December 2013
Intermediate to advanced
496 pages
8h 57m
Chinese
Content preview from LINUX系統程式設計 第二版
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第五章
讓不具 root 身份的行程得以來回切換。真實用戶識別碼就是有效用戶識別碼被設定成實
際執行程式的用戶，而暫存用戶識別碼就是在 exec 進行期間，一個 suid 二元檔導致有
效用戶識別碼變更之前的有效用戶識別碼。
變更真實或暫存的用戶或群組識別碼
下面的系統呼叫可分別用於設定用戶與群組識別碼：
#include <sys/types.h>
#include <unistd.h>
int setuid (uid_t uid);
int setgid (gid_t gid);
setuid()
可用於設定當前行程的有效用戶識別碼。如果該行程目前的有效用戶識別碼為
0（root），則也會設定真實及暫存用戶識別碼。root 用戶可以替
uid
提供任何值，因此
可以把這三個用戶識別碼的值設定成
uid
。一個非 root 的用戶只能替
uid
提供真實或暫
存的用戶識別碼。換言之，一個非 root 的用戶只可以把有效用戶識別碼設定成這兩個值
其中之一。
執行成功時，
setuid()
會傳回
0
。發生錯誤時，此呼叫會傳回
-1
並把
errno
設定成下面
其中一個值：
EAGAIN
uid
與真實用戶識別碼不同，而且將真實用戶識別碼設定成
uid
，會使得用戶逾越其
NPROC
rlimit（用於指定一個用戶所能擁有之行程的數目）。
EPERM
用戶不是
root
，而
uid
既不是有效用戶識別碼也不是暫存用戶識別碼。
前面的討論同樣適用於群組—僅需要把
setuid()
代換成
setgid()
，以及把
uid
代換成
gid
。
變更有效的用戶或群組識別碼 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.