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Permisos del sistema de archivos

Resumen de permisos

Como hemos dicho, Slackware Linux es un sistema operativo multiusuario. Debido a esto, sus sistemas de archivos también son de usuario múltiple. Esto significa que cada archivo o directorio tiene un conjunto de permisos que pueden otorgar o denegar privilegios a diferentes usuarios. Hay tres permisos básicos y tres conjuntos de permisos para cada archivo. Echemos un vistazo a un archivo de ejemplo.

darkstar:~$ ls -l /bin/ls
-rwxr-xr-x 1 root root 81820 2007-06-08 21:12 /bin/ls

Recuerde del capítulo 4 que ls - l enumera los permisos para un archivo o directorio junto con el usuario y el grupo que “posee” el archivo. En este caso, los permisos son rwxr-xr-x, el usuario es root y el grupo también es root. La sección de permisos, aunque agrupada, es en realidad tres piezas separadas. El primer conjunto de tres letras son los permisos otorgados al usuario que posee el archivo. El segundo conjunto de tres son los otorgados al propietario del grupo, y los últimos tres son permisos para todos los demás.

Tabla 10.1. Permisos de /bin/ls

Grupo Listado Significado
Propietario rwx El propietario “root” puede leer, escribir y ejecutar
Grupo r-x El grupo “root” puede leer y ejecutar
Otros r-x Todos los demás pueden leer y ejecutar

Los permisos son bastante autoexplicativos, por supuesto, al menos para archivos. Leer, escribir y ejecutar le permite leer un archivo, escribir en él o ejecutarlo. Pero, ¿qué significan estos permisos para los directorios? En pocas palabras, los permisos de lectura otorgan la posibilidad de listar los contenidos del directorio (digamos con ls ). El permiso de escritura otorga la posibilidad de crear nuevos archivos en el directorio, así como de eliminar todo el directorio, incluso si de otra forma no podría eliminar algunos de los otros archivos que contiene. El permiso de ejecución otorga la posibilidad de ingresar al directorio (por ejemplo, con cd , comando incorporado de bash ).

Echemos un vistazo a los permisos en un directorio ahora.

darkstar:~$ ls -ld /home/alan
drwxr-x--- 60 alan users 3040 2008-06-06 17:14 /home/alan/

Aquí vemos los permisos en mi directorio personal y su propiedad. El directorio es propiedad del usuario alan y los usuarios del grupo. Al usuario se le otorgan todos los derechos (rwx), al grupo solo se le otorgan permisos de lectura y ejecución (r-x), y todos los demás tienen prohibido hacer cualquier cosa.

chmod, chown, and chgrp

So now that we know what permissions are, how do we change them? And for that matter, how do we assign user and group ownership? The answer is right here in this section.

The first tool we'll discuss is the useful chown (1) command. Using chown, we can (you guessed it), change the ownership of a file or directory. chown is historically used only to change the user ownership, but can change the group ownership as well.

darkstar:~# ls -l /tmp/foo
total 0
-rw-r--r-- 1 alan users 0 2008-06-06 22:29 a
-rw-r--r-- 1 alan users 0 2008-06-06 22:29 b
darkstar:~# chown root /tmp/foo/a
darkstar:~# ls -l /tmp/foo
total 0
-rw-r--r-- 1 root users 0 2008-06-06 22:29 a
-rw-r--r-- 1 alan users 0 2008-06-06 22:29 b

By using a colon after the user account, you may also specify a new group account.

darkstar:~# chown root:root /tmp/foo/b
darkstar:~#  ls -l /tmp/foo
total 0
-rw-r--r-- 1 root users 0 2008-06-06 22:29 a
-rw-r--r-- 1 root root  0 2008-06-06 22:29 b

chown can also be used recursively to change the ownership of all files and directories below a target directory. The following command would change all the files under the directory /tmp/foo to have their ownership set to root:root.

darkstar:~# chown -R root:root /tmp/foo/b

Specifying a colon and a group name without a user name will simply change the group for a file and leave the user ownership intact.

darkstar:~# chown :wheel /tmp/foo/a
darkstar:~# ls -l /tmp/foo
ls -l /tmp/foo
total 0
-rw-r--r-- 1 root wheel 0 2008-06-06 22:29 a
-rw-r--r-- 1 root root  0 2008-06-06 22:29 b

The younger brother of chown is the slightly less useful chgrp(1). This command works just like chown, except it can only change the group ownership of a file. Since chown can already do this, why bother with chgrp? The answer is simple. Many other operating systems use a different version of chown that cannot change the group ownership, so if you ever come across one of those, now you know how.

There's a reason we discussed changing ownership before changing permissions. The first is a much easier concept to grasp. The tool for changing permissions on a file or directory is chmod(1). The syntax for it is nearly identical to that for chown, but rather than specify a user or group, the administrator must specify either a set of octal permissions or a set of alphabetic permissions. Neither one is especially easy to grasp the first time. We'll begin with the less complicated octal permissions.

Octal permissions derive their name from being assigned by one of eight digits, namely the numbers 0 through 7. Each permissions is assigned a number that is a power of 2, and those numbers are added together to get the final permissions for one of the permission sets. If this sounds confusing, maybe this table will help.

Table 10.2. Octal Permissions

Permission Meaning
Read 4
Write 2
Execute 1

By adding these values together, we can reach any number between 0 and 7 and specify all possible permission combinations. For example, to grant both read and write privileges while denying execute, we would use the number 6. The number 3 would grant write and execute permissions, but deny the ability to read the file. We must specify a number for each of the three sets when using octal permissions. It's not possible to specify only a set of user or group permissions this way for example.

darkstar:~# ls -l /tmp/foo/a
-rw-r--r-- 1 root root  0 2008-06-06 22:29 a
darkstar:~# chmod 750 /tmp/foo/a
darkstar:~# ls -l /tmp/foo/a
-rwxr-x--- 1 root root  0 2008-06-06 22:29 a

chmod can also use letter values along with + or to grant or deny permissions. While this may be easier to remember, it's often easier to use the octal permissions.

Table 10.3. Alphabetic Permissions

Permission Letter Value
Read r
Write w
Execute x

Table 10.4. Alphabetic Users and Groups

Accounts Affected Letter Value
User/Owner u
Group g
Others/World o

To use the letter values with chmod, you must specify which set to use them with, either “u” for user, “g” for group, and “o” for all others. You must also specify whether you are adding or removing permissions with the “+” and “-” signs. Multiple sets can be changed at once by separating each with a comma.

darkstar:/tmp/foo# ls -l
total 0
-rw-r--r-- 1 alan users 0 2008-06-06 23:37 a
-rw-r--r-- 1 alan users 0 2008-06-06 23:37 b
-rw-r--r-- 1 alan users 0 2008-06-06 23:37 c
-rw-r--r-- 1 alan users 0 2008-06-06 23:37 d
darkstar:/tmp/foo# chmod u+x a
darkstar:/tmp/foo# chmod g+w b
darkstar:/tmp/foo# chmod u+x,g+x,o-r c
darkstar:/tmp/foo# chmod u+rx-w,g+r,o-r d
darkstar:/tmp/foo# ls -l
-rwxr--r-- 1 alan users 0 2008-06-06 23:37 a*
-rw-rw-r-- 1 alan users 0 2008-06-06 23:37 b
-rwxr-x--- 1 alan users 0 2008-06-06 23:37 c*
-r-xr----- 1 alan users 0 2008-06-06 23:37 d*

Which you prefer to use is entirely up to you. There are places where one is better than the other, so a real Slacker will know both inside out.

SUID, SGID, and the "Sticky" Bit

We're not quite done with permissions just yet. There are three other “special” permissions in addition to those mentioned above. They are SUID, SGID, and the sticky bit. When a file has one or more of these permissions set, it behaves in special ways. The SUID and SGID permissions change the way an application is run, while the sticky bit restricts deletion of files. These permissions are applied with chmod like read, write, and execute, but with a twist.

SUID and SGID stand for “Set User ID” and “Set Group ID” respectively. When an application with one of these bits is set, the application runs with the user or group ownership permissions of that application regardless of what user actually executed it. Let's take a look at a common SUID application, the humble passwd and the files it modifies.

darkstar:~# ls -l /usr/bin/passwd \
  /etc/passwd \
-rw-r--r-- 1 root root    1106 2008-06-03 22:23 /etc/passwd
-rw-r----- 1 root shadow   627 2008-06-03 22:22 /etc/shadow
-rws--x--x 1 root root   34844 2008-03-24 16:11 /usr/bin/passwd*

Notice the permissions on passwd. Instead of an x in the user's execute slot, we have an s. This tells us that passwd is a SUID program, and when we run it, the process will run as the user “root” rather than as the user that actually executed it. The reason for this is readily apparent as soon as you look at the two files it modifies. Neither /etc/passwd nor /etc/shadow are writable by anyone other than root. Since users need to change their personal information, passwd must be run as root in order to modify those files.

So what about the sticky bit? The sticky bit restricts the ability to move or delete files and is only ever set on directories. Non-root users cannot move or delete any files under a directory with the sticky bit set unless they are the owner of that file. Normally anyone with write permission to the file can do this, but the sticky bit prevents it for anyone but the owner (and of course, root). Let's take a look at a common “sticky” directory.

darkstar:~# ls -ld /tmp
drwxrwxrwt 1 root root   34844 2008-03-24 16:11 /tmp

Naturally, being a directory for the storage of temporary files system wide, /tmp needs to be readable, writable, and executable by anyone and everyone. Since any user is likely to have a file or two stored here at any time, it only makes good sense to prevent other users from deleting those files, so the sticky bit has been set. You can see it by the presence of the t in place of the x in the world permissions section.

Table 10.5. SUID, SGID, and “Sticky” Permissions

Permission Type Octal Value Letter Value
SUID 4 s
SGID 2 s
Sticky 1 t

When using octal permissions, you must specify an additional leading octal value. For example, to recreate the permission on /tmp, we would use 1777. To recreate those permissions on /usr/bin/passwd, we would use 4711. Essentially, any time this leading fourth octet isn't specified, chmod assumes its value to be 0.

darkstar:~# chmod 1777 /tmp
darkstar:~# chmod 4711 /usr/bin/passwd

Using the alphabetic permission values is slightly different. Assuming the two files above have permissions of 0000 (no permissions at all), here is how we would set them.

darkstar:~# chmod ug+rwx,o+rwt /tmp
darkstar:~# chmod u+rws,go+x /usr/bin/passwd

Chapter Navigation

Previous Chapter: Users and Groups

Next Chapter: Working with Filesystems


  • Originally written by Alan Hicks, Chris Lumens, David Cantrell, Logan Johnson

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