4.7. access FunctionAs we described earlier, when we open a file, the kernel performs its access tests based on the effective user and group IDs. There are times when a process wants to test accessibility based on the real user and group IDs. This is useful when a process is running as someone else, using either the set-user-ID or the set-group-ID feature. Even though a process might be set-user-ID to root, it could still want to verify that the real user can access a given file. The access function bases its tests on the real user and group IDs. (Replace effective with real in the four steps at the end of Section 4.5.)
ExampleFigure 4.8 shows the use of the access function.Here is a sample session with this program:$ ls -l a.out -rwxrwxr-x 1 sar 15945 Nov 30 12:10 a.out $ ./a.out a.out read access OK open for reading OK $ ls -l /etc/shadow -r-------- 1 root 1315 Jul 17 2002 /etc/shadow $ ./a.out /etc/shadow access error for /etc/shadow: Permission denied open error for /etc/shadow: Permission denied $ su become superuser Password: enter superuser password # chown root a.out change file's user ID to root # chmod u+s a.out and turn on set-user-ID bit # ls -l a.out check owner and SUID bit -rwsrwxr-x 1 root 15945 Nov 30 12:10 a.out # exit go back to normal user $ ./a.out /etc/shadow access error for /etc/shadow: Permission denied open for reading OK In this example, the set-user-ID program can determine that the real user cannot normally read the file, even though the open function will succeed. Figure 4.8. Example of access function#include "apue.h" #include <fcntl.h> int main(int argc, char *argv[]) { if (argc != 2) err_quit("usage: a.out <pathname>"); if (access(argv[1], R_OK) < 0) err_ret("access error for %s", argv[1]); else printf("read access OK\n"); if (open(argv[1], O_RDONLY) < 0) err_ret("open error for %s", argv[1]); else printf("open for reading OK\n"); exit(0); } In the preceding example and in Chapter 8, we'll sometimes switch to become the superuser, to demonstrate how something works. If you're on a multiuser system and do not have superuser permission, you won't be able to duplicate these examples completely.  |
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