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+[[!meta copyright="Copyright © 2021 Free Software Foundation, Inc."]]
+
+[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
+id="license" text="Permission is granted to copy, distribute and/or modify this
+document under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with no Invariant
+Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
+is included in the section entitled [[GNU Free Documentation
+License|/fdl]]."]]"""]]
+
+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.
+
+
+# Code
+
+The translator is available in the checkperm-deferred-authorization branch in [the hurd repository](https://git.savannah.gnu.org/cgit/hurd/hurd.git).
+
+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 $(realpath ~/Dev/hurd/trans/checkperms) --groupname=user
+    su - user --shell /bin/bash -c 'cat /hello'
+    # ⇒ HELLOWORLD # user has the group user
+    cat /hello # root does not have the group user, so
+               # this blocks until positive reply in the other shell
+
+**Second shell** (run the program):
+
+    Process 732 tries to access file /hello but is not in the required group user.
+    USER       PID %CPU %MEM    SZ   RSS TT STAT   START     TIME COMMAND
+    root       732  0.0  0.1  148M 3.55M p2 Sso  Mon 1AM  0:01.10 -bash
+    Grant permission and add group "user" for 5 minutes? [y/N]> y
+
+**First shell** as root:
+
+    # ⇒ HELLOWORLD
+    # only blocks once despite getting two reads from cat,
+    # because for the second read cat already has the group `user`.
+
+
+
+## Trying it yourself
+
+Setup the development environment with the code at ~/Dev similar to
+https://www.draketo.de/software/hurd-development-environment
+
+
+Compile and setup the translator:
+
+    cd ~/Dev/hurd && \
+    patch -p1 < checkperms.patch && \
+    autoreconf -i  && \
+    ./configure --without-parted && \
+    make && \
+    touch trans/checkperms.c && \
+    CFLAGS="$CFLAGS -g" make && \
+    echo HELLOWORLD > /hello && \
+    settrans -cga /hello $(realpath ~/Dev/hurd/trans/checkperms) --groupname=user
+
+Create the FIFOs:
+
+    USER=root
+    GROUP=user
+    mkdir -p /run/$USER/request-permission
+    mkdir -p /run/$USER/grant-permission
+    mkfifo /run/$USER/request-permission/$GROUP
+    mkfifo /run/$USER/grant-permission/$GROUP
+
+Setup the permission-granting program in a separate shell:
+
+    USER=root
+    GROUP=user
+    while true; do
+      PID="$(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 $GROUP 2>/dev/null) &
+      else
+        echo 1 > /run/$USER/grant-permission/$GROUP
+      fi
+    done
+
+
+Access the translator as user without the required group and with the group:
+
+    su - user --shell /bin/bash -c cat /hello'
+    cat /hello &
+
+
+# Concept
+
+## 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
+
+    /run/USER/request-permission/GROUP
+
+Then it reads from
+
+    /run/USER/grant-permission/GROUP
+
+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
+
+    /run/USER/request-permission/GROUP
+
+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
+
+    /run/USER/grant-permission/GROUP
+
+## 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.
+
+# Possibilities
+
+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
+
+## read-only
+
+[[!tag open_issue_hurd]]
+
+The current implementation only provides read-access, writing is
+prevented. This is not an intrinsic limitation, only an implementation
+artefact.
+
+## delegate
+
+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.  
+REQUEST: PID LABEL  
+GRANT: RET LABEL (RET=0 is success)  
+LABEL=$RANDOM
+
+
+## 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.