pmb.install.format: add support for setting FDE passphrase

This adds support for using an environment variable to set the FDE
passphrase, allowing us to automate image creation when using FDE.
The method used here was borrowed from how we set the password with when
using the --password arg: write to a temp file, call something in the
chroot to read/use it, then remove it.

Part-of: !2538

pmb/install/format.py

@@ -4,6 +4,8 @@ from pmb.helpers import logging
 import pmb.chroot
 from pmb.core import Chroot
 from pmb.types import PartitionLayout, PmbArgs, PathString
+import os
+import tempfile
 
 
 def install_fsprogs(filesystem: str) -> None:
@@ -52,21 +54,36 @@ def format_luks_root(args: PmbArgs, device: str) -> None:
     # Avoid cryptsetup warning about missing locking directory
     pmb.chroot.root(["mkdir", "-p", "/run/cryptsetup"])
 
-    pmb.chroot.root(
-        [
-            "cryptsetup",
-            "luksFormat",
-            "-q",
-            "--cipher",
-            args.cipher,
-            "--iter-time",
-            args.iter_time,
-            "--use-random",
-            device,
-        ],
-        output="interactive",
-    )
-    pmb.chroot.root(["cryptsetup", "luksOpen", device, "pm_crypt"], output="interactive")
+    format_cmd = [
+        "cryptsetup",
+        "luksFormat",
+        "-q",
+        "--cipher",
+        args.cipher,
+        "--iter-time",
+        args.iter_time,
+        "--use-random",
+        device,
+    ]
+    open_cmd = ["cryptsetup", "luksOpen"]
+
+    path_outside = None
+    fde_key = os.environ.get("PMB_FDE_PASSWORD", None)
+    if fde_key:
+        # Write passphrase to a temp file, to avoid printing it in any log
+        path = tempfile.mktemp(dir="/tmp")
+        path_outside = Chroot.native() / path
+        with open(path_outside, "w", encoding="utf-8") as handle:
+            handle.write(f"{fde_key}")
+        format_cmd += [str(path)]
+        open_cmd += ["--key-file", str(path)]
+
+    try:
+        pmb.chroot.root(format_cmd, output="interactive")
+        pmb.chroot.root([*open_cmd, device, "pm_crypt"], output="interactive")
+    finally:
+        if path_outside:
+            os.unlink(path_outside)
 
     if not (Chroot.native() / mountpoint).exists():
         raise RuntimeError("Failed to open cryptdevice!")