Solutions to Selected Exercises 1427Chapter 9
9-1. In considering the following, remember that changes to the effective user ID
always also change the file-system user ID.
a) real=2000, effective=2000, saved=2000, file-system=2000
b) real=1000, effective=2000, saved=2000, file-system=2000
c) real=1000, effective=2000, saved=0, file-system=2000
d) real=1000, effective=0, saved=0, file-system=2000
e) real=1000, effective=2000, saved=3000, file-system=2000
9-2. Strictly speaking, such a process is unprivileged, since its effective user ID is
nonzero. However, an unprivileged process can use the setuid(), setreuid(), seteuid(),
or setresuid() calls to set its effective user ID to the same value as its real user ID or
saved set-user-ID. Thus, this process could use one of these calls to regain privilege.
9-4. The following code shows the steps for each system call.e = geteuid(); /* Save initial value of effective user ID */setuid(getuid()); /* Suspend privileges */
setuid(e); /* Resume privileges */
/* Can't permanently drop the set-user-ID identity with setuid() */seteuid(getuid()); /* Suspend privileges */
seteuid(e); /* Resume privileges */
/* Can't permanently drop the set-user-ID identity with seteuid() */setreuid(-1, getuid()); /* Temporarily drop privileges */
setreuid(-1, e); /* Resume privileges */
setreuid(getuid(), getuid()); /* Permanently drop privileges */setresuid(-1, getuid(), -1); /* Temporarily drop privileges */
setresuid(-1, e, -1); /* Resume privileges */
setresuid(getuid(), getuid(), getuid()); /* Permanently drop privileges */9-5. With the exception of setuid(), the answers are the same as for the previous
exercise, except that we can substitute the value 0 for the variable e. For setuid(), the
following holds:/* (a) Can't suspend and resume privileges with setuid() */setuid(getuid()); /* (b) Permanently drop privileges */Chapter 10
10-1. The maximum unsigned 32-bit integer value is 4,294,967,295. Divided by 100 clock
ticks per second, this corresponds to slightly more than 497 days. Divided by
1 million (CLOCKS_PER_SEC), this corresponds to 71 minutes and 35 seconds.