PROGRAMMING - Hacking: The Art of Exploitation - page 88

reader@hacking:~/booksrc $ chmod 731 simplenote.c
reader@hacking:~/booksrc $ ls -l simplenote.c
-rwx-wx--x 1 reader reader 1826 2007-09-07 02:51 simplenote.c
reader@hacking:~/booksrc $ chmod ugo-wx simplenote.c
reader@hacking:~/booksrc $ ls -l simplenote.c
-r-------- 1 reader reader 1826 2007-09-07 02:51 simplenote.c
reader@hacking:~/booksrc $ chmod u+w simplenote.c
reader@hacking:~/booksrc $ ls -l simplenote.c
-rw------- 1 reader reader 1826 2007-09-07 02:51 simplenote.c
reader@hacking:~/booksrc $

The first command ( chmod 721 ) gives read, write, and execute permissions to
the user, since the first number is 7 (4 + 2 + 1), write and execute permissions
to group, since the second number is 3 (2 + 1), and only execute permis-
sion to other, since the third number is 1. Permissions can also be added or
subtracted using chmod . In the next chmod command, the argument ugo-wx
means Subtract write and execute permissions from user, group, and other . The final
chmod u+w command gives write permission to user.
In the simplenote program, the open() function uses S_IRUSR|S_IWUSR for
its additional permission argument, which means the /tmp/notes file should
only have user read and write permission when it is created.

reader@hacking:~/booksrc $ ls -l /tmp/notes
-rw------- 1 reader reader 36 2007-09-07 02:52 /tmp/notes
r eader@hacking:~/booksrc $

0x283 User IDs

Every user on a Unix system has a unique user ID number. This user ID can
be displayed using the id command.

reader@hacking:~/booksrc $ id reader
uid=999(reader) gid=999(reader)
groups=999(reader),4(adm),20(dialout),24(cdrom),25(floppy),29(audio),30(dip),4
4(video),46(plugdev),104(scanner),112(netdev),113(lpadmin),115(powerdev),117(a
dmin)
reader@hacking:~/booksrc $ id matrix
uid=500(matrix) gid=500(matrix) groups=500(matrix)
reader@hacking:~/booksrc $ id root
uid=0(root) gid=0(root) groups=0(root)
r eader@hacking:~/booksrc $

The root user with user ID 0 is like the administrator account, which has
full access to the system. The su command can be used to switch to a differ-
ent user, and if this command is run as root, it can be done without a pass-
word. The sudo command allows a single command to be run as the root user.
On the LiveCD, sudo has been configured so it can be executed without a pass-
word, for simplicity's sake. These commands provide a simple method to
quickly switch between users.