92 lines
2.3 KiB
C
92 lines
2.3 KiB
C
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/* This file is part of the sample code and exercises
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* used by the class "Advanced Programming in the UNIX
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* Environment" taught by Jan Schaumann
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* <jschauma@netmeister.org> at Stevens Institute of
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* Technology.
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*
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* This file is in the public domain.
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*
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* You don't have to, but if you feel like
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* acknowledging where you got this code, you may
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* reference me by name, email address, or point
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* people to the course website:
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* https://stevens.netmeister.org/631/
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*/
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/* A simple program to illustrate the difference
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* between effective and real user IDs.
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*
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* Compile, then run as user1 and user2:
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* user1$ cc myuids.c
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* user1$ ./a.out
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* user2$ ./a.out
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* user1$ chmod u+s a.out
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* user1$ ./a.out
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* user2$ ./a.out
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*
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* For a more complete, albeit notably more complex
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* illustration of the various uids and the handling
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* of saved-setuid bits, see setuid.c and the comments
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* and links in there.
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*/
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#include <errno.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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void
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printUids() {
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uid_t ruid = getuid();
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uid_t euid = geteuid();
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printf("My effective uid is: %d\n", euid);
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printf("My real uid is: %d\n", ruid);
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}
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int
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main() {
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printUids();
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uid_t ruid = getuid();
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uid_t euid = geteuid();
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if (ruid == euid) {
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/* Nothing meaningful to do here. */
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exit(EXIT_SUCCESS);
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/* NOTREACHED */
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}
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printf("\nLet's drop elevated privs and set our effective uid to our real uid.\n");
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printf("Calling 'seteuid(%d)'...\n", ruid);
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if (seteuid(ruid) < 0) {
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fprintf(stderr, "Unable to seteuid(%d): %s\n", ruid, strerror(errno));
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} else {
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printUids();
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}
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printf("\nBecause we used seteuid(%d) and not setuid(%d), we can get back our earlier euid %d.\n", ruid, ruid, euid);
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printf("Calling 'seteuid(%d)'...\n", euid);
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if (seteuid(euid) < 0) {
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fprintf(stderr, "Unable to seteuid(%d): %s\n", euid, strerror(errno));
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} else {
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printUids();
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}
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printf("\nNow let's try to 'setuid(%d)'...\n", ruid);
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if (setuid(ruid) < 0) {
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fprintf(stderr, "Unable to setuid(%d): %s\n", ruid, strerror(errno));
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} else {
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printUids();
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}
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printf("\nNow let's try to gain back our effective uid %d...\n", euid);
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printf("Calling 'seteuid(%d)'...\n", euid);
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if (seteuid(euid) < 0) {
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fprintf(stderr, "Unable to seteuid(%d): %s\n", euid, strerror(errno));
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}
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printUids();
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return EXIT_SUCCESS;
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}
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