什么是 ASan
ASan 是 Address Sanitizer 简称,它是是一种基于编译器用于快速检测原生代码中内存错误的工具。
简而言之,ASan 就是一个用于快速检测内存错误的工具。这里很多朋友有误解,ASan 其实并不能用于内存泄漏检测,Android 平台内存泄漏检测推荐 MallocDebug 。
另外需要注意的是 Android O(API >= 27)及以上版本才支持 ASan ,NDK 需要选用 r20 及以上版本。
ASan 可以检测到内存错误类型如下:
- Stack and heap buffer overflow/underflow 栈和堆缓冲区上溢/下溢;
- Heap use after free 堆内存被释放之后还在使用其指针;
- Stack use outside scope 在某个局部变量的作用域之外,使用其指针;
- Double free/wild free 指针重复释放的情况。
ASan 支持 arm 和 x86 平台,使用 ASan 时,APP 性能会变慢且内存占用会飙升。针对 arm64 平台,Android 官方推荐使用 HWAddress Sanitizer (HWASan),后面会介绍。
关于 ASan 的原理本文不做深入讨论,该文章的主要目的是帮助开发者快速上手 ASan 的使用。
这里感性地介绍下 ASan 的工作原理:ASan 相当于接管了内存的分配,当分配一块内存时,会在这块内存的前后添加"标志位",然后再次使用该内存的时候检查"标志位"是否被修改,当发现"标志位"被修改时,判断出现内存错误。
怎么使用 ASan
之所以写这篇文件,就是因为发现一些文章介绍 ASan 使用方法搞得非常复杂,不易上手。
其实 Android 官方的使用说明非常简洁,就是复制黏贴,添加两行代码就搞定。
官方文档:developer.android.com/ndk/guides/…
修改编译脚本
CMake
APP 下面的 build.gradle 添加:
android {
defaultConfig {
externalNativeBuild {
cmake {
# Can also use system or none as ANDROID_STL.
arguments "-DANDROID_ARM_MODE=arm", "-DANDROID_STL=c++_shared"
}
}
}
}
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CMakeLists.txt 脚本添加:
target_compile_options(${libname} PUBLIC -fsanitize=address -fno-omit-frame-pointer)
set_target_properties(${libname} PROPERTIES LINK_FLAGS -fsanitize=address)
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NDK-BUILD
Application.mk 文件添加:
APP_STL := c++_shared # Or system, or none.
APP_CFLAGS := -fsanitize=address -fno-omit-frame-pointer
APP_LDFLAGS := -fsanitize=address
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Android.mk 文件添加:
APP_STL := c++_shared # Or system, or none.
APP_CFLAGS := -fsanitize=address -fno-omit-frame-pointer
APP_LDFLAGS := -fsanitize=address
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拷贝 Asan 库到 jniLibs 目录下
Asan 库位于下面路径下:
android-ndk-r21\toolchains\llvm\prebuilt\windows-x86_64\lib64\clang\9.0.8\lib\linux
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64 位 libclang_rt.asan-aarch64-android.so , 32 位 libclang_rt.asan-arm-android.so ,分别拷贝两个库到 jniLibs 相应的目录下。
新建 wrap.sh 文件,拷贝下面内容到文件中:
#!/system/bin/sh
HERE="$(cd "$(dirname "$0")" && pwd)"
export ASAN_OPTIONS=log_to_syslog=false,allow_user_segv_handler=1
ASAN_LIB=$(ls $HERE/libclang_rt.asan-*-android.so)
if [ -f "$HERE/libc++_shared.so" ]; then
# Workaround for https://github.com/android-ndk/ndk/issues/988.
export LD_PRELOAD="$ASAN_LIB $HERE/libc++_shared.so"
else
export LD_PRELOAD="$ASAN_LIB"
fi
"$@"
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在 main 文件夹下新建目录 resources\lib 然后将 wrap.sh 文件拷贝到相应的目录下面,最终的目录结构是这样的:
<project root>
└── app
└── src
└── main
├── jniLibs
│ ├── arm64-v8a
│ │ └── libclang_rt.asan-aarch64-android.so
│ ├── armeabi-v7a
│ │ └── libclang_rt.asan-arm-android.so
│ ├── x86
│ │ └── libclang_rt.asan-i686-android.so
│ └── x86_64
│ └── libclang_rt.asan-x86_64-android.so
└── resources
└── lib
├── arm64-v8a
│ └── wrap.sh
├── armeabi-v7a
│ └── wrap.sh
├── x86
│ └── wrap.sh
└── x86_64
└── wrap.sh
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自此 ASan 接入完成,是不是很简单?
ASan 检测内存错误
这一节我们在代码中故意设置一些常见的内存错误(内存越界等)用来测试 ASan 检测出来的结果是否正确。需要注意的是,当 ASan 检测出内存错误,程序就会立即 crash ,不再往下执行,log 中会出现关键字 AddressSanitizer 。
堆内存溢出
static void HeapBufferOverflow() {
int *arr = new int[1024];
arr[0] = 11;
arr[1024] = 12;
LOGCATE("HeapBufferOverflow arr[0]=%d, arr[1024]",arr[0], arr[1024]);
}
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ASan 检测结果(crash log)中出现关键字 heap-buffer-overflow :
05-13 19:52:16.247 4194 4194 I com.byteflow.learnffmpeg: =================================================================
05-13 19:52:16.247 4194 4194 I com.byteflow.learnffmpeg: ==4194==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x004f0d5a8100 at pc 0x0074f822b648 bp 0x007ff0227a00 sp 0x007ff02279f8
05-13 19:52:16.247 4194 4194 I com.byteflow.learnffmpeg: WRITE of size 4 at 0x004f0d5a8100 thread T0
05-13 19:52:16.257 23334 23334 D Launcher_UnlockAnimationStateMachine: mResetIdleStateRunnable
05-13 19:52:16.265 4194 4194 I com.byteflow.learnffmpeg: #0 0x74f822b644 (/data/app/com.byteflow.learnffmpeg-mVg7CcQSTXVnJhfo7u0XLA==/lib/arm64/liblearn-ffmpeg.so+0x146644)
05-13 19:52:16.265 4194 4194 I com.byteflow.learnffmpeg: #1 0x74f8229b20 (/data/app/com.byteflow.learnffmpeg-mVg7CcQSTXVnJhfo7u0XLA==/lib/arm64/liblearn-ffmpeg.so+0x144b20)
05-13 19:52:16.265 4194 4194 I com.byteflow.learnffmpeg: #2 0x74f8229a7c (/data/app/com.byteflow.learnffmpeg-mVg7CcQSTXVnJhfo7u0XLA==/lib/arm64/liblearn-ffmpeg.so+0x144a7c)
05-13 19:52:16.265 4194 4194 I com.byteflow.learnffmpeg: #3 0x74fdaac0a0 (/data/app/com.byteflow.learnffmpeg-mVg7CcQSTXVnJhfo7u0XLA==/oat/arm64/base.odex+0xb0a0)
05-13 19:52:16.265 4194 4194 I com.byteflow.learnffmpeg: ........
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栈内存溢出
//stack-buffer-overflow
static void StackBufferOverflow() {
int arr[1024];
arr[0] = 11;
arr[1024] = 12;
LOGCATE("StackBufferOverflow arr[0]=%d, arr[1024]",arr[0], arr[1024]);
}
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ASan 检测结果(crash log)中出现关键字 stack-buffer-overflow :
05-13 19:54:30.371 5002 5002 I com.byteflow.learnffmpeg: =================================================================
05-13 19:54:30.371 5002 5002 I com.byteflow.learnffmpeg: ==5002==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x007ff02278c0 at pc 0x0074f8bd6640 bp 0x007ff0226890 sp 0x007ff0226888
05-13 19:54:30.372 5002 5002 I com.byteflow.learnffmpeg: WRITE of size 4 at 0x007ff02278c0 thread T0
05-13 19:54:30.389 5002 5002 I com.byteflow.learnffmpeg: #0 0x74f8bd663c (/data/app/com.byteflow.learnffmpeg-aaLGh4G_-2c2WC7sX0ibag==/lib/arm64/liblearn-ffmpeg.so+0x14663c)
05-13 19:54:30.389 5002 5002 I com.byteflow.learnffmpeg: #1 0x74f8bd4a20 (/data/app/com.byteflow.learnffmpeg-aaLGh4G_-2c2WC7sX0ibag==/lib/arm64/liblearn-ffmpeg.so+0x144a20)
05-13 19:54:30.389 5002 5002 I com.byteflow.learnffmpeg: #2 0x74f8bd497c (/data/app/com.byteflow.learnffmpeg-aaLGh4G_-2c2WC7sX0ibag==/lib/arm64/liblearn-ffmpeg.so+0x14497c)
......
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使用已释放的指针
//heap-use-after-free
static void UseAfterFree() {
int *arr = new int[1024];
arr[0] = 11;
delete [] arr;
LOGCATE("UseAfterFree arr[0]=%d, arr[1024]",arr[0], arr[1024]);
}
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ASan 检测结果(crash log)中出现关键字 heap-use-after-free :
05-13 19:56:44.430 5235 5235 I com.byteflow.learnffmpeg: =================================================================
05-13 19:56:44.430 5235 5235 I com.byteflow.learnffmpeg: ==5235==ERROR: AddressSanitizer: heap-use-after-free on address 0x004f0d5a7100 at pc 0x0074f7ac945c bp 0x007ff02279d0 sp 0x007ff02279c8
05-13 19:56:44.430 5235 5235 I com.byteflow.learnffmpeg: READ of size 4 at 0x004f0d5a7100 thread T0
05-13 19:56:44.447 5235 5235 I com.byteflow.learnffmpeg: #0 0x74f7ac9458 (/data/app/com.byteflow.learnffmpeg-w2WNuKKPLEj7i4_8_Oj3Sw==/lib/arm64/liblearn-ffmpeg.so+0x146458)
05-13 19:56:44.448 5235 5235 I com.byteflow.learnffmpeg: #1 0x74f7ac7920 (/data/app/com.byteflow.learnffmpeg-w2WNuKKPLEj7i4_8_Oj3Sw==/lib/arm64/liblearn-ffmpeg.so+0x144920)
05-13 19:56:44.448 5235 5235 I com.byteflow.learnffmpeg: #2 0x74f7ac787c (/data/app/com.byteflow.learnffmpeg-w2WNuKKPLEj7i4_8_Oj3Sw==/lib/arm64/liblearn-ffmpeg.so+0x14487c)
......
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局部变量的作用域之外使用其指针
//stack-use-after-scope
static int *p;
static void UseAfterScope()
{
{
int a = 0;
p = &a;
}
*p = 1111;
LOGCATE("UseAfterScope *p=%d",*p);
}
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ASan 检测结果(crash log)中出现关键字 stack-use-after-scope :
https://developer.android.com/ndk/guides/asan#cmake05-13 20:01:19.447 5907 5907 I com.byteflow.learnffmpeg: =================================================================
05-13 20:01:19.447 5907 5907 I com.byteflow.learnffmpeg: ==5907==ERROR: AddressSanitizer: stack-use-after-scope on address 0x007ff02279a0 at pc 0x0074f8236438 bp 0x007ff0227970 sp 0x007ff0227968
05-13 20:01:19.448 5907 5907 I com.byteflow.learnffmpeg: WRITE of size 4 at 0x007ff02279a0 thread T0
05-13 20:01:19.462 23334 23334 D Launcher_UnlockAnimationStateMachine: mResetIdleStateRunnable
05-13 20:01:19.464 5907 5907 I com.byteflow.learnffmpeg: #0 0x74f8236434 (/data/app/com.byteflow.learnffmpeg-jx_Xi14rwGHag_VZ9KWXYA==/lib/arm64/liblearn-ffmpeg.so+0x146434)
05-13 20:01:19.465 5907 5907 I com.byteflow.learnffmpeg: #1 0x74f8234860 (/data/app/com.byteflow.learnffmpeg-jx_Xi14rwGHag_VZ9KWXYA==/lib/arm64/liblearn-ffmpeg.so+0x144860)
05-13 20:01:19.465 5907 5907 I com.byteflow.learnffmpeg: #2 0x74f82347bc (/data/app/com.byteflow.learnffmpeg-jx_Xi14rwGHag_VZ9KWXYA==/lib/arm64/liblearn-ffmpeg.so+0x1447bc)
05-13 20:01:19.465 5907 5907 I com.byteflow.learnffmpeg: #3 0x74fdabe0a0 (/data/app/com.byteflow.learnffmpeg-jx_Xi14rwGHag_VZ9KWXYA==/oat/arm64/base.odex+0xb0a0)
.....
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重复释放指针
//double-free
static void DoubleFree() {
int *arr = new int[1024];
arr[0] = 11;
delete [] arr;
delete [] arr;
LOGCATE("UseAfterFree arr[0]=%d",arr[0]);
}
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ASan 检测结果(crash log)中出现关键字 double-free :
05-13 20:02:16.474 6102 6102 I com.byteflow.learnffmpeg: =================================================================
05-13 20:02:16.475 6102 6102 I com.byteflow.learnffmpeg: ==6102==ERROR: AddressSanitizer: attempting double-free on 0x004f0d5a7100 in thread T0:
05-13 20:02:16.492 6102 6102 I com.byteflow.learnffmpeg: #0 0x74f9f2b7b0 (/data/app/com.byteflow.learnffmpeg-kjj44NZxl-eyA06gf3E2MA==/lib/arm64/libclang_rt.asan-aarch64-android.so+0xd57b0)
05-13 20:02:16.492 6102 6102 I com.byteflow.learnffmpeg: #1 0x74f88cd210 (/data/app/com.byteflow.learnffmpeg-kjj44NZxl-eyA06gf3E2MA==/lib/arm64/liblearn-ffmpeg.so+0x146210)
05-13 20:02:16.492 6102 6102 I com.byteflow.learnffmpeg: #2 0x74f88cb720 (/data/app/com.byteflow.learnffmpeg-kjj44NZxl-eyA06gf3E2MA==/lib/arm64/liblearn-ffmpeg.so+0x144720)
05-13 20:02:16.492 6102 6102 I com.byteflow.learnffmpeg: #3 0x74f88cb67c (/data/app/com.byteflow.learnffmpeg-kjj44NZxl-eyA06gf3E2MA==/lib/arm64/liblearn-ffmpeg.so+0x14467c)
......
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ASan 基本上可以覆盖到常见的内存错误问题,还有其他 Case 就不一一展示了,
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