linux下system函数的简单分析

jidonghui 2017-05-26

简单分析了linux下system函数的相关内容,具体内容如下

int
__libc_system (const char *line)
{
 if (line == NULL)
  /* Check that we have a command processor available. It might
    not be available after a chroot(), for example. */
  return do_system ("exit 0") == 0;

 return do_system (line);
}
weak_alias (__libc_system, system)

代码位于glibc/sysdeps/posix/system.c,这里system是__libc_system的弱别名,而__libc_system是do_system的前端函数,进行了参数的检查,接下来看do_system函数。

static int
do_system (const char *line)
{
 int status, save;
 pid_t pid;
 struct sigaction sa;
#ifndef _LIBC_REENTRANT
 struct sigaction intr, quit;
#endif
 sigset_t omask;

 sa.sa_handler = SIG_IGN;
 sa.sa_flags = 0;
 __sigemptyset (&sa.sa_mask);

 DO_LOCK ();
 if (ADD_REF () == 0)
  {
   if (__sigaction (SIGINT, &sa, &intr) < 0)
  {
   (void) SUB_REF ();
   goto out;
  }
   if (__sigaction (SIGQUIT, &sa, &quit) < 0)
  {
   save = errno;
   (void) SUB_REF ();
   goto out_restore_sigint;
  }
  }
 DO_UNLOCK ();

 /* We reuse the bitmap in the 'sa' structure. */
 __sigaddset (&sa.sa_mask, SIGCHLD);
 save = errno;
 if (__sigprocmask (SIG_BLOCK, &sa.sa_mask, &omask) < 0)
  {
#ifndef _LIBC
   if (errno == ENOSYS)
  __set_errno (save);
   else
#endif
  {
   DO_LOCK ();
   if (SUB_REF () == 0)
    {
     save = errno;
     (void) __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL);
    out_restore_sigint:
     (void) __sigaction (SIGINT, &intr, (struct sigaction *) NULL);
     __set_errno (save);
    }
  out:
   DO_UNLOCK ();
   return -1;
  }
  }

#ifdef CLEANUP_HANDLER
 CLEANUP_HANDLER;
#endif

#ifdef FORK
 pid = FORK ();
#else
 pid = __fork ();
#endif
 if (pid == (pid_t) 0)
  {
   /* Child side. */
   const char *new_argv[4];
   new_argv[0] = SHELL_NAME;
   new_argv[1] = "-c";
   new_argv[2] = line;
   new_argv[3] = NULL;

   /* Restore the signals. */
   (void) __sigaction (SIGINT, &intr, (struct sigaction *) NULL);
   (void) __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL);
   (void) __sigprocmask (SIG_SETMASK, &omask, (sigset_t *) NULL);
   INIT_LOCK ();

   /* Exec the shell. */
   (void) __execve (SHELL_PATH, (char *const *) new_argv, __environ);
   _exit (127);
  }
 else if (pid < (pid_t) 0)
  /* The fork failed. */
  status = -1;
 else
  /* Parent side. */
  {
   /* Note the system() is a cancellation point. But since we call
   waitpid() which itself is a cancellation point we do not
   have to do anything here. */
   if (TEMP_FAILURE_RETRY (__waitpid (pid, &status, 0)) != pid)
  status = -1;
  }

#ifdef CLEANUP_HANDLER
 CLEANUP_RESET;
#endif

 save = errno;
 DO_LOCK ();
 if ((SUB_REF () == 0
    && (__sigaction (SIGINT, &intr, (struct sigaction *) NULL)
    | __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL)) != 0)
   || __sigprocmask (SIG_SETMASK, &omask, (sigset_t *) NULL) != 0)
  {
#ifndef _LIBC
   /* glibc cannot be used on systems without waitpid. */
   if (errno == ENOSYS)
  __set_errno (save);
   else
#endif
  status = -1;
  }
 DO_UNLOCK ();

 return status;
}

do_system

首先函数设置了一些信号处理程序,来处理SIGINT和SIGQUIT信号,此处我们不过多关心,关键代码段在这里

#ifdef FORK
 pid = FORK ();
#else
 pid = __fork ();
#endif
 if (pid == (pid_t) 0)
  {
   /* Child side. */
   const char *new_argv[4];
   new_argv[0] = SHELL_NAME;
   new_argv[1] = "-c";
   new_argv[2] = line;
   new_argv[3] = NULL;

   /* Restore the signals. */
   (void) __sigaction (SIGINT, &intr, (struct sigaction *) NULL);
   (void) __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL);
   (void) __sigprocmask (SIG_SETMASK, &omask, (sigset_t *) NULL);
   INIT_LOCK ();

   /* Exec the shell. */
   (void) __execve (SHELL_PATH, (char *const *) new_argv, __environ);
   _exit (127);
  }
 else if (pid < (pid_t) 0)
  /* The fork failed. */
  status = -1;
 else
  /* Parent side. */
  {
   /* Note the system() is a cancellation point. But since we call
   waitpid() which itself is a cancellation point we do not
   have to do anything here. */
   if (TEMP_FAILURE_RETRY (__waitpid (pid, &status, 0)) != pid)
  status = -1;
  }

首先通过前端函数调用系统调用fork产生一个子进程,其中fork有两个返回值,对父进程返回子进程的pid,对子进程返回0。所以子进程执行6-24行代码,父进程执行30-35行代码。

子进程的逻辑非常清晰,调用execve执行SHELL_PATH指定的程序,参数通过new_argv传递,环境变量为全局变量__environ。

其中SHELL_PATH和SHELL_NAME定义如下

#define  SHELL_PATH  "/bin/sh"  /* Path of the shell. */
#define  SHELL_NAME  "sh"    /* Name to give it. */

其实就是生成一个子进程调用/bin/sh -c "命令"来执行向system传入的命令。 

下面其实是我研究system函数的原因与重点:

在CTF的pwn题中,通过栈溢出调用system函数有时会失败,听师傅们说是环境变量被覆盖,但是一直都是懵懂,今天深入学习了一下,总算搞明白了。

在这里system函数需要的环境变量储存在全局变量__environ中,那么这个变量的内容是什么呢。

__environ是在glibc/csu/libc-start.c中定义的,我们来看几个关键语句。

# define LIBC_START_MAIN __libc_start_main

__libc_start_main是_start调用的函数,这涉及到程序开始时的一些初始化工作,对这些名词不了解的话可以看一下这篇文章。接下来看LIBC_START_MAIN函数。

STATIC int
LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
     int argc, char **argv,
#ifdef LIBC_START_MAIN_AUXVEC_ARG
     ElfW(auxv_t) *auxvec,
#endif
     __typeof (main) init,
     void (*fini) (void),
     void (*rtld_fini) (void), void *stack_end)
{
 /* Result of the 'main' function. */
 int result;

 __libc_multiple_libcs = &_dl_starting_up && !_dl_starting_up;

#ifndef SHARED
 char **ev = &argv[argc + 1];

 __environ = ev;

 /* Store the lowest stack address. This is done in ld.so if this is
   the code for the DSO. */
 __libc_stack_end = stack_end;

    ......
 /* Nothing fancy, just call the function. */
 result = main (argc, argv, __environ MAIN_AUXVEC_PARAM);
#endif

 exit (result);
}

我们可以看到,在没有define SHARED的情况下,在第19行定义了__environ的值。启动程序调用LIBC_START_MAIN之前,会先将环境变量和argv中的字符串保存起来(其实是保存到栈上),然后依次将环境变量中各项字符串的地址,argv中各项字符串的地址和argc入栈,所以环境变量数组一定位于argv数组的正后方,以一个空地址间隔。所以第17行的&argv[argc + 1]语句就是取环境变量数组在栈上的首地址,保存到ev中,最终保存到__environ中。第203行调用main函数,会将__environ的值入栈,这个被栈溢出覆盖掉没什么问题,只要保证__environ中的地址处不被覆盖即可。

所以,当栈溢出的长度过大,溢出的内容覆盖了__environ中地址中的重要内容时,调用system函数就会失败。具体环境变量距离溢出地址有多远,可以通过在_start中下断查看。

以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持安科网。

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