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(PHP 4, PHP 5)
umask — Changes the current umask
umask() setzt die umask von PHP auf mask & 0777 und gibt die alte umask zurück. Wird PHP als Servermodul benutzt, wird die umask wiederhergestellt, wenn die jeweilige Anfrage beendet ist.
Die neue umask.
umask() ohne Parameter gibt nur die aktuelle umask zurück; andernfalls wird die alte umask zurückgegeben.
Beispiel #1 umask()-Beispiel
<?php
$old = umask(0);
chmod("/pfad/ein_verzeichnis/eine_datei.txt", 0755);
umask($old);
// Prüfung
if ($old != umask()) {
die('Ein Fehler trat beim Wiederherstellen der umask auf');
}
?>
Hinweis:
Sie sollten diese Funktion nicht in Multithreaded-Webservern nutzen. Der bessere Weg ist, die Dateiberechtigungen mit chmod() zu ändern nachdem die Datei erstellt wurde. Die Nutzung von umask() kann zu unerwartetem Verhalten von gleichzeitig ausgeführten Skripten und des Webservers führen, weil die gleiche umask genutzt wird.
"It is better to change the file permissions with chmod() after creating the file."
If you take that advice seriously, consider setting your umask so that files are created private to your user, then use chmod to open them up.
<?php
// files will create as -rw-------
umask(0077);
// create a file, eg fopen()
// give access: -rw-r--r--
chmod('/path/to/file', 0644);
?>
Whenever reasonable, default to shut and open as needed (like above) instead of default to open and shut as needed. The above still has a race condition, but the race condition will deny appropriate access instead of granting inappropriate access.
You can use umask to solve the PHP session bug that appears in several PHP versions.
<?php
umask(0022);
session_start();
?>
This will prevent sessions being created with inadequate permissions.
I think that the best way to understand umask is to say that umask is used to revoke permissions, not to set permissions.
umask sets which permissions must be removed from the system default when you create a file or a directory.
For example, a mask 0022 means that you don't want group and others modify the file.
default 0666 rw-.rw-.rw-
umask 0022 ---.-w-.-w-
Final 0644 rw-.r--.r--
That means that any file from now on will have 0644 permissions.
It is important to understand that umask revokes, deletes permissions from system default, so it can´t grant permissions the system default hasn't. In the example above, with the 666 system default, there is no way you can use umask to create a file with execute permission. If you want to grant more permissions, use chmod.
Be aware that system default permissions are not related to PHP (they depends upon server configuration). PHP has a default umask that is applied after system default base permissions. And there are different system default base permissions for files and directories.
Usually, system default permissions for files are 666 and for directories 0777. And usually, default PHP umask is 0022
Understanding umask() properly is made more difficult for the novice programmer by the spurious flitting between UNIX's octal representation of file permissions and PHP's requirements for (decimal) integer arguments to most of its functions (including umask).
-
Instead of making mental pictures of chains of rwx|rwx|rw- type words and then tying yourself in cerebral knots manipulating them between the two different number systems, look at the issue as one involving simple bit patterns:
-
On most hosts, the baseline permission for created data files is the well-known OCTAL 0666, which means a bit pattern: 110110110.
-
The baseline pattern for new directories is OCTAL 0777, which is 111111111.
-
Both of these are subject to modification by the current system-wide umask setting, which is normally OCTAL 0022 or, bitwise: 000010010.
-
The mask works like this:
1. INVERT the binary umask
2. Perform an AND operation with the baseline permission pattern
3. Result is the bit pattern which will be applied to your new files or
directories
Example:
System-wide umask: 000010010 (OCTAL 0022)
INVERTED: 111101101
& with baseline: 110110110 (OCTAL 0666)
Result: 110100100
... which is OCTAL 0644.
Let us say you want write permissions on files for your workgroup:
The bit pattern you are aiming for is: 110110100 (OCTAL 0664)
What needs to be AND-ed with our baseline 110110110 to give us this ?
Any AND relationship is reversible, so the answer is given by:
baseline: 110110110 (OCTAL 0666)
target: 110110100 (OCTAL 0664)
result: 111110101
inversion gives: 000001010 (OCTAL 0012)
... which is our desired umask.
Lining up bits in a binary AND is conceptually easier than juggling
with the rw-rwx--x notation when it comes to masks. Remember that
an AND only delivers a binary '1' when a binary '1' is present in BOTH
strings.
Always pass this to PHP in the form:
umask(octdec("0012"));
... so that the umask() function receives a true INT type.
On most UNIX environments the recommended default umask for files, defined in /home/user/.profile or /etc/profile, is 022 (chmod: 644). On trusted systems it is 002. Exercise caution when applying more liberal settings.
umask takes away the given values from the standard mask 777.
A graphical view shows this better:
standard:
rwxrwxrwx = 777
will get with umask 002:
rwxrwxr-x = 775
or will get with umask 077:
rwx------ = 700
and so on.
"It is better to change the file permissions with chmod() after creating the file."
The usual lacking of security knowledge within the PHP team rears its head once again. You *always* want to have the file created with the proper permission. Let me illustrate why:
(a) you create new file with read permissions
(b) an attacking script opens the file
(c) you chmod the file to remove read permissions
(d) you write sensitive data to the file
Now, you might think that the changes of an attacking script getting to open the file before you chmod them are low. And you're right. But low changes are never low enough - you want zero chance.
When creating a file that needs increased permissions, you always need to create the file with the proper permissions, and also create it with O_EXCL set. If you don't do an exclusive create, you end up with this scenario:
(a) attacker creates the file, makes it writable to everyone
(b) you open the file with restricted permissions, but since it already exists, the file is merely opened and the permissions left alone
(c) you write sensitive data into the insecure file
Detecting the latter scenario is possible, but it requires a bit of work. You have to check that the file's owner and group match the script's (that is, posix_geteuid(), not myuid()) and check the permissions - if any of those are incorrect, then the file is insecure - you can attempt to unlink() it and try again while logging a warning, of course.
The only time when it is reasonable or safe to chmod() a file after creating it is when you want to grant extra permissions instead of removing them. For example, it is completely safe to set the umask to 0077 and then chmoding the files you create afterward.
Doing truly secure programming in PHP is difficult as is, and advice like this in the documentation just makes things worse. Remember, kids, anything that applies to security in the C or UNIX worlds is 100% applicable to PHP. The best thing you can possibly do for yourself as a PHP programmer is to learn and understand secure C and UNIX programming techniques.
Notice that directory(s) and file(s) sometimes have different results.
<?php
umask(0670); //- set umask
$handle = fopen('file', 'w'); //- 0006
mkdir("/path/dir"); //- 0107
?>
calculate the result:
<?php
$umask = 0670;
umask($umask);
//- if you are creating a new directory, $permission = 0777;
//- if you are creating a new file, $permission = 0666.
printf( "result: %04o", $permission & ( 0777 - $umask) );
?>
BTW, as the manual said, the form of umask() is "int umask ( [int mask] )", so if you want to print/echo any umask, don't forget to convert it from DEC (because it returns a "int") to OCT.
<?php
$umask = umask(); //- returns the current umask, which is a "int"
$umask = decoct($umask); //- Now, $umask is a "string"
echo $umask;
?>
Don't forget that the argument(parameter) is a "int", too.
<?php
umask(777); //- WRONG! Even though you maybe use "umask 777" in some OS.
umask(0777); //- OK
?>
If there was any mistake, please correct my statement.
Using (cmask - umask) is a wrong way to calculate the new mask:
0022 - 0700 = 0656 WRONG
0700 & ~0022 = 0700 CORRECT
Correct php code:
<?php
$rmask = ($cmask & ~$umask);
?>
To play around with umasks and permissions use this little fragment:
<?
$umask = 0012;
$perm = 0777;
printf("umask: %04o perm: %04o result: %04o\n",
$umask,$perm,$perm & (0777 - $umask));
?>
I thought I would clarify the numbering scheme used here, as it confused me at first.
On the UNIX console, the command:
umask "blah"
In this instance, the umask command forces "blah" to be an octal number, regardless of how many digits you use and regardless of any leading zeroes. In PHP, umask() does not default to octal as the console command does, it uses whatever numeric format you specify.
For example:
umask(213);
This uses the decimal integer 213 and not the octal number 213 as you would expect when using the console command. In this case, it would set the umask to the octal number "325".
To enter the number as octal, just add one or more zeroes to the left of the number:
umask(0213);
umask(07);
umask(0044);
etc.
The first comment perhaps didn't quite make clear what's on with your umask and the permissions.
The permission passed to a command is first bitwise ANDed with the _INVERSE_ of the current umask, then applied to the file.
For example, umask = 0011 and permission = 0775
The inverse of 0011 = 0766
0775 AND 0766
= 111.111.101 AND 111.110.110
= 111.110.100
= 0764
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