关于设置uid位和seteuid的问题

[skineffect]

程序如下：

#include <stdio.h>
#include <sys/types.h>
#include <pwd.h>

int main() {
	struct passwd *pwd, *rpwd;
	rpwd = getpwuid(getuid());
	pwd = getpwuid(geteuid());
	printf("real UID:[%s]\n", rpwd->pw_name);
	printf("effective UID:[%s]\n", pwd->pw_name);

	system("touch /tmp/foo.txt;ls -l /tmp/foo.txt; rm -rf /tmp/foo.txt;");
	printf("\nset EUID to 'skin'..\n");
	
	pwd = getpwnam("skin");
	seteuid(pwd->pw_uid);

	rpwd = getpwuid(getuid());
	printf("real UID:[%s]\n", rpwd->pw_name);
	pwd = getpwuid(geteuid());
	printf("effective UID:[%s]\n", pwd->pw_name);
	system("touch /tmp/foo.txt;ls -l /tmp/foo.txt; rm -rf /tmp/foo.txt;");
	
	printf("\nset EUID to 'root'\n");
	seteuid(0);

	rpwd = getpwuid(getuid());
	printf("real UID:[%s]\n", rpwd->pw_name);
	pwd = getpwuid(geteuid());
	printf("effective UID:[%s]\n", pwd->pw_name);
	system("touch /tmp/foo.txt;ls -l /tmp/foo.txt; rm -rf /tmp/foo.txt;");

	return 0;
}

编译如下：
skin@ubuntu:~$ sudo gcc -D_GNU_SOURCE -o euid euid.c; sudo chmod +s euid; ls -l euid
-rwsr-sr-x 1 root root 7410 2011-03-30 11:06 euid
运行结果如下：
skin@ubuntu:~/$ ./euid
real UID:[root] //问题1
effective UID:[root]
-rw-r--r-- 1 root root 0 2011-03-30 11:06 /tmp/foo.txt

set EUID to 'skin'..
real UID:[skin] //问题2
effective UID:[skin]
-rw-r--r-- 1 skin root 0 2011-03-30 11:06 /tmp/foo.txt //问题3

set EUID to 'root'
real UID:[skin]
effective UID:[root]
-rw-r--r-- 1 root root 0 2011-03-30 11:06 /tmp/foo.txt

有几个地方不明白：
1.euid文件设置了设置uid位, 用skin用户运行时, 应该euid变成了root, 但为什么real uid也会变成root呢？设置uid位不是 只改变euid吗？
2.程序中调用seteuid(skin的uid）后, 因为是root调的, 所以连real uid都变成了skin吗？不是说只有root调用setuid后才会三个uid都改变吗，而seteuid只改变euid吗？
3.就算调用seteuid(skin的uid）后real uid和euid都变成了skin, 但是system后为什么euid变成了root呢？不也应该是skin吗？
实在是搞不明白，请大家指教，谢谢啦～

[justkk]

1、2 两个问题，程序有误，你的假设不正确

rpwd = getpwuid(getuid());
pwd = getpwuid(geteuid());
printf("real UID:[%s]\n", rpwd->pw_name);
printf("effective UID:[%s]\n", pwd->pw_name);

因为getpwuid使用静态变量保存数据，后面的调用会覆盖之前的数据，这段的显示结果是错误的，误导你了
改为：
rpwd = getpwuid(getuid());
printf("real UID:[%s]\n", rpwd->pw_name);
pwd = getpwuid(geteuid());
printf("effective UID:[%s]\n", pwd->pw_name);

3、哪儿看到是euid变成root了？？那儿的root是组，不是用户

[skineffect]

谢谢你，确实是因为getpwuid使用静态变量保存数据。还有最后将group id看成了euid,惭愧啊～