---
name: "userbindmount"
section: "3"
platform: "linux"
package: "libuserbindmount-dev"
version: "0.1-3"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/userbindmount/3"
---

# userbindmount(3)

bind-mount in user-namespaces

## NAME

userbindmount, userbindmount\_unshare, userbindmount\_set\_cap\_sysadm, userbindmount\_string, userbindmount\_data, userbindmount\_fd - bind-mount in user-namespaces

## SYNOPSIS

**#include \<userbindmount.h\>**\
 **int userbindmount(const char \****source***, const char \****target***);**\
 **int userbindmount\_unshare(void);**\
 **int userbindmount\_set\_cap\_sysadm(void);**\
 **int userbindmount\_string(const char \****string***, const char \****target***, mode\_t** *mode***);**\
 **int userbindmount\_data(const void \****data***, size\_t** *count***, const char \****target***, mode\_t** *mode***);**\
 **int userbindmount\_fd(int** *fd***, const char \****target***, mode\_t** *mode***);**

These functions are provided by libuserbindmount. Link with *-luserbindmount*.

## DESCRIPTION

Libuserbindmount is a library providing support for bind mount in user userspaces.

**userbindmount** bind mounts *source* on *destination*.  If it is not permitted in the current namespace it creates (unshare) a new user-namespace.

**userbindmount\_unshare** creates a new user-namespace where bind mount is allowed.

**userbindmount\_set\_cap\_sysadm** add the CAP\_SYS\_ADMIN ambient capability to the current namespace so that the  ability of bind mount files and directories can be exported to new programs (ambient capabilities survive to [execve(2)](/execve/2.md)).

**userbindmount\_string** bind-mounts on *destination* a temporary file whose (text) contents is provided by *string*. The temporary file is automatically deleted when the namespace is closed or the file/directory unmounted.

**userbindmount\_data** bind-mounts on *destination* a temporary file whose (binary) contents is provided by *data* and has the size of *count* bytes. The temporary file is automatically deleted when  the namespace is closed or the file/directory unmounted.

**userbindmount\_fd** bind-mounts on *destination* a temporary file whose contents is read from the by the file descriptor *fd* (up to the end of file). The temporary file is automatically  deleted when the namespace is closed or the file/directory unmounted.

## RETURN VALUE

All the functions provided by libuserbindmount return 0 in case of success.   -1 is returned elseways and errno is set appropriately.

## NOTES

Libuserbindmount fails if user namespaces have not been configured in the running kernel and enabled for users.  In Debian the sysctl knob **kernel.unprivileged\_userns\_clone** must be set to 1.

## EXAMPLES

The following excerpts of C code shows the use of **libuserbindmount**:  the inclusion of the header file for this library is required:

```
#include <usrbindmount.h>
```

Bind-mount /tmp/resolv.conf on /etc/resolv.conf:

```
userbindmount("/tmp/resolv.conf", "/etc/resolv.conf");
```

Bind-mount a string on /etc/resolv.conf:

```
userbindmount_string("nameserver 9.9.9.90, "/etc/resolv.conf", 0600);
```

Bind-mount a binary data on /proc/self/cmdline:

```
static char fakeargv[] = {'c','m','d',0,
	'a','r','g','1',0,
	'a','r','g','2',0,
	0};
userbindmount_data(fakeargv, sizeof(fakeargv), "/proc/self/cmdline", 0600);
```

Bind-mount the data read from a file descriptor on /etc/resolv.conf:

```
userbindmount_fd(STDIN_FILENO, "/etc/resolv.conf", 0600);
```

Bind-mount several files or directories:

```
userbindmount("/tmp/resolv.conf", "/etc/resolv.conf");
userbindmount("/tmp/passwd", "/etc/passwd");
userbindmount("/tmp/hosts", "/etc/hosts");
```

Only the first userbindmount creates a new namespace if needed.

The following program creates a namespace and runs a program in it. In the new namespace bind-mount is allowed.

```
#include <stdio.h>
#include <unistd.h>
#include <userbindmount.h>
int main(int argc, char *argv[]) {
	userbindmount_unshare();
	userbindmount_set_cap_sysadm();
	execvp(argv[1], argv+1);
}
```

It can be compiled and tested in the following way:

```
$ gcc -o unshare_sysadm unshare_sysadm.c -luserbindmount
$ unshare_sysadm bash
$ cat /etc/resolv.conf
nameserver 127.0.0.1
$ echo "nameserver 9.9.9.9" > /tmp/resolv.conf
$ busybox mount --bind /tmp/resolv.conf /etc/resolv.conf
$ cat /etc/resolv.conf
nameserver 9.9.9.9
$ exit
$
```

please note that in the example the mount command by busybox has been used instead of the standard mount by util-linux. In fact the standard mount command has not been updated to support the capabilities, and forbids the access to the mount system call if the effective user is not root, denying in this way a legal operation.

## SEE ALSO

[**mount**(2)](/mount/2.md), **mount**(8), **user\_namespaces**(7)

## BUGS

Bug reports should be addressed to \<info@virtualsquare.org\>

## AUTHORS

Renzo Davoli \<renzo@cs.unibo.it\>