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UNVEIL(2) System Calls Manual UNVEIL(2)

unveil parts of a restricted filesystem view

#include <unistd.h>
unveil(const char *path, const char *permissions);

The first call to unveil removes visibility of the entire filesystem from all other filesystem-related system calls (such as open(2), chmod(2) and rename(2)), except for the specified path and permission. Subsequent calls to unveil can expose additional paths with specified permissions in the filesystem.
The unveil call itself is treated specially and can continue to see the filesystem for subsequent calls.
Future calls to unveil can be blocked by passing two NULL arguments. If the veil is not yet active, this does not activate it. Alternatively, pledge(2) may be used to remove the unveil promise.
The permissions argument points to a string consisting of the following characters:
Make path available for read operations, corresponding to the pledge(2) promise rpath.
Make path available for write operations, corresponding to the pledge(2) promise wpath.
Make path available for execute operations, corresponding to the pledge(2) promise exec.
Allow path to be created and removed, corresponding to the pledge(2) promise cpath.
A path that is a directory will enable all filesystem access underneath path using permissions if and only if no more specific matching unveil() exists at a lower level. Directories are remembered at the time of a call to unveil(). This means that a directory that is removed and recreated after a call to unveil() will appear to not exist.
Non-directories paths are remembered by name within their containing directory, and so may be created, removed, or re-created after a call to unveil() and still appear to exist.
Attempts to access paths not allowed by unveil will result in an error of EACCES when the permissions argument does not match the attempted operation. ENOENT is returned for paths for which no unveil permissions qualify.
As with pledge(2), the use of unveil() in an application will require lots of study and understanding of the interfaces called. In most cases it is best practice to unveil the directories in which an application makes use of files.

unveil() returns 0 on success or -1 on failure.

The addition of path would exceed the per-process limit for unveiled paths.
A directory in path did not exist.
An invalid value of permissions was used.
An attempt to increase permissions was made, or the path was not accessible, or unveil was called after locking.

The unveil() system call first appeared in OpenBSD 6.4.

Filesystem lookups work today when they cross an unveil() during namei(9) lookup in the kernel. A program that does relative operations below a higher unveil() may currently not see the parts of the filesystem underneath the high level unveil. This is actively being worked on.
July 30, 2018 OpenBSD-current