Documentation:Unix Section 4

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I/O Restrictions

I/O Restrictions are feature introduced in POV-Ray for Unix 3.5. The purpose of this feature is to attempt to at least partially protect a machine running POV-Ray from having files read or written outside of a given set of directories.

The need for this is related to the fact that the POV-Ray scene language has, over the years, become something more akin to a scripting language combined with a scene-description model. It is now possible to write obfuscated POV-Ray code, and to open, create, read and write arbitrary files anywhere on the target system's hard disk (subject to operating system permission).

The basic idea of I/O Restrictions is to attempt to protect the user from a script that may have been downloaded from an untrusted source, and which may attempt to create or modify files that it should not.

The I/O Restriction facility hooks the file open and creation functions in the core POV-Ray renderer code, and allows the Unix version to allow or deny any particular file operation.

Warning: We do not guarantee that the I/O Restriction facility will actually stop anything from happening. There is always the chance that, like almost all software, it could have a bug in it that causes it to malfunction. Therefore, the onus is on the person who chooses to load an INI or scene file into POV-Ray to ensure that it does not do anything that it should not do. Please consider I/O Restrictions just a sometimes-helpful backup for manual checks.

Please read this section in full so that you understand the caveats and conditions of the facility (such as the fact that some directories are allowed by default).

Configuration file format

Now to the details: how to configure the I/O Restrictions (or get them out of your hair if you want them turned off).

The I/O Restrictions are configured by a separate configuration file. This can be a system-wide configuration file (PREFIX/etc/povray/3.7/povray.conf, PREFIX by default is /usr/local) or a user configuration file ($HOME/.povray/3.7/povray.conf). POV-Ray will always use the most strict version of what is specified; user settings can only make security more strict.

If neither of these files exists I/O Restrictions are deactivated.

The general syntax of these files is:

; Comment

[Section]
setting

File I/O Security

The [File I/O Security] section only contains a single setting which is either none, read-only or restricted.

none means that there are no restrictions other than those enforced by the file system, i.e. normal Unix file and directory permissions.

read-only means that files may be read without restriction.

restricted means that files access is subject to restrictions as specified in the rest of this file. See below for details.

Shellout Security

[Shellout Security] determines whether POV-Ray will be allowed to call scripts (e.g. Post_Frame_Command etc., see POV-Ray Options).

This section contains a single setting which is either allowed or forbidden.

allowed means that shellout will work as specified in the documentation.

forbidden means that shellout will be disabled.

Permitted Paths

The [Permitted Paths] section contains a list of directories which are specifically allowed for either reading or reading and writing. These paths are only used when the setting for [File I/O Security] is either read-only or restricted.

Directories that are only allowed for reading are added with read=directory. For allowing reading and writing use read+write=directory.

If [File I/O Security] is set to read-only, any directory can be used to read in a file, and read+write entries must specify which directories are allowed for writing.

If [File I/O Security] is set to restricted, reading and writing is allowed only in the directories given by the read and read+write entries.

If the directory name contains spaces it has to be quoted or doubly-quoted. There can be spaces before and after the equal sign. Read-only and read/write entries can be specified in any order.

If you want the permissions for a specified directory to also extend to all of its subdirectories you have to add a * (like read*=directory or read+write*=directory).

Both relative and absolute paths are permitted (making . especially useful). The install directory (typically /usr/local/share/povray-3.7 or /usr/share/povray-3.7) can be specified with %INSTALLDIR%, the user home directory with %HOME%. The install directory and its descendents are typically only writable by root; therefore it does not make sense to have %INSTALLDIR% in read/write directory paths.

Note: Since user-level permissions are at least as strict as system-level restrictions, any paths specified in the system-wide povray.conf will also need to be specified in ~/.povray/3.7/povray.conf if this file exists.

Examples for path settings

[Permitted Paths]
read=%INSTALLDIR%

would permit reading from the directory where the POV-Ray supplementary files are installed.

Note that the installdir location does not relate to where the binary is run from - it relates to the information defined at compile-time. Relative paths are legal as well, and will be resolved only once at load time (but relative to the current directory, not the installdir). For example, a relative path like the following ...

[Permitted Paths]
read+write=../output

would be resolved with relation to the current directory at the time POV-Ray for Unix was started, so if you started povray while in the directory ~/myscenes/newscene, then the above path would be resolved as ~/myscenes/output. Please note that the actual location of the povray binary is not relevent here - it is the current directory that matters (which is typically not that of the program).

Example configuration file

Here is a complete example for a povray.conf file:

[File I/O Security]
; none
; read-only
restricted

[Shellout Security]
allowed
; forbidden

[Permitted Paths]
read*=%INSTALLDIR%/include
read*=%INSTALLDIR%/scenes
read=%INSTALLDIR%/../../etc
read+write=.
read+write*=/tmp

Post-processing Images

Understanding File Types