The checkperms translator implements deferred authorization.

It is part of a project to enable asking for a grant of authorization when processes access a file. It is built as a translator and a simple permission granting program.

The translator can delegate permission-granting to the program via two FIFO files. The goal is to create a simple replacement for the use-case of polkit of granting privilege to a process to access some resource after user-interaction with a permission-granting daemon.


The translator is available in the checkperm-deferred-authorization branch in the hurd repository.

The code for the program is provided in this article

Usage Example

We restrict a the node /hello to require explicit permission for every PID that does not have the group user. This notably does include processes started by root.

How it looks

First shell as root:

settrans -cga /hello $(realpath ~/Dev/hurd/trans/checkperms) --groupname=user
su - user --shell /bin/bash -c 'cat /hello'
# ⇒ HELLOWORLD # user has the group user
cat /hello # root does not have the group user, so
           # this blocks until positive reply in the other shell

Second shell (run the program):

Process 732 tries to access file /hello but is not in the required group user.
root       732  0.0  0.1  148M 3.55M p2 Sso  Mon 1AM  0:01.10 -bash
Grant permission and add group "user" for 5 minutes? [y/N]> y

First shell as root:

# only blocks once despite getting two reads from cat,
# because for the second read cat already has the group `user`.

Trying it yourself

Setup the development environment with the code at ~/Dev similar to

Compile and setup the translator:

cd ~/Dev/hurd && \
patch -p1 < checkperms.patch && \
autoreconf -i  && \
./configure --without-parted && \
make && \
touch trans/checkperms.c && \
CFLAGS="$CFLAGS -g" make && \
echo HELLOWORLD > /hello && \
settrans -cga /hello $(realpath ~/Dev/hurd/trans/checkperms) --groupname=user

Create the FIFOs:

mkdir -p /run/$USER/request-permission
mkdir -p /run/$USER/grant-permission
mkfifo /run/$USER/request-permission/$GROUP
mkfifo /run/$USER/grant-permission/$GROUP

Setup the permission-granting program in a separate shell:

while true; do
  PID="$(cat /run/$USER/request-permission/$GROUP)"
  echo Process $PID tries to access file /hello but is not in the required group $GROUP.
  ps-hurd -p $PID -aeux
  if [[ "$(read -e -p 'Grant permission and add group "'$GROUP'" for 5 minutes? [y/N]> '; echo $REPLY)" == [Yy]* ]]; then
    addauth -p $PID -g $GROUP
    echo 0 > /run/$USER/grant-permission/$GROUP
    (sleep 300 && rmauth -p $PID -g $GROUP 2>/dev/null) &
    echo 1 > /run/$USER/grant-permission/$GROUP

Access the translator as user without the required group and with the group:

su - user --shell /bin/bash -c cat /hello'
cat /hello &


The translator

The translator is started with a GROUP as argument. When the file is accessed, the translator checks whether the process has the given group. If it does, it returns data read from the underlying file.

If the process lacks the required group, the translator retrieves its USER and PID and writes the PID into a FIFO located at


Then it reads from


It blocks until it gets a reply. If it reads a 0 (=success), it reads from the file and returns the data.

The permission granting program

The permission granting program reads the PID from


retrieves information about the PID and asks the user whether to allow the program.

If the USER answers no, the RET value is non-zero.

If the USER answers yes, the RET value is zero (0) and the program adds the GROUP to the process at PID (using addauth).

It also starts a daemon that will remove the group again after 5 minutes (modelled after the temporary permissions to run privileged without password granted by sudo).

The program then writes the RET value into


What if the translator crashes?

If the translator crashes, the permissions return to those of the underlying node. For every user except root this usually means that the process does not have access to the file.

The failure-mode should therefore be safe.


The most important use-case for this translator is to make it easier to start programs with reduced permissions and only add these when required.

To setup deferred permissions for a single file, you can create a group just for that file. Then each file can have its own permission granting program. Having dedicated groups decouples authentication and authorization while staying in the conventional *nix permissions scheme.

You can also set this translator on a file that gets accessed first when a process accesses a set of related files that all have the same group. Since the authorization-program here adds the group for 5 minutes, the other files can afterwards be accessed, too.

Since the translator simply defers to a program, that program could do any action to get authorization, including curl. Administrators for a local network could therefore set up terminals for unprivileged users that request permissions from a local server when accessing a file. That way permissions can easily be coordinated over multiple machines. (naturally this does not restrict root who can always use settrans -g to get raw access to the file)

Open Issues


The current implementation only provides read-access, writing is prevented. This is not an intrinsic limitation, only an implementation artefact.


The underlying file is currently read by the translator and the data returned to the reading process. To reduce delays, it could directly delegate to the underlying file. With the long term goal to provide multiplexing of access, for example for audio, reading via the translator could be preferable, though.

writing via system shell

Writing to and reading from the FIFOs is currently done with system(). It would be nicer to move to an implementation that does not rely on the system-shell.

potential race-condition

Accesses from two different translators can currently race for the reply. To fix this, the translator should write the PID and a random LABEL into the request. The program should repeat that label for replies to ensure that the reply and request can be matched. If receiving a non-matching reply, it MUST be written into the grant again after a random delay to enable a matching translator to retrieve the grant.
GRANT: RET LABEL (RET=0 is success)

multiple permission-granting programs

The system assumes having a single permission granting program per user. For a setup with multiple unconnected sessions per user (like several TTYs) the permission granting program needs to coordinate between these.