## Name

unveil - restrict filesystem access

## Synopsis

```**c++
#include <unistd.h>

int unveil(const char* path, const char* permissions);
```

## Description

`unveil()` manipulates the process veil. The veil is a whitelist of paths on
the file system the calling process is allowed to access.

A process that has not made any `unveil()` calls is allowed to access the whole
filesystem (subject to the regular permission checks). A process that has made
one or more `unveil()` calls cannot access any paths except those that were
explicitly unveiled.

Calling `unveil()` allows the process to access the given `path`, which must be
an absolute path, according to the given `permissions` string, which may
include the following characters:

* `r`: May read a file at this path
* `w`: May write to a file at this path
* `x`: May execute a program image at this path
* `c`: May create or remove a file at this path
* `b`: May browse directories at this path

A single `unveil()` call may specify multiple permission characters at once.
Subsequent `unveil()` calls may take away permissions from the ones allowed
earlier for the same file. Note that unveilng a path with any set of
permissions does not turn off the regular permission checks: access to a file
which the process has unvelied for itself, but has otherwise no appropriate
permissions for, will still be rejected. Unveiling a directory allows the
process to access any files inside the directory.

Calling `unveil()` with both `path` and `permissions` set to null locks the
veil; no further `unveil()` calls are allowed after that.

The veil state is reset after the program successfully performs an `execve()`
call.

`unveil()` is intended to be used in programs that want to sandbox themselves,
either to limit the impact of a possible vulnerability exploitation, or before
intentionally executing untrusted code.

## Return value

If successful, returns 0. Otherwise, returns -1 and sets `errno` to describe
the error.

## Errors

* `EFAULT`: `path` and/or `permissions` are not null and not in readable
  memory.
* `EPERM`: The veil is locked, or an attempt to add more permissions for an
  already unvelied path was rejected.
* `EINVAL`: `path` is not an absolute path, or `permissions` are malformed.

All of the usual path resolution errors may also occur.

## History

The `unveil()` system call was first introduced by OpenBSD.

## Examples

```c++
// Allow the process to read from /res:
unveil("/res", "r");

// Allow the process to read, write, and create the config file:
unveil("/etc/WindowServer/WindowServer.ini", "rwc");

// Allow the process to execute Calendar:
unveil("/bin/Calendar", "x");

// Allow the process to browse files from /usr/share:
unveil("/usr/share", "b");

// Disallow any further veil manipulation:
unveil(nullptr, nullptr);
```

## See also

* [`pledge`(2)](pledge.md)
* [`chroot`(2)](chroot.md)
