Preserve random seed across reboots
At first I tried to just use systemd's systemd-random-seed, but that one
does not actually use an appropriate ioctl for persuading kernel that
entropy is there. There's a patch [1] for this, but its fate is far from
certain, and even with it, I am not completely sure that I got
everything working correctly (some boots were quick, others waited for
systemd-resolved in the same manner as before this patch).
In the end, just seeding stuff from the initrd [2] is much easier. A
downside is that the systemd unit systemd-random-seed.service will add
the contents of that file once again -- but that doesn't matter much
because we do not patch it, and therefore it does not increase kernel's
idea about available entropy.
Changes from that upstream rndaddentropy:
- larger buffer
- more conservative entropy contribution
[1] https://github.com/systemd/systemd/pull/10621 .
[2] https://github.com/rfinnie/twuewand/blob/master/rndaddentropy/rndaddentropy.c
Change-Id: Ibe3c926c241d75fb7d7c40f3df2a96813931971b
diff --git a/board/czechlight/common/patches/systemd/random-seed.patch b/board/czechlight/common/patches/systemd/random-seed.patch
new file mode 100644
index 0000000..5eed6f5
--- /dev/null
+++ b/board/czechlight/common/patches/systemd/random-seed.patch
@@ -0,0 +1,26 @@
+diff --git a/meson.build b/meson.build
+index b338886c38..a968af87ee 100644
+--- a/meson.build
++++ b/meson.build
+@@ -146,7 +146,7 @@ bootlibdir = join_paths(prefixdir, 'lib/systemd/boot/efi')
+ testsdir = join_paths(prefixdir, 'lib/systemd/tests')
+ systemdstatedir = join_paths(localstatedir, 'lib/systemd')
+ catalogstatedir = join_paths(systemdstatedir, 'catalog')
+-randomseeddir = join_paths(localstatedir, 'lib/systemd')
++randomseeddir = '/cfg/random-seed'
+ profiledir = join_paths(rootlibexecdir, 'portable', 'profile')
+
+ docdir = get_option('docdir')
+diff --git a/src/random-seed/random-seed.c b/src/random-seed/random-seed.c
+index 0c5f329756..f30f32fb61 100644
+--- a/src/random-seed/random-seed.c
++++ b/src/random-seed/random-seed.c
+@@ -17,7 +17,7 @@
+ #include "string-util.h"
+ #include "util.h"
+
+-#define POOL_SIZE_MIN 512
++#define POOL_SIZE_MIN 4096
+ #define POOL_SIZE_MAX (10*1024*1024)
+
+ static int run(int argc, char *argv[]) {