compose: remove lastchg values from [/usr]/etc/shadow

The format of /etc/shadow contains several fields relating to
password aging. One of these, `lastchg`, contains a timestamp
that represents the epoch day of the last password change. Tools
that don't respect SOURCE_DATE_EPOCH, which includes systemd's
`sysusers` tool, will set this to a value based on the current
time of the build. This causes a lack of reproducibility.

The `lastchg` field can be safely made empty. This disables the
password aging features.

This change rewrites /etc/shadow to remove all `lastchg` values.

This removal could be made conditional on the entry containing
an encrypted password field indicating that the account is either
locked or otherwise restricted from using passwords, such as `*`
or anything starting with `!`.

rust/src/normalization.rs

@@ -13,7 +13,9 @@ use lazy_static::lazy_static;
 use ostree_ext::gio;
 use std::convert::TryInto;
 use std::ffi::{OsStr, OsString};
-use std::io::{Read, Seek, Write};
+use std::io::{BufRead, BufReader, Read, Seek, SeekFrom, Write};
+use std::path::Path;
+use subprocess::{Exec, Redirection};
 
 lazy_static! {
     static ref SOURCE_DATE_EPOCH_RAW: Option<OsString> = std::env::var_os("SOURCE_DATE_EPOCH");
@@ -129,6 +131,26 @@ pub(crate) fn normalize_rpmdb<P: AsRef<Path>>(
     }
 }
 
+#[context("Rewriting /etc/shadow to remove lastchg field")]
+pub(crate) fn normalize_etc_shadow(rootfs: &openat::Dir) -> Result<()> {
+    let mut shadow = rootfs.update_file("usr/etc/shadow", 0o400)?;
+    let entries = BufReader::new(&mut shadow)
+        .lines()
+        .map(|l| {
+            let l = l?;
+            Ok(l.split(':').map(|s| s.to_string()).collect())
+        })
+        .collect::<std::io::Result<Vec<Vec<String>>>>()?;
+    shadow.seek(SeekFrom::Start(0))?;
+    shadow.set_len(0)?;
+    for mut entry in entries {
+        entry[2] = String::new();
+        writeln!(shadow, "{}", entry.join(":"))?;
+    }
+
+    Ok(())
+}
+
 mod rpmdb_bdb {
     use super::SOURCE_DATE_EPOCH;
     use anyhow::{anyhow, Context, Result};

rust/src/passwd.rs

@@ -5,6 +5,7 @@
 use crate::cxxrsutil::*;
 use crate::ffiutil;
 use crate::nameservice;
+use crate::normalization;
 use crate::treefile::{CheckGroups, CheckPasswd, Treefile};
 use anyhow::{anyhow, Context, Result};
 use fn_error_context::context;
@@ -518,6 +519,11 @@ fn complete_pwgrp(rootfs: &openat::Dir) -> Result<()> {
     // In actuality, nothing should change /etc/shadow or /etc/gshadow, so
     // we'll just have to pay the (tiny) cost of re-checksumming.
 
+    // /etc/shadow ends up with a timestamp in it thanks to the `lastchg`
+    // field. This can be made empty safely, especially for accounts that
+    // are locked.
+    normalization::normalize_etc_shadow(rootfs)?;
+
     Ok(())
 }