c - 第二个getpwuid调用似乎覆盖了旧值

这是一个小的C程序，当该文件设置了setuid标志时，该程序会打印(当然应该打印)进程的真实和有效ID。在此程序中，当我第二次调用getpwuid(L.No 38)时，它趋向于覆盖在L.No 24中获得的realUserName变量的值。我无法解释这种现象。这是预期的行为，为什么？我正在Linux机器(RHEL 2.6.18-371.1.2.el5)中尝试此操作。

  1 /* Filename: test.c
  2  * Notes:
  3  * 1] ./test owned by user cadmn (userID: 3585)
  4  * 2] ./test run by user pmn (4471)
  5  * 3] ./test has the setuid bit switched-on.
  6  */
  7 #include <stdio.h>
  8 #include <pwd.h>
  9 #include <sys/types.h>
 10 #include <unistd.h>
 11 int main()
 12 {
 13
 14     uid_t realId, effectiveId;
 15     struct passwd *realUser, *effUser;
 16
 17     realId = getuid(); // realId = 4471
 18     effectiveId = geteuid(); //effectiveId = 3585
 19
 20     printf("Real ID is %i and Effective ID is %i\n", (int)realId, (int)effectiveId);
 21     //prints 4472 and 3585, respectively
 22
 23     realUser = getpwuid(realId);
 24     char *realUserName = realUser->pw_name; //realUserName = pmn
 25
 26     printf("Real ID (name) at this point is %s\n", realUserName);
 27     // prints pmn.
 28
 29     /*
 30      *********************************************************
 31      *                                                       *
 32      *    everything works as expected up to this point      *
 33      *                                                       *
 34      *********************************************************
 35      */
 36
 37     // The value obtained from this call is not used anywhere in this program
 38     effUser = getpwuid(effectiveId);
 39     printf("\nCalled getpwuid with the effectiveId\n\n");
 40
 41     printf("Real ID is %i and Effective ID is %i\n", (int)realId, (int)effectiveId);
 42     //prints 4472 and 3585, respectively
 43
 44     printf("Real ID (name) at this point is %s.\n", realUserName);
 45     // Expect to still see 'pmn' printed; though see 'cadmn' as the output!
 46     // Why does this happen?
 47
 48     return 0;
 49 }
 50

输出:

pmn@rhel /tmp/temp > id pmn
uid=4471(pmn) gid=1000(nusers) groups=1000(nusers)
pmn@rhel /tmp/temp >

pmn@rhel /tmp/temp > id cadmn
uid=3585(cadmn) gid=401(cusers) groups=401(cusers)
pmn@rhel /tmp/temp >

pmn@rhel /tmp/temp > ls -l ./test
-r-sr-xr-x 1 cadmn cusers 9377 Dec 24 19:48 ./test
pmn@rhel /tmp/temp >

pmn@rhel /tmp/temp > ./test
Real ID is 4471 and Effective ID is 3585
Real ID (name) at this point is pmn

Called getpwuid with the effectiveId

Real ID is 4471 and Effective ID is 3585
Real ID (name) at this point is cadmn.
pmn@rhel /tmp/temp >

最佳答案

您观察到的行为是预期的行为。
getpwuid()返回值所引用的结构是静态地在后者内部定义的，因此，每次调用getpwuid()时都希望将其填充(并覆盖)。

这条线

char * realUserName = realUser->pw_name;

只是存储对该静态内部结构所保存的值的引用。如果静态内部结构被下一次对getpwuid()的调用覆盖，则此值也会被覆盖。

为了解决这个问题，有两种可能性:

使用可重入的getpwuid()版本，即 getpwuid_r() 。为了能够使用它，添加

#define _POSIX_SOURCE

在程序源代码中的第一个#include语句之前。

创建所需成员的副本，在这种情况下为pw_name。可以通过执行以下操作来实现:

char * realUserName = strdup(realUser->pw_name);

知道realUserName现在指向动态分配的内存，如果不再需要该内存，则需要由程序本身对其进行free()编码。为此打电话

free(realUserName);

关于c - 第二个getpwuid调用似乎覆盖了旧值，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/20757940/