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C/C++内存泄露分析过程

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本文字数： 2.4k 阅读时长 ≈ 2 分钟

前言

最近在工作中遇到了一些内存泄露问题，虽然泄露速度很慢，但是对于小型嵌入式设备而言资源本身就很紧张而且过72h老化测试过不了。于是寻找一些内存泄露检测工具辅助查找内存泄露问题，此处只使用了leaktracer这个开源库来帮助查找内存泄露问题。

leaktracer概述

LeakTracer 是在检查 C/C++ 程序内存泄漏时编写的一个小工具。 要使用 LeakTracer，请使用提供的 LeakCheck 脚本运行您的程序。它使用 LD_PRELOAD 特性在你的函数之上“覆盖”一些函数（不需要重新编译）。如果您的平台不支持 LD_PRELOAD，您可以将 LeakTracer.o 对象文件添加到 Makefile 中的对象并运行您的应用程序。 LeakTracer利用gdb去输出发生内存泄露所发生的位置，它是通过override operator new, operator delete, operator malloc, operator free来实现检测。

用法

加载 leaktracer 库的 3 种方法：

  • 将您的程序链接到 libleaktracer.a
  • 将您的程序链接到 libleaktracer.so。您需要将 -lleaktracer 选项作为链接命令的第一个选项。
  • 使用 LD_PRELOAD 环境变量以确保它在任何其他库之前加载。

将leaktracer添加到程序中：

  • 添加头文件MemoryTrace.hpp
  • 添加leaktracer::MemoryTrace::GetInstance().startMonitoringAllThreads()函数，作为起始检测位置
  • 添加leaktracer::MemoryTrace::GetInstance().writeLeaksToFile("/mnt/extsd/leaks.out")函数，作为结束位置，生成检测报告
  • 编译选项中添加-funwind-tables生成backtrace信息表
  • 链接选项中添加-Wl,-Bstatic -lleaktracer静态链接libleaktracer，最好加上-g3，处理报告时可以显示对应的代码
  • 编译后保存生成的bin文件，备用

生成报告及处理：

生成的报告中一行信息如下：

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# LeakTracer report diff_utc_mono=1588164717.365890
leak, time=1434.190531, stack=0x7f05d63801d5 0x7f05d638e594 0x7f05d638d8f8 0x7f05d638c28d 0x7f05d6382738, size=1024, data=***********************************************...
leak, time=1444.194638, stack=0x7f05d70f9c58 0x4027f5 0x7f05d6333830 0x402369, size=100, data=."%...............................................
leak, time=1434.190625, stack=0x4024d7 0x7f05d6333830 0x402369, size=100, data=..................................................
leak, time=1444.194222, stack=0x7f05d70f9a5c 0x4027f0 0x7f05d6333830 0x402369, size=100, data=. %...............................................
leak, time=1434.190651, stack=0x402500 0x7f05d6333830 0x402369, size=100,
  • leak：代表内存泄露
  • time：代表调用分配内存函数的时间（开机到当前的时间）
  • stack：调用栈
  • size：泄露的内存大小
  • data：申请时内存中的数据

明显可以看到调用栈，callstack全是地址，我们以使用helpers文件夹中的leak-analyze-addr2line，leak-analyze-gdb二个工具进行解析。或者借助gdb、objdump或map文件等手段得到该泄露源的真正文件/行号或函数范围。

报告处理

  • 使用addr2line处理leaks.out，生成可读的调用栈
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leak-analyze-addr2line {bin} leaks.out > leaks.addr2line.txt

生成的报告示例如下：

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$ ./helpers/leak-analyze-addr2line memleak_test_so leaks.out 
Processing "leaks.out" log for "memleak_test_so"
Matching addresses to "memleak_test_so"
found 49 leak(s)
100 bytes lost in 1 blocks (one of them allocated at 1444.194222), from following call stack:
  ??:0
  /home/cll/99_temp/memory_leak/leaktracer/memleak_way1/memleak_test.cpp:110
  ??:0
  ??:?
400 bytes lost in 1 blocks (one of them allocated at 1444.194550), from following call stack:
  ??:0
  /home/cll/99_temp/memory_leak/leaktracer/memleak_way1/memleak_test.cpp:110
  ??:0
  ??:?
328 bytes lost in 1 blocks (one of them allocated at 1434.190960), from following call stack:
  /home/cll/99_temp/memory_leak/leaktracer/memleak_way1/memleak_test.cpp:83
  ??:0
  ??:?

解析：49个重复调用，泄露100个字节的内存，其中一个泄漏点在leaks.out中的时间是 06219.879013，调用栈从下往上看。

  • 使用gdb处理leaks.out，生成可读的调用栈及源码对应
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leak-analyze-gdb {bin} leaks.out > leaks.gdb.txt

生成的报告示例如下：

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$ ./helpers/leak-analyze-gdb memleak_test_so  leaks.out 
found 49 leak(s)
(gdb) Reading symbols from memleak_test_so...done.
16 bytes lost in 1 blocks (one of them allocated at 1434.190803), from following call stack:
main + 364 in section .text
0x4025e4 is in main() (memleak_test.cpp:80).
80		new_delete_test *p_new_class_no_free =  new new_delete_test;
No symbol matches 0x7f05d6333830.
_start + 41 in section .text
 
1 bytes lost in 1 blocks (one of them allocated at 1444.194798), from following call stack:
No symbol matches 0x7f05d70f9cee.
main + 893 in section .text
0x4027f5 is in main() (memleak_test.cpp:111).
111		leaktracer::MemoryTrace::GetInstance().stopAllMonitoring();
No symbol matches 0x7f05d6333830.

无论是leak-analyze-addr2line，leak-analyze-gdb二个工具进行解析。或者借助gdb、objdump或map文件等手段得到该泄露源的真正文件/行号或函数范围。其中都会碰上各种库引用等问题导致？？？。都会很麻烦，然而leaktracer提供了相关源码，我们可以对leaktracer进行改造成符合我们的定制化需求。

原理

leaktracer主要的设计思路为：

  • 实现一组内存的分配/释放函数，这组函数的函数原型与系统的那一组完全一样，让被trace的library对于内存的分配/释放函数的调用都链接到自己实现的这一组函数中以override掉系统的那组内存/分配释放函数；
  • 自己实现的这组函数中的内存分配函数记录分配相关的信息，包括分配的内存的大小，callstack等，并调用系统本来的内存分配函数去分配内存；
  • 自己实现的这组函数中的内存释放函数则销毁内存分配的相关记录，并使用系统的内存释放函数真正的释放内存；
  • 在trace结束时，遍历所有保存的内存分配记录的信息，并把这些信息保存进文件以供进一步的分析;

override系统内存分配/释放函数

LeakTracer实现的用于override系统内存分配/释放函数的那组函数在AllocationHandlers.cpp中定义：

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c++
void* operator new(size_t size)
void* operator new[] (size_t size)
void operator delete (void *p)
void operator delete[]
c
void *malloc(size_t size)
void free(void* ptr)
void* realloc(void *ptr, size_t size)
void* calloc(size_t nmemb, size_t size)
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//
// LeakTracer
// Contribution to original project by Erwin S. Andreasen
// site: http://www.andreasen.org/LeakTracer/
//
// Added by Michael Gopshtein, 2006
// mgopshtein@gmail.com
//
// Any comments/suggestions are welcome
// 
 
#include "MemoryTrace.hpp"
#include "LeakTracer_l.hpp"
 
void* (*lt_malloc)(size_t size);
void  (*lt_free)(void* ptr);
void* (*lt_realloc)(void *ptr, size_t size);
void* (*lt_calloc)(size_t nmemb, size_t size);
 
void* operator new(size_t size) {
  void *p;
  leaktracer::MemoryTrace::Setup();
 
  p = LT_MALLOC(size);
  leaktracer::MemoryTrace::GetInstance().registerAllocation(p, size, false);
 
  return p;
}
 
 
void* operator new[] (size_t size) {
  void *p;
  leaktracer::MemoryTrace::Setup();
 
  p = LT_MALLOC(size);
  leaktracer::MemoryTrace::GetInstance().registerAllocation(p, size, true);
 
  return p;
}
 
 
void operator delete (void *p) {
  leaktracer::MemoryTrace::Setup();
 
  leaktracer::MemoryTrace::GetInstance().registerRelease(p, false);
  LT_FREE(p);
}
 
 
void operator delete[] (void *p) {
  leaktracer::MemoryTrace::Setup();
 
  leaktracer::MemoryTrace::GetInstance().registerRelease(p, true);
  LT_FREE(p);
}
 
/** -- libc memory operators -- **/
 
/* malloc
 * in some malloc implementation, there is a recursive call to malloc
 * (for instance, in uClibc 0.9.29 malloc-standard )
 * we use a InternalMonitoringDisablerThreadUp that use a tls variable to prevent several registration
 * during the same malloc
 */
void *malloc(size_t size)
{
  void *p;
  leaktracer::MemoryTrace::Setup();
 
  leaktracer::MemoryTrace::GetInstance().InternalMonitoringDisablerThreadUp();
  p = LT_MALLOC(size);
  leaktracer::MemoryTrace::GetInstance().InternalMonitoringDisablerThreadDown();
  leaktracer::MemoryTrace::GetInstance().registerAllocation(p, size, false);
 
  return p;
}
 
void free(void* ptr)
{
  leaktracer::MemoryTrace::Setup();
 
  leaktracer::MemoryTrace::GetInstance().registerRelease(ptr, false);
  LT_FREE(ptr);
}
 
void* realloc(void *ptr, size_t size)
{
  void *p;
  leaktracer::MemoryTrace::Setup();
 
  leaktracer::MemoryTrace::GetInstance().InternalMonitoringDisablerThreadUp();
 
  p = LT_REALLOC(ptr, size);
 
  leaktracer::MemoryTrace::GetInstance().InternalMonitoringDisablerThreadDown();
 
  if (p != ptr)
  {
    if (ptr)
      leaktracer::MemoryTrace::GetInstance().registerRelease(ptr, false);
    leaktracer::MemoryTrace::GetInstance().registerAllocation(p, size, false);
  }
  else
  {
    leaktracer::MemoryTrace::GetInstance().registerReallocation(p, size, false);
  }
 
  return p;
}
 
void* calloc(size_t nmemb, size_t size)
{
  void *p;
  leaktracer::MemoryTrace::Setup();
 
  leaktracer::MemoryTrace::GetInstance().InternalMonitoringDisablerThreadUp();
  p = LT_CALLOC(nmemb, size);
  leaktracer::MemoryTrace::GetInstance().InternalMonitoringDisablerThreadDown();
  leaktracer::MemoryTrace::GetInstance().registerAllocation(p, nmemb*size, false);
 
  return p;
}

内存分配函数记录分配相关的信息

registerAllocation记录每一次内存分配的相关信息:

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/** registers new memory allocation, should be called by the
	 *  function intercepting "new" calls */
  inline void registerAllocation(void *p, size_t size, bool is_array);
 
  /** registers memory reallocation, should be called by the
	 *  function intercepting realloc calls */
  inline void registerReallocation(void *p, size_t size, bool is_array);

registerReallocation记录每一次内存分配的相关信息:

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/** registers memory reallocation, should be called by the
	 *  function intercepting realloc calls */
  inline void registerReallocation(void *p, size_t size, bool is_array);
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// adds all relevant info regarding current allocation to map
inline void MemoryTrace::registerAllocation(void *p, size_t size, bool is_array)
{
  allocation_info_t *info = NULL;
  if (!AllMonitoringIsDisabled() && (__monitoringAllThreads || getThreadOptions().monitoringAllocations) && p != NULL) {
    MutexLock lock(__allocations_mutex);
    info = __allocations.insert(p);
    if (info != NULL) {
      info->size = size;
      info->isArray = is_array;
      storeTimestamp(info->timestamp);
    }
  }
 	// we store the stack without locking __allocations_mutex
  // it should be safe enough
  // prevent a deadlock between backtrave function who are now using advanced dl_iterate_phdr function
 	// and dl_* function which uses malloc functions
  if (info != NULL) {
    storeAllocationStack(info->allocStack);
  }
 
  if (p == NULL) {
    InternalMonitoringDisablerThreadUp();
    // WARNING
    InternalMonitoringDisablerThreadDown();
  }
}
 
 
// adds all relevant info regarding current allocation to map
inline void MemoryTrace::registerReallocation(void *p, size_t size, bool is_array)
{
  if (!AllMonitoringIsDisabled() && (__monitoringAllThreads || getThreadOptions().monitoringAllocations) && p != NULL) {
    MutexLock lock(__allocations_mutex);
    allocation_info_t *info = __allocations.find(p);
    if (info != NULL) {
      info->size = size;
      info->isArray = is_array;
      storeAllocationStack(info->allocStack);
      storeTimestamp(info->timestamp);
    }
  }
 
  if (p == NULL) {
    InternalMonitoringDisablerThreadUp();
    // WARNING
    InternalMonitoringDisablerThreadDown();
  }
}

内存释放函数则销毁内存分配的相关记录

registerReallocation记录每一次内存释放的相关信息

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/** registers new memory allocation, should be called by the
	 *  function intercepting "new" calls */
  inline void registerAllocation(void *p, size_t size, bool is_array);
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// removes allocation's info from the map
inline void MemoryTrace::registerRelease(void *p, bool is_array)
{
  if (!AllMonitoringIsDisabled() && __monitoringReleases && p != NULL) {
    MutexLock lock(__allocations_mutex);
    allocation_info_t *info = __allocations.find(p);
    if (info != NULL) {
      if (info->isArray != is_array) {
        InternalMonitoringDisablerThreadUp();
        // WARNING
        InternalMonitoringDisablerThreadDown();
      }
      __allocations.release(p);
    }
  }
}

遍历所有保存的内存分配记录的信息，并把这些信息保存

writeLeaksToFile保存内存分配记录信息到文件:

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/** writes report with all memory leaks */
  void writeLeaksToFile(const char* reportFileName);
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// writes all memory leaks to given stream
void MemoryTrace::writeLeaksToFile(const char* reportFilename)
{
  MutexLock lock(__allocations_mutex);
  InternalMonitoringDisablerThreadUp();
 
  std::ofstream oleaks;
  if (!isFolderExist(reportFilename)) {
    createDirectory(reportFilename);
  }
  if (__allocations.empty()) {
    return; //no memory leak, not need to create leak file
  }
 
  oleaks.open(reportFilename, std::ios_base::out);
  if (oleaks.is_open())
  {
    writeLeaksPrivate(oleaks);
    oleaks.close();
  }
  else
  {
    std::cerr << "Failed to write to \"" << reportFilename << "\"\n";
  }
  InternalMonitoringDisablerThreadDown();
}

遍历自定义MapMemoryInfo 中所有元素

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// writes all memory leaks to given stream
void MemoryTrace::writeLeaksPrivate(std::ostream &out)
{
  struct timespec mono, utc, diff;
  allocation_info_t *info;
  void *p;
  double d;
  const int precision = 6;
  int maxsecwidth;
 
  clock_gettime(CLOCK_REALTIME, &utc);
  clock_gettime(CLOCK_MONOTONIC, &mono);
 
  if (utc.tv_nsec > mono.tv_nsec) {
    diff.tv_nsec = utc.tv_nsec - mono.tv_nsec;
    diff.tv_sec = utc.tv_sec - mono.tv_sec;
  } else {
    diff.tv_nsec = 1000000000 - (mono.tv_nsec - utc.tv_nsec);
    diff.tv_sec = utc.tv_sec - mono.tv_sec -1;
  }
 
  maxsecwidth = 0;
  while(mono.tv_sec > 0) {
    mono.tv_sec = mono.tv_sec/10;
    maxsecwidth++;
  }
  if (maxsecwidth == 0) maxsecwidth=1;
 
  out << "# LeakTracer report";
  d = diff.tv_sec + (((double)diff.tv_nsec)/1000000000);
  out << " diff_utc_mono=" << std::fixed << std::left << std::setprecision(precision) << d ;
  out << "\n";
 
  __allocations.beginIteration();
  while (__allocations.getNextPair(&info, &p)) {
    d = info->timestamp.tv_sec + (((double)info->timestamp.tv_nsec)/1000000000);
    out << "leak, ";
    out << "time="  << std::fixed << std::right << std::setprecision(precision) << std::setfill('0') << std::setw(maxsecwidth+1+precision) << d << ", "; // setw(16) ?
    out << "stack=";
    #ifdef BACKTRACE_SYMBOLS_USED
    unsigned int i_depth = 0;
    for (i_depth = 0; i_depth < ALLOCATION_STACK_DEPTH; i_depth++) {
      if (info->allocStack[i_depth] == NULL) break;
 
      if (i_depth > 0) out << ' ';
      out << info->allocStack[i_depth];
    }
    out << '\n';
    char **trace_symbols = (char **)backtrace_symbols (info->allocStack, i_depth);
    if (NULL != trace_symbols) {
      size_t name_size = 64;
      char *name = (char*)malloc(name_size);
      for (unsigned int j = 0; j < i_depth; j++) {
        char *begin_name = 0;
        char *begin_offset = 0;
        char *end_offset = 0;
        for (char *p = trace_symbols[j]; *p; ++p) {
          if (*p == '(') {
            begin_name = p;
          } else if (*p == '+' && begin_name) {
            begin_offset = p;
          } else if (*p == ')' && begin_offset) {
            end_offset = p;
            break;
          }
        }
        if (begin_name && begin_offset && end_offset ) {
          *begin_name++ = '\0';
          *begin_offset++ = '\0';
          *end_offset = '\0';
          int status = -4;
          char *ret = abi::__cxa_demangle(begin_name, name, &name_size, &status);
          if (0 == status) {
            name = ret;
            out << trace_symbols[j] << ":" << name << "+" << begin_offset;
          } else {
            out << trace_symbols[j] << ":" << begin_name << "()+" << begin_offset;
          }
        } else {
          out << trace_symbols[j];
        }
        out << '\n';
      }
      free(trace_symbols);
    }
    #else
    for (unsigned int i = 0; i < ALLOCATION_STACK_DEPTH; i++) {
      if (info->allocStack[i] == NULL) break;
 
      if (i > 0) out << ' ';
      out << info->allocStack[i];
    }
    out << ", ";
    #endif
    out << "size=" << info->size << ", ";
 
    out << "data=";
    const char *data = reinterpret_cast<const char *>(p);
    for (unsigned int i = 0; i < PRINTED_DATA_BUFFER_SIZE && i < info->size; i++)
      out << (isprint(data[i]) ? data[i] : '.');
    out << '\n';
  }
}

整体来看LeakTracer的设计与实现都并不复杂，因而能够trace的memory issue也就有限。比如，LeakTracer就无法trace多次释放等问题。但我们可以通过源码编写更强大的内存相关的trace工具。

注意：对于应用，无论是静态链接或动态链接的库，若想能够生成callstack，都需要添加-funwind-tables重新编译一次 leak-analyze-addr2line和leak-analyze-gdb在libleaktracer.tar.gz中，需要将其中的addr2line和gdb改成交叉编译工具链中的arm-xxx-addr2line和arm-xxx-gdb

参考文献

https://blog.csdn.net/xiaoting451292510/article/details/105850409