NAME
unveil —
unveil parts of a restricted filesystem
view
SYNOPSIS
#include
<unistd.h>
int
unveil(const
char *path, const char
*permissions);
DESCRIPTION
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
permissions.
The
unveil()
system call remains capable of traversing to any path
in the filesystem, so additional calls can set permissions at other points
in the filesystem hierarchy.
After establishing a collection of
path and permissions rules,
future calls to
unveil()
can be disabled by passing two NULL arguments.
Alternatively,
pledge(2) may be used to remove the "unveil" promise.
The permissions argument points to a string consisting of the following characters:
r- Make path available for read operations, corresponding to the pledge(2) promise "rpath".
w- Make path available for write operations, corresponding to the pledge(2) promise "wpath".
x- Make path available for execute operations, corresponding to the pledge(2) promise "exec".
c- 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-directory 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. After a process has
terminated,
lastcomm(1) will mark it with the ‘U’ flag if file
access was prevented by unveil().
unveil()
use can be tricky because programs misbehave badly when their files
unexpectedly disappear. In many cases it is easier to unveil the directories
in which an application makes use of files.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
ERRORS
E2BIG- The addition of path would exceed the per-process limit for unveiled paths.
ENOENT- A directory in path did not exist.
EINVAL- An invalid value of permissions was used.
EPERM- An attempt to increase permissions was made, or the
path was not accessible, or
unveil() was called after locking.
HISTORY
The unveil() system call first appeared in
OpenBSD 6.4.
BUGS
readlink(2) partially bypasses
unveil() restrictions required by
realpath(3). Future changes intend to repair this problem.