PAM: Add an Hacking document to explain better the architecture

pam/Hacking.md

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+## Overview
+
+The authd PAM module works in a quite complex way due to some Go limitations
+when used as shared library, so we basically have two different modes to use
+it.
+
+ 1. When used in GDM, we just compile the module as a shared library that
+    the gdm pam worker will load and open through the `gdm-authd` PAM service.
+    <br/>
+    In such mode, the module also communicates to GDM via the PAM extended
+    binary protocol, using [JSON format](https://gitlab.gnome.org/GNOME/gdm/-/merge_requests/227).
+ 2. When used in normal PAM transactions by any other PAM application we
+    instead compile the module as an executable application that is launched
+    by our [`go-exec`](./go-exec/module.c) PAM module implemented in C
+    and that does some DBus communication over a private bus with the actual
+    PAM application.
+
+
+### Compiling
+
+#### GDM PAM module
+
+The module can be both generated in release or debug mode:
+
+    go generate -C pam -x
+
+or:
+
+    go generate -C pam -x -tags pam_debug
+
+Either way the PAM library to be loaded by the `gdm-authd` service will be in
+`./pam/pam_authd.so`
+
+This can be technically be used also by any other PAM application, but due to
+some threading race (as per the go multithreading nature), it's known to be
+hanging when loaded by some applications (such as `sshd`).
+
+#### Generic PAM Module
+
+As mentioned in order to make our Go PAM implementation of the PAM module to be
+reliable, we relied on a simpler C module that only acts as a wrapper between
+the actual PAM APIs and a go program that is called and that communicates with
+the actual module using a DBus private connection (where the security of that
+is both grantee by the fact that it's almost impossible to predict its address,
+but also by native DBus credentials checks and on child PID verification).
+
+In order to build the module and its relative companion PAM program you can do:
+
+    go generate -C pam -x
+    go build -C pam -o $PWD/pam/pam_authd
+
+or:
+
+    go generate -C pam -x -tags pam_debug
+    go build -C pam -tags pam_debug -o $PWD/pam/pam_authd
+
+This will build two binaries:
+ - `./pam/go-exec/pam_authd_exec.so`: That is the actual PAM module
+ - `./pam/pam_authd`: Its companion child PAM application
+
+It's possible to use these in a PAM service file using something like:
+
+
+    auth    sufficient $AUTHD_SOURCE_PATH/pam/go-exec/pam_authd_exec.so $AUTHD_SOURCE_PATH/pam/pam_authd
+
+It's also possible to provide other arguments that may have effect either on the
+module or in its wrapper, for example:
+
+    auth    sufficient $AUTHD_SOURCE_PATH/pam/go-exec/pam_authd_exec.so $AUTHD_SOURCE_PATH/pam/pam_authd --exec-debug debug=true force_native_client=true logfile=/dev/stderr
+
+Se [`go-exec-module.c`](./go-exec/module.c) or [`pam.go`](./pam.go) source
+code for further arguments.
+
+
+## Manual Testing the PAM module
+
+While there's no yet an interactive way to test the GDM mode of the module,
+without an actual gnome-shell implementation, it's possible to do some manual
+testing of the PAM module as it works when called by tools such as `su`, `sudo`,
+`login`, `passwd`, `ssh`...
+
+To perform such testing we've a tool, [`pam-runner.go`](./tools/pam-runner/pam-runner.go),
+that is in charge of:
+ - Compiling the PAM Exec module
+ - Compile the PAM Child application
+ - Generate a temporary PAM service file with those files defined
+ - Start a PAM transaction via libpam
+ - Handle the PAM conversations
+
+The basic login mode can be simulated by running:
+
+    go run -tags=withpamrunner ./pam/tools/pam-runner login
+
+You can specify the `socket` to be used with `socket=$authd_socket_path` or
+enable debugging via `debug=true`:
+
+    go run -tags=withpamrunner ./pam/tools/pam-runner login \
+        socket=/tmp/authd.sock debug=true
+
+Password change simulation (as done by `passwd`) can be done using the `passwd`
+argument:
+
+    go run -tags=withpamrunner ./pam/tools/pam-runner passwd
+
+In order to trigger the native PAM UI (as in `ssh`) instead it's needed to
+explicitly request it (as otherwise it's only triggered when a TTY is not
+available):
+
+    AUTHD_PAM_CLI_SUPPORTS_CONVERSATION=1 go run -tags withpamrunner \
+        ./pam/tools/pam-runner login force_native_client=true
+
+
+### Troubleshooting
+
+In all these cases, when enabled through `debug=true` parameter, it's possible
+to see the debug log both passing the `logfile=` parameter pointing to a
+writable path (`/dev/stderr` is valid too!) or leaving it as default and relying
+on systemd journal that is possible to watch in realtime with:
+
+    journalctl -ef _COMM=exec-child
+
+
+Note that to test this with an installed version of authd.