Linux UID

在 Linux 系統中使用者透過 UID 區分身份，其中又分為

EUID (Effective UID) 當建立、存取檔案時發揮作用
RUID (Real UID) 實際行程 (Process) 的真正擁有者、影響傳送訊號 (Signal) 的權限
SUID (Saved UID) 當降權時、將原本的權限暫存的。非 root 下 EUID 只能設定更改成 RUID、SUID 兩種

當系統登入時會檢查 /etc/passwd 下的內容、其中每行又透過冒號 (:) 區隔分為 7 欄，分別為

username
password 其中 x 表示密碼儲存在 /etc/shadow 下、* 表示不允許登入
UID
GID
Description
home directory
login shell

C

這是一個 C 版本的 setuid 簡單程式：

/* Simple get/set UID C program. The program set the set-uid-bit with root owner, the progam
 * first has the root uid and can downgrade the privilege by the seteuid.
 *
 * > gcc test.c
 * > chown root a.out
 * > chmod +s a.out
 * > ./a.out
 */
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

int main() {
	uid_t uid;
	gid_t gid;

	fprintf(stdout, "uid: %d, gid: %d\n", getuid(), getgid());
	fprintf(stdout, "euid: %d, egid: %d\n", geteuid(), getegid());

	fprintf(stdout, "seteuid to 0: %d %m\n", seteuid(getuid()));
	fprintf(stdout, "uid: %d, gid: %d\n", getuid(), getgid());
	fprintf(stdout, "euid: %d, egid: %d\n", geteuid(), getegid());

	fprintf(stdout, "seteuid to 0: %d %m\n", seteuid(0));
	fprintf(stdout, "uid: %d, gid: %d\n", getuid(), getgid());
	fprintf(stdout, "euid: %d, egid: %d\n", geteuid(), getegid());
	return 0;
}