Merge pull request #54980 from danbst/etc-relative

nixos: make symlinks in `/etc` relative (except `/etc/static`)

nixos/doc/manual/release-notes/rl-1903.xml

@@ -535,6 +535,20 @@
       use <literal>nixos-rebuild boot; reboot</literal>.
      </para>
    </listitem>
+   <listitem>
+    <para>
+     Symlinks in <filename>/etc</filename> (except <filename>/etc/static</filename>)
+     are now relative instead of absolute. This makes possible to examine
+     NixOS container's <filename>/etc</filename> directory from host system
+     (previously it pointed to host <filename>/etc</filename> when viewed from host,
+     and to container <filename>/etc</filename> when viewed from container chroot).
+    </para>
+    <para>
+     This also makes <filename>/etc/os-release</filename> adhere to
+     <link xlink:href="https://www.freedesktop.org/software/systemd/man/os-release.html">the standard</link>
+     for NixOS containers.
+    </para>
+   </listitem>
    <listitem>
     <para>
       Flat volumes are now disabled by default in <literal>hardware.pulseaudio</literal>.

nixos/modules/system/etc/make-etc.sh

@@ -10,6 +10,11 @@ users_=($users)
 groups_=($groups)
 set +f
 
+# Create relative symlinks, so that the links can be followed if
+# the NixOS installation is not mounted as filesystem root.
+# Absolute symlinks violate the os-release format
+# at https://www.freedesktop.org/software/systemd/man/os-release.html
+# and break e.g. systemd-nspawn and os-prober.
 for ((i = 0; i < ${#targets_[@]}; i++)); do
     source="${sources_[$i]}"
     target="${targets_[$i]}"
@@ -19,14 +24,14 @@ for ((i = 0; i < ${#targets_[@]}; i++)); do
         # If the source name contains '*', perform globbing.
         mkdir -p $out/etc/$target
         for fn in $source; do
-            ln -s "$fn" $out/etc/$target/
+            ln -s --relative "$fn" $out/etc/$target/
         done
 
     else
 
         mkdir -p $out/etc/$(dirname $target)
         if ! [ -e $out/etc/$target ]; then
-            ln -s $source $out/etc/$target
+            ln -s --relative $source $out/etc/$target
         else
             echo "duplicate entry $target -> $source"
             if test "$(readlink $out/etc/$target)" != "$source"; then

nixos/modules/system/etc/setup-etc.pl

@@ -4,6 +4,7 @@ use File::Copy;
 use File::Path;
 use File::Basename;
 use File::Slurp;
+use File::Spec;
 
 my $etc = $ARGV[0] or die;
 my $static = "/etc/static";
@@ -17,6 +18,20 @@ sub atomicSymlink {
     return 1;
 }
 
+# Create relative symlinks, so that the links can be followed if
+# the NixOS installation is not mounted as filesystem root.
+# Absolute symlinks violate the os-release format
+# at https://www.freedesktop.org/software/systemd/man/os-release.html
+# and break e.g. systemd-nspawn and os-prober.
+sub atomicRelativeSymlink {
+    my ($source, $target) = @_;
+    my $tmp = "$target.tmp";
+    unlink $tmp;
+    my $rel = File::Spec->abs2rel($source, dirname $target);
+    symlink $rel, $tmp or return 0;
+    rename $tmp, $target or return 0;
+    return 1;
+}
 
 # Atomically update /etc/static to point at the etc files of the
 # current configuration.
@@ -103,7 +118,7 @@ sub link {
     if (-e "$_.mode") {
         my $mode = read_file("$_.mode"); chomp $mode;
         if ($mode eq "direct-symlink") {
-            atomicSymlink readlink("$static/$fn"), $target or warn;
+            atomicRelativeSymlink readlink("$static/$fn"), $target or warn;
         } else {
             my $uid = read_file("$_.uid"); chomp $uid;
             my $gid = read_file("$_.gid"); chomp $gid;
@@ -117,7 +132,7 @@ sub link {
         push @copied, $fn;
         print CLEAN "$fn\n";
     } elsif (-l "$_") {
-        atomicSymlink "$static/$fn", $target or warn;
+        atomicRelativeSymlink "$static/$fn", $target or warn;
     }
 }