[OSSTEST PATCH 09/38] Bodge systemd random seed arrangements

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

systemd does not regard the contents of the random seed file as useful
for the purposes of placating the kernel's entropy tracker.  As a
result, the system hangs at boot waiting for entropy.

Fix this by providing a small program which can be used to load a seed
file into /dev/random and also call RNDADDTOENTCNT to add the
appropriate amount to the kernel's counter.

Arrange to run this program instead of
   /lib/systemd/systemd-random-seed load

With systemd the random seed file is in /var/lib/systemd/random-seed
rather than /var/lib/urandom/random-seed.

Unfortunately we must hardcode the actual numerical value of
RNDADDTOENTCNT because we don't have a suitable compiler anywhere
nearby.  It seems to have the same value on i386, amd64, armhf and
arm64, our currently supported architectures.

Thanks to Colin Watson for pointers to the systemd random unit and
Matthew Vernon for instructions on overriding just ExecStart.

I think this change should be a no-op on non-systemd systems.

Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---
 .../override.conf                             |  3 ++
 overlay/usr/local/bin/random-seed-add         | 33 +++++++++++++++++++
 2 files changed, 36 insertions(+)
 create mode 100644 
overlay/etc/systemd/system/systemd-random-seed.service.d/override.conf
 create mode 100755 overlay/usr/local/bin/random-seed-add

diff --git 
a/overlay/etc/systemd/system/systemd-random-seed.service.d/override.conf 
b/overlay/etc/systemd/system/systemd-random-seed.service.d/override.conf
new file mode 100644
index 00000000..f6cc0f84
--- /dev/null
+++ b/overlay/etc/systemd/system/systemd-random-seed.service.d/override.conf
@@ -0,0 +1,3 @@
+[Service]
+ExecStart=
+ExecStart=/usr/local/bin/random-seed-add /var/lib/systemd/random-seed
diff --git a/overlay/usr/local/bin/random-seed-add 
b/overlay/usr/local/bin/random-seed-add
new file mode 100755
index 00000000..89e75c4d
--- /dev/null
+++ b/overlay/usr/local/bin/random-seed-add
@@ -0,0 +1,33 @@
+#!/usr/bin/perl -w
+use strict;
+
+open R, '>', '/dev/random' or die "open /dev/random: $!\n";
+R->autoflush(1);
+
+sub rndaddtoentcnt ($) {
+    my ($bits) = @_;
+    my $x = pack 'L', $bits;
+    my $r = ioctl R, 0x40045201, $x;
+    defined $r or die "RNDADDTOENTCNT: $!\n";
+}
+
+sub process_stdin ($) {
+    my ($f) = @_;
+    my $got = read STDIN, $_, 512;
+    defined $got or die "read $f: $!\n";
+    last if !$got;
+    print R $_ or die "write /dev/random: $!\n";
+    my $bits = length($_) * 8;
+    rndaddtoentcnt($bits);
+}
+
+if (!@ARGV) {
+    process_stdin('stdin');
+} else {
+    die "no options supported\n" if $ARGV[0] =~ m/^\-/;
+    foreach my $f (@ARGV) {
+        open STDIN, '<', $f or die "open for reading $f: $!\n";
+        process_stdin($f);
+    }
+}
+
-- 
2.20.1