼎的 valgrind 据说会导致应用程序性能下降到正常情况的十几分之一，但引入 AddressSanitizer 只会减慢运行速度的一半。

AddressSanitizer 的使用

自 LLVM 的版本 3.1 和 GCC 的版本 4.8 开始， AddressSanitizer
就是它们的一部分。如果需要的话，也可以从源码编译 AddressSanitizerHowToBuild
。
查看自己的 LLVM 版本和 GCC 版本来确认是否内置了对 AddressSanitizer 的支持：

$ clang --version
clang version 3.9.1-4ubuntu3~16.04.2 (tags/RELEASE_391/rc2)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

$ gcc --version
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.10) 5.4.0 20160609
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

我本地的工具虽然版本比较老，但对 AddressSanitizer 还是支持的。
看一下前面提到的 AddressSanitizer 的运行时库：

$ locate asan
/usr/lib/gcc/x86_64-linux-gnu/5/libasan.a
/usr/lib/gcc/x86_64-linux-gnu/5/libasan.so
/usr/lib/gcc/x86_64-linux-gnu/5/libasan_preinit.o
/usr/lib/gcc/x86_64-linux-gnu/5/32/libasan.a
/usr/lib/gcc/x86_64-linux-gnu/5/32/libasan.so
/usr/lib/gcc/x86_64-linux-gnu/5/32/libasan_preinit.o
/usr/lib/gcc/x86_64-linux-gnu/5/include/sanitizer/asan_interface.h
/usr/lib/gcc/x86_64-linux-gnu/5/x32/libasan.a
/usr/lib/gcc/x86_64-linux-gnu/5/x32/libasan.so
/usr/lib/gcc/x86_64-linux-gnu/5/x32/libasan_preinit.o
/usr/lib/gcc/x86_64-linux-gnu/7/libasan.a
/usr/lib/gcc/x86_64-linux-gnu/7/libasan.so
/usr/lib/gcc/x86_64-linux-gnu/7/libasan_preinit.o
/usr/lib/gcc/x86_64-linux-gnu/7/include/sanitizer/asan_interface.h
/usr/lib/gcc-cross/arm-linux-gnueabihf/5/libasan.a
/usr/lib/gcc-cross/arm-linux-gnueabihf/5/libasan.so
/usr/lib/gcc-cross/arm-linux-gnueabihf/5/libasan_preinit.o
/usr/lib/gcc-cross/arm-linux-gnueabihf/5/include/sanitizer/asan_interface.h
/usr/lib/gcc-cross/arm-linux-gnueabihf/5/sf/libasan.a
/usr/lib/gcc-cross/arm-linux-gnueabihf/5/sf/libasan.so
/usr/lib/gcc-cross/arm-linux-gnueabihf/5/sf/libasan_preinit.o
/usr/lib/llvm-3.9/lib/clang/3.9.1/asan_blacklist.txt
/usr/lib/llvm-3.9/lib/clang/3.9.1/include/sanitizer/asan_interface.h
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-i386.a
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-i386.so
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-i686.a
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-i686.so
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-preinit-i386.a
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-preinit-i686.a
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-preinit-x86_64.a
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-x86_64.a
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-x86_64.a.syms
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan-x86_64.so
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan_cxx-i386.a
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan_cxx-i686.a
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan_cxx-x86_64.a
/usr/lib/llvm-3.9/lib/clang/3.9.1/lib/linux/libclang_rt.asan_cxx-x86_64.a.syms
/usr/lib/llvm-5.0/lib/clang/5.0.2/asan_blacklist.txt
/usr/lib/llvm-5.0/lib/clang/5.0.2/include/sanitizer/asan_interface.h
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-i386.a
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-i386.so
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-i686.a
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-i686.so
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-preinit-i386.a
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-preinit-i686.a
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-preinit-x86_64.a
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-x86_64.a
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-x86_64.a.syms
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan-x86_64.so
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan_cxx-i386.a
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan_cxx-i686.a
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan_cxx-x86_64.a
/usr/lib/llvm-5.0/lib/clang/5.0.2/lib/linux/libclang_rt.asan_cxx-x86_64.a.syms
/usr/lib/x86_64-linux-gnu/libasan.so.2
/usr/lib/x86_64-linux-gnu/libasan.so.2.0.0
/usr/lib/x86_64-linux-gnu/libasan.so.4
/usr/lib/x86_64-linux-gnu/libasan.so.4.0.0
/usr/lib32/libasan.so.2
/usr/lib32/libasan.so.2.0.0
/usr/libx32/libasan.so.2
/usr/libx32/libasan.so.2.0.0

为了使用 AddressSanitizer
，需要在使用 GCC 或 Clang 编译链接程序时加上 -fsanitize=address
开关。为了获得合理的性能，可以加上 -O1
或更高。为了在错误信息中获得更友好的栈追踪信息可以加上 -fno-omit-frame-pointer
。为了获得完美的栈追踪信息，还可以禁用内联（使用 -O1
）和尾调用消除（ -fno-optimize-sibling-calls
）
下面是一段存在内存访问错误的代码：

// main.cpp
int main(int argc, char **argv){
  int *array = new int[100];
  delete [] array;
  return array[argc];  // BOOM
}

使用 GCC 编译并运行：

$ gcc -fsanitize=address -fno-omit-frame-pointer -O1 -g -o main main.cpp
$ ./main 
=================================================================
==5385==ERROR: AddressSanitizer: heap-use-after-free on address 0x61400000fe44 at pc 0x0000004007d4 bp 0x7ffddf0bafb0 sp 0x7ffddf0bafa0
READ of size 4 at 0x61400000fe44 thread T0
    #0 0x4007d3 in main addresssanitizer_demo/main.cpp:4
    #1 0x7f297fc2282f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #2 0x4006b8 in _start (addresssanitizer_demo/main+0x4006b8)

0x61400000fe44 is located 4 bytes inside of 400-byte region [0x61400000fe40,0x61400000ffd0)
freed by thread T0 here:
    #0 0x7f2980065caa in operator delete[](void*) (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x99caa)
    #1 0x4007a8 in main /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:3
    #2 0x7f297fc2282f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)

previously allocated by thread T0 here:
    #0 0x7f29800656b2 in operator new[](unsigned long) (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x996b2)
    #1 0x400798 in main /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:2
    #2 0x7f297fc2282f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)

SUMMARY: AddressSanitizer: heap-use-after-free addresssanitizer_demo/main.cpp:4 main
Shadow bytes around the buggy address:
  0x0c287fff9f70: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9f80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9f90: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9fa0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9fb0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c287fff9fc0: fa fa fa fa fa fa fa fa[fd]fd fd fd fd fd fd fd
  0x0c287fff9fd0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff9fe0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff9ff0: fd fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa
  0x0c287fffa000: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fffa010: 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
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  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
==5385==ABORTING

AddressSanitizer 在探测到内存错误之后，向 stderr 打印了错误信息并以非 0 值返回码退出。AddressSanitizer 在发现第一个错误时退出程序。这主要是基于如下的设计：

• 这种方法允许 AddressSanitizer 产生更快和更小的生成码（总共 ~5%）。
• 解决 bug 变得无法避免。AddressSanitizer 不产生误报。一旦内存崩溃发生，则程序进入不一致的状态，这可能导致令人费解的结果和潜在的误导性的后续报告。

如果进程运行在沙盒中且运行在 OS X 10.10 或更早的版本上，则需要设置 DYLD_INSERT_LIBRARIES
环境变量并把它指向由编译器打包用于构建可执行文件的 ASan 库。（可以搜索名字中包含 asan
的动态链接库来找到这个库。）如果没有设置环境变量，则进程将试图重新执行。同时记住，当把可执行文件移动到另一台机器时，ASan 库也需要复制过去。

编译时如果遗漏了 -g
参数，导致可执行文件中缺乏调试信息，则探测到内存错误时，AddressSanitizer 吐出来的错误信息中，无法显示具体的出错的代码行，就像下面这样：

$ gcc -fsanitize=address -fno-omit-frame-pointer -O1  -o main main.cpp 
$ ./main 
=================================================================
==5403==ERROR: AddressSanitizer: heap-use-after-free on address 0x61400000fe44 at pc 0x0000004007d4 bp 0x7ffd981fce20 sp 0x7ffd981fce10
READ of size 4 at 0x61400000fe44 thread T0
    #0 0x4007d3 in main (addresssanitizer_demo/main+0x4007d3)
    #1 0x7f9de8af482f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #2 0x4006b8 in _start (addresssanitizer_demo/main+0x4006b8)

0x61400000fe44 is located 4 bytes inside of 400-byte region [0x61400000fe40,0x61400000ffd0)
freed by thread T0 here:
    #0 0x7f9de8f37caa in operator delete[](void*) (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x99caa)
    #1 0x4007a8 in main (addresssanitizer_demo/main+0x4007a8)
    #2 0x7f9de8af482f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)

previously allocated by thread T0 here:
    #0 0x7f9de8f376b2 in operator new[](unsigned long) (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x996b2)
    #1 0x400798 in main (addresssanitizer_demo/main+0x400798)
    #2 0x7f9de8af482f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)

SUMMARY: AddressSanitizer: heap-use-after-free ??:0 main
Shadow bytes around the buggy address:
  0x0c287fff9f70: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9f80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9f90: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9fa0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9fb0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c287fff9fc0: fa fa fa fa fa fa fa fa[fd]fd fd fd fd fd fd fd
  0x0c287fff9fd0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff9fe0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff9ff0: fd fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa
  0x0c287fffa000: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fffa010: 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
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  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
==5403==ABORTING

上面的 -g
选项也可以用 -ggdb
选项（尽可能的生成 gdb 的可以使用的调试信息）替换。

$ gcc -fsanitize=address -fno-omit-frame-pointer -O1 -ggdb -o main main.cpp 
$ ./main 
=================================================================
==5495==ERROR: AddressSanitizer: heap-use-after-free on address 0x61400000fe44 at pc 0x0000004007d4 bp 0x7fff7014ca10 sp 0x7fff7014ca00
READ of size 4 at 0x61400000fe44 thread T0
    #0 0x4007d3 in main /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:4
    #1 0x7fdddb19982f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #2 0x4006b8 in _start (addresssanitizer_demo/main+0x4006b8)

0x61400000fe44 is located 4 bytes inside of 400-byte region [0x61400000fe40,0x61400000ffd0)
freed by thread T0 here:
    #0 0x7fdddb5dccaa in operator delete[](void*) (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x99caa)
    #1 0x4007a8 in main /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:3
    #2 0x7fdddb19982f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)

previously allocated by thread T0 here:
    #0 0x7fdddb5dc6b2 in operator new[](unsigned long) (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x996b2)
    #1 0x400798 in main /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:2
    #2 0x7fdddb19982f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)

SUMMARY: AddressSanitizer: heap-use-after-free /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:4 main
Shadow bytes around the buggy address:
  0x0c287fff9f70: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9f80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9f90: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9fa0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9fb0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c287fff9fc0: fa fa fa fa fa fa fa fa[fd]fd fd fd fd fd fd fd
  0x0c287fff9fd0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff9fe0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff9ff0: fd fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa
  0x0c287fffa000: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fffa010: 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
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  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
==5495==ABORTING

使用 LLVM/Clang 编译与上面使用 GCC 编译基本相同：

$ clang++ -fsanitize=address -fno-omit-frame-pointer -O1 -g -o test main.cpp
$ ./test 
=================================================================
==5508==ERROR: AddressSanitizer: heap-use-after-free on address 0x61400000fe44 at pc 0x00000050770f bp 0x7ffea20137b0 sp 0x7ffea20137a8
READ of size 4 at 0x61400000fe44 thread T0
    #0 0x50770e in main /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:4:10
    #1 0x7fdeb802382f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/../csu/libc-start.c:291
    #2 0x4192b8 in _start (addresssanitizer_demo/test+0x4192b8)

0x61400000fe44 is located 4 bytes inside of 400-byte region [0x61400000fe40,0x61400000ffd0)
freed by thread T0 here:
    #0 0x504c20 in operator delete[](void*) (addresssanitizer_demo/test+0x504c20)
    #1 0x5076de in main /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:3:3
    #2 0x7fdeb802382f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/../csu/libc-start.c:291

previously allocated by thread T0 here:
    #0 0x5045a0 in operator new[](unsigned long) (addresssanitizer_demo/test+0x5045a0)
    #1 0x5076d3 in main /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:2:16
    #2 0x7fdeb802382f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/../csu/libc-start.c:291

SUMMARY: AddressSanitizer: heap-use-after-free /home/hanpfei0306/data/MyProjects/addresssanitizer_demo/main.cpp:4:10 in main
Shadow bytes around the buggy address:
  0x0c287fff9f70: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9f80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9f90: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9fa0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff9fb0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c287fff9fc0: fa fa fa fa fa fa fa fa[fd]fd fd fd fd fd fd fd
  0x0c287fff9fd0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff9fe0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff9ff0: fd fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa
  0x0c287fffa000: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fffa010: 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
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  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
==5508==ABORTING

可以通过 readelf 看一下 GCC 和 LLVM 生成的可执行文件有什么差别：

# GCC 生成的可执行文件
$ readelf -s -W main | grep asan
Symbol table '.dynsym' contains15entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     1:0000000000000000   0FUNC    GLOBAL DEFAULT  UND __asan_report_load4
    10:0000000000400650   0FUNC    GLOBAL DEFAULT  UND __asan_init_v4
    44:0000000000000000   0FILE    LOCAL  DEFAULT  ABS asan_preinit.cc
    57:0000000000000000   0FUNC    GLOBAL DEFAULT  UND __asan_report_load4
    62:0000000000400650   0FUNC    GLOBAL DEFAULT  UND __asan_init_v4
    69: 0000000000600dd0    8OBJECT  GLOBAL HIDDEN   19__local_asan_preinit

# LLVM 生成的可执行文件
$ readelf -s -W test | grep asan
     3:0000000000000000   0NOTYPE  WEAK   DEFAULT  UND __asan_default_options
    16:0000000000000000   0NOTYPE  WEAK   DEFAULT  UND __asan_on_error
    49:0000000000000000   0NOTYPE  WEAK   DEFAULT  UND __asan_default_suppressions
    66:0000000000426740 466FUNC    GLOBAL DEFAULT  13__asan_stack_malloc_0
    70: 00000000004269b0  504FUNC    GLOBAL DEFAULT  13__asan_stack_malloc_1
. . . . . .
   271:0000000000428000 134FUNC    GLOBAL DEFAULT  13__asan_stack_free_8
   272: 000000000042b1e0   44FUNC    GLOBAL DEFAULT  13__asan_register_image_globals
   273: 00000000004d7550   44FUNC    GLOBAL DEFAULT  13__asan_report_load4_noabort
   275: 00000000004282a0  134FUNC    GLOBAL DEFAULT  13__asan_stack_free_9
. . . . . .
   635: 00000000004d7460   44FUNC    GLOBAL DEFAULT  13__asan_report_load2
   636: 00000000004d74f0   44FUNC    GLOBAL DEFAULT  13__asan_report_load4
   644: 00000000004d7580   44FUNC    GLOBAL DEFAULT  13__asan_report_load8
. . . . . .
    37:0000000000000000   0FILE    LOCAL  DEFAULT  ABS asan_allocator.cc.o
    38:0000000000419370 254FUNC    LOCAL  DEFAULT  13_ZN6__asanL10RZSize2LogEj
    39: 0000000000421c90  214FUNC    LOCAL  DEFAULT  13_ZN11__sanitizer20SizeClassAllocator64ILm105553116266496ELm4398046511104ELm0ENS_12SizeClassMapILm17ELm128ELm16EEEN6__asan20AsanMapUnmapCallbackEE15DeallocateBatchEPNS_14AllocatorStatsEmPNS2_13TransferBatchE.isra.32
    40:00000000004194701241FUNC    LOCAL  DEFAULT  13_ZN6__asan9AsanChunk8UsedSizeEb.part.19
  1162:00000000004226701101FUNC    WEAK   HIDDEN   13_ZN11__sanitizer28SizeClassAllocatorLocalCacheINS_20SizeClassAllocator64ILm105553116266496ELm4398046511104ELm0ENS_12SizeClassMapILm17ELm128ELm16EEEN6__asan20AsanMapUnmapCallbackEEEE6RefillEPS6_m
  1178: 00000000004d74f0   44FUNC    GLOBAL DEFAULT  13__asan_report_load4
  1194: 00000000004cff10   57FUNC    GLOBAL HIDDEN   13_ZN6__asan10AsanTSDGetEv
  1196: 00000000004d6ea0    8FUNC    GLOBAL DEFAULT  13__asan_get_report_sp
  1202: 00000000004a75e0   63FUNC    GLOBAL HIDDEN   13_ZN6__asan13SetThreadNameEPKc
  1212: 00000000004d7b50   96FUNC    GLOBAL DEFAULT  13__asan_load1_noabort
. . . . . .
  2202: 00000000004d9780   96FUNC    GLOBAL DEFAULT  13__asan_init
  2208: 000000000041a720 2468FUNC    GLOBAL HIDDEN   13_ZN6__asan22FindHeapChunkByAddressEm
  2219: 00000000004d9800    7FUNC    GLOBAL HIDDEN   13_ZN6__asan20GetMallocContextSizeEv
. . . . . .
  3790: 0000000000419ac0   19FUNC    GLOBAL HIDDEN   13_ZN6__asan13AsanChunkView11IsAllocatedEv
  3796: 00000000004d08e0   97FUNC    GLOBAL HIDDEN   13_ZN6__asan25ThreadNameWithParenthesisEPNS_17AsanThreadContextEPcm

主要看两个可执行文件中都有的符号 __asan_report_load4
和 __asan_init
，可以看到 GCC 生成的可执行文件动态链接 ASan 库，LLVM 静态链接。

符号化输出

AddressSanitizer
收集如下事件的调用栈：

• malloc
  和 malloc
• 线程创建
• 失败

malloc
和 malloc
发生的相对频繁，且它对于快速解开调用栈非常重要。 AddressSanitizer
使用一个依赖帧指针的简单的 unwinder。

如果不关心 malloc
/ free
调用栈，简单地完全禁用（使用 malloc_context_size=0
运行时标记）unwinder。
每个栈帧需要被符号化（当然，如果二进制文件编译时带有调试信息）。给定一台 PC，我们需要输出：

#0xabcdf function_name file_name.cc:1234

AddressSanitizer 使用 Clang 包中的 llvm-symbolizer
符号化栈追踪信息（注意理想的 llvm-symbolizer 版本必须与 ASan 运行时库匹配）。为了使 AddressSanitizer 符号化它的输出，需要设置 ASAN_SYMBOLIZER_PATH
环境变量指向 llvm-symbolizer
二进制文件，或确保 llvm-symbolizer
在 $PATH
中：

$ ASAN_SYMBOLIZER_PATH=/usr/local/bin/llvm-symbolizer ./a.out
==9442== ERROR: AddressSanitizer heap-use-after-free on address 0x7f7ddab8c084 at pc 0x403c8c bp 0x7fff87fb82d0 sp 0x7fff87fb82c8
READ of size 4 at 0x7f7ddab8c084 thread T0
    #0 0x403c8c in main example_UseAfterFree.cc:4
    #1 0x7f7ddabcac4d in __libc_start_main ??:0
0x7f7ddab8c084 is located 4 bytes inside of 400-byte region [0x7f7ddab8c080,0x7f7ddab8c210)
freed by thread T0 here:
    #0 0x404704 in operator delete[](void*) ??:0
    #1 0x403c53 in main example_UseAfterFree.cc:4
    #2 0x7f7ddabcac4d in __libc_start_main ??:0
previously allocated by thread T0 here:
    #0 0x404544 in operator new[](unsigned long) ??:0
    #1 0x403c43 in main example_UseAfterFree.cc:2
    #2 0x7f7ddabcac4d in __libc_start_main ??:0
==9442== ABORTING

llvm-symbolizer
符号化工具属于 llvm 包，Ubuntu 下具体的安装方法可以参考 LLVM Debian/Ubuntu nightly packages
。

如果上面的方法不起作用，可以使用一个单独的脚本来离线地符号化结果（在线的符号化可以通过设置 ASAN_OPTIONS=symbolize=0
，或者设置一个空的 ASAN_SYMBOLIZER_PATH
环境变量（ $ export ASAN_SYMBOLIZER_PATH=
）来强制禁用）：

$ ASAN_OPTIONS=symbolize=0 ./a.out 2> log
$ projects/compiler-rt/lib/asan/scripts/asan_symbolize.py/ <log|c++filt
==9442== ERROR: AddressSanitizer heap-use-after-free on address 0x7f7ddab8c084 at pc 0x403c8c bp 0x7fff87fb82d0 sp 0x7fff87fb82c8
READ of size 4 at 0x7f7ddab8c084 thread T0
    #0 0x403c8c in main example_UseAfterFree.cc:4
    #1 0x7f7ddabcac4d in __libc_start_main ??:0
...

这个脚本接收一个可选的参数 -- a file prefix
。子串 .*prefix
将被从文件名中移除。

上面的 c++filt
用于解函数名的符号重组，这也可以通过给 asan_symbolize.py
脚本添加 -d
参数来完成。

在 OS X 上可能需要对二进制文件运行 dsymutil
以在 AddressSanitizer 报告中获得 file:line
信息。

还可以引入自己的栈追踪格式，使用 stack_trace_format
运行时标记完成。例如：

% ./a.out
    ...
    #0 0x4b615d in main /home/you/use-after-free.cc:12:3
    ...
% ASAN_OPTIONS='stack_trace_format="[frame=%n, function=%f, location=%S]"' ./a.out
    ...
    [frame=0, function=main, location=/home/you/use-after-free.cc:12:3]

AddressSanitizer 算法

简单的版本

运行时库替换 malloc
和 free
函数。把 malloc 分配的内存区域（红色区域）附近放入一些特定的字节（使中毒）。把 free
的内存放入隔离区，并且也放入一些特定的字节（使中毒）。程序中的每次内存访问由编译器以下面的方式做一个转换。
之前：

*address = ...;  // or: ... = *address;

之后：

if (IsPoisoned(address)){
  ReportError(address,kAccessSize, kIsWrite);
}
*address= ...; // or: ... = *address;

棘手的部分是如何把 IsPoisoned
实现的高效，且 ReportError
紧凑。同时，对某些访问的插桩可能被证明是冗余的。

内存映射

虚拟地址空间被分割为 2 个互斥的类别：

• 主应用内存（ Mem
  ）：这种内存由常规的应用代码使用。
• 阴影内存（ Shadow
  ）：这种内存包含阴影值（或元数据）。阴影和主应用程序内存之间存在对应关系。在主内存中 使中毒
  一个字节意味着在对应的阴影区写入一些特殊的值。

这两种类别的内存应该以阴影内存（ MemToShadow
）可以被快速计算出来的方式进行组织。
编译器执行的插桩如下：

shadow_address= MemToShadow(address);
if (ShadowIsPoisoned(shadow_address)){
  ReportError(address,kAccessSize, kIsWrite);
}

映射

AddressSanitizer
把 8 个字节的应用内存映射为 1 个字节的阴影内存。
对于任何 8 字节对齐的应用内存只有 9 个不同的值：

• qword 中的所有 8 字节是未中毒的（比如，可寻址）。阴影值为 0。
• qword 中的所有 8 字节是中毒的（比如，不可寻址）。阴影值为负数。
• 起始的 k
  字节是未中毒的，其余的 8-k
  字节是中毒的。阴影值为 k
  。这主要由 malloc
  的行为保证，即 malloc
  总是返回 8 字节对齐的内存块。一个 qword 对齐的不同字节具有不同状态的仅有的情况是 malloc 的区域的尾部。比如，如果我们调用 malloc(13)
  ，我们将拥有一个完整的未中毒的 qword 和一个开头 5 字节未中毒的 qword。

插桩看起来像下面这样：

byte*shadow_address= MemToShadow(address);
byteshadow_value= *shadow_address;
if (shadow_value){
  if (SlowPathCheck(shadow_value,address,kAccessSize)) {
    ReportError(address,kAccessSize, kIsWrite);
  }
}

// Check the cases where we access first k bytes of the qword
// and these k bytes are unpoisoned.
bool SlowPathCheck(shadow_value, address, kAccessSize){
  last_accessed_byte = (address & 7) + kAccessSize - 1;
  return (last_accessed_byte >= shadow_value);
}

MemToShadow(ShadowAddr)
落入不可寻址的 ShadowGap
区域。因此，如果程序试图直接访问阴影区域中的内存位置，它将崩溃。

64-bit

Shadow = (Mem >> 3) + 0x7fff8000;

32 bit

Shadow = (Mem >> 3) + 0x20000000;

超紧凑的阴影区

使用更紧凑的阴影区内存也是可能的，比如：

Shadow = (Mem >> 7) | kOffset;

还在实验中。

报告错误

ReportError
可以被实现为一个调用（当前默认就是这样），但也有一些其它的，稍微更加高效和/或更加紧凑的方案。此刻默认的行为 是
：

• 把失败地址拷贝到 %rax
  ( %eax
  )
• 执行 ud2
  （产生 SIGILL）
• 在 ud2
  之后的一个字节指令中编码访问类型和大小。整体上这 3 个指令需要 5-6 个字节的机器码。

仅使用单个指令（比如 ud2
）也是可能的，但这需要在运行时库中有一个完整的反汇编器（或一些其它的 hacks）。

栈

为了捕获栈溢出， AddressSanitizer
插桩的代码像这样：
原始的代码：

void foo(){
  char a[8];
  ...
  return;
}

插桩后的代码：

void foo() {
  char redzone1[32];  // 32-byte aligned
  char a[8];          // 32-byte aligned
  char redzone2[24];
  char redzone3[32];  // 32-byte aligned
  int  *shadow_base = MemToShadow(redzone1);
  shadow_base[0] = 0xffffffff;  // poison redzone1
  shadow_base[1] = 0xffffff00;  // poison redzone2, unpoison 'a'
  shadow_base[2] = 0xffffffff;  // poison redzone3
  ...
  shadow_base[0] = shadow_base[1] = shadow_base[2] = 0; // unpoison all
  return;
}

插桩的代码示例（x86_64）

# long load8(long *a) { return *a; }
0000000000000030 :
  30:   48 89 f8                mov    %rdi,%rax
  33:   48 c1 e8 03             shr    $0x3,%rax
  37:   80 b8 00 80 ff 7f 00    cmpb   $0x0,0x7fff8000(%rax)
  3e:   75 04                   jne    44 
  40:   48 8b 07                mov    (%rdi),%rax   <<<<<< original load
  43:   c3                      retq   
  44:   52                      push   %rdx
  45:   e8 00 00 00 00          callq  __asan_report_load8

# int load4(int *a) { return *a; }
0000000000000000 :
   0:   48 89 f8                mov    %rdi,%rax
   3:   48 89 fa                mov    %rdi,%rdx
   6:   48 c1 e8 03             shr    $0x3,%rax
   a:   83 e2 07                and    $0x7,%edx
   d:   0f b6 80 00 80 ff 7f    movzbl 0x7fff8000(%rax),%eax
  14:   83 c2 03                add    $0x3,%edx
  17:   38 c2                   cmp    %al,%dl
  19:   7d 03                   jge    1e 
  1b:   8b 07                   mov    (%rdi),%eax    <<<<<< original load
  1d:   c3                      retq   
  1e:   84 c0                   test   %al,%al
  20:   74 f9                   je     1b 
  22:   50                      push   %rax
  23:   e8 00 00 00 00          callq  __asan_report_load4

未对齐的访问

当前紧凑的映射将不捕获未对齐的部分越界访问：

int *x = new int[2]; // 8 bytes: [0,7].
int *u = (int*)((char*)x + 6);
*u = 1;  // Access to range [6-9]

https://github.com/google/sanitizers/issues/100
中描述了一个可行的方案，但它付出了性能的代价。

运行时库

Malloc

运行时库替换 malloc
/ free
，并提供错误报告函数，如 __asan_report_load8
。

malloc
分配由红区围绕的请求数量的内存。阴影值对应的红区被下毒，主内存区域的阴影值被清除。

free
用阴影值对整个区域下毒，并把内存块放入一个隔离区（这样在一定时间内这个内存块将不会再次被 malloc 返回）。

参考文档

• Clang AddressSanitizer
• AddressSanitizerAlgorithm
• llvm-symbolizer
• Address Sanitizer 用法
• sanitizers
• AddressSanitizerExampleHeapOutOfBounds
• AddressSanitizerCallStack