[PATCH v2 2/3] common: machine_id: introduce machine id generation and pass id on

[Bastian Krause <bst@pengutronix.de>]

By default systemd generates a machine id on first boot and tries to
persist it (see `man machine-id`). When the root file system is read-only
systemd cannot persist the machine id. In case multiple redundant slots
are used the machine id will vary. When not handled explicitly the
machine id will also change during updates.

It is possible to pass a machine id to the kernel which will be used by
systemd (systemd.machine_id=).

This adds functionality to pass device-specific information that will be
hashed to generate a persistent unique machine id. The machine id will
be finally added to the kernel parameters via the
linux.bootargs.machine_id global variable.

Note: if multiple sources provide hashable device-specific information
(via machine_id_set_hashable()) the information provided by the last call
prior to the late initcall set_machine_id() is used to generate the
machine id from. Thus when updating barebox the machine id might change.

Signed-off-by: Bastian Krause <bst@pengutronix.de>
---
Changes since (implicit) v1:
  - depend on SHA1 instead of selecting DIGEST/DIGEST_SHA1_GENERIC
  - add note about multiple sources providing hashables to kconfig
  - add note about no hashable provided to kconfig
  - remove warning about "no hashable info provided" along with pr_fmt
  - make machine_id_set_hashable()'s hashable parameter const
  - make a copy of the hashable provided rather than storing the pointer
  - hash data instead of pointer address
  - use basprintf() and dedicated variables for hex machine id/machine id
    bootarg
  - add static inline wrapper if CONFIG_MACHINE_ID is disabled
---
 common/Kconfig       | 18 +++++++++++++
 common/Makefile      |  1 +
 common/machine_id.c  | 63 ++++++++++++++++++++++++++++++++++++++++++++
 include/machine_id.h | 16 +++++++++++
 4 files changed, 98 insertions(+)
 create mode 100644 common/machine_id.c
 create mode 100644 include/machine_id.h

diff --git a/common/Kconfig b/common/Kconfig
index 8aad5baecd..7be2487a20 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -982,6 +982,24 @@ config RESET_SOURCE
 	  of the reset and why the bootloader is currently running. It can be
 	  useful for any kind of system recovery or repair.
 
+config MACHINE_ID
+	bool "pass machine-id to kernel"
+	depends on FLEXIBLE_BOOTARGS
+	depends on SHA1
+	help
+	  Sets the linux.bootargs.machine_id global variable with a value of
+	  systemd.machine_id=UID. The UID is a persistent device-specific
+	  id. It is a hash over device-specific information provided by various
+	  sources.
+
+	  Note: if multiple sources provide hashable device-specific information
+	  (via machine_id_set_hashable()) the information provided by the last call
+	  prior to the late initcall set_machine_id() is used to generate the
+	  machine id from. Thus when updating barebox the machine id might change.
+
+	  Note: if no hashable information is available no machine id will be passed
+	  to the kernel.
+
 endmenu
 
 menu "Debugging"
diff --git a/common/Makefile b/common/Makefile
index a284655fc1..10960169f9 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -11,6 +11,7 @@ obj-y				+= bootsource.o
 obj-$(CONFIG_ELF)		+= elf.o
 obj-y				+= restart.o
 obj-y				+= poweroff.o
+obj-$(CONFIG_MACHINE_ID)	+= machine_id.o
 obj-$(CONFIG_AUTO_COMPLETE)	+= complete.o
 obj-y				+= version.o
 obj-$(CONFIG_BAREBOX_UPDATE)	+= bbu.o
diff --git a/common/machine_id.c b/common/machine_id.c
new file mode 100644
index 0000000000..f2eeea0f8e
--- /dev/null
+++ b/common/machine_id.c
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2019 Pengutronix, Bastian Krause <kernel@pengutronix.de>
+ */
+
+#include <common.h>
+#include <init.h>
+#include <digest.h>
+#include <globalvar.h>
+#include <crypto/sha.h>
+#include <machine_id.h>
+
+#define MACHINE_ID_LENGTH 32
+
+static void *__machine_id_hashable;
+static size_t __machine_id_hashable_length;
+
+
+void machine_id_set_hashable(const void *hashable, size_t len)
+{
+
+	__machine_id_hashable = xmemdup(hashable, len);
+	__machine_id_hashable_length = len;
+}
+
+static int machine_id_set_bootarg(void)
+{
+	struct digest *digest = NULL;
+	unsigned char machine_id[SHA1_DIGEST_SIZE];
+	char hex_machine_id[MACHINE_ID_LENGTH];
+	char *machine_id_bootarg;
+	int ret = 0;
+
+	/* nothing to do if no hashable information provided */
+	if (!__machine_id_hashable)
+		goto out;
+
+	digest = digest_alloc_by_algo(HASH_ALGO_SHA1);
+	ret = digest_init(digest);
+	if (ret)
+		goto out;
+
+	ret = digest_update(digest, __machine_id_hashable,
+			    __machine_id_hashable_length);
+	if (ret)
+		goto out;
+
+	ret = digest_final(digest, machine_id);
+	if (ret)
+		goto out;
+
+	/* use the first 16 bytes of the sha1 hash as the machine id */
+	bin2hex(hex_machine_id, machine_id, MACHINE_ID_LENGTH/2);
+
+	machine_id_bootarg = basprintf("systemd.machine_id=%.*s", MACHINE_ID_LENGTH, hex_machine_id);
+	globalvar_add_simple("linux.bootargs.machine_id", machine_id_bootarg);
+
+out:
+	digest_free(digest);
+	return ret;
+
+}
+late_initcall(machine_id_set_bootarg);
diff --git a/include/machine_id.h b/include/machine_id.h
new file mode 100644
index 0000000000..31d5e0bb28
--- /dev/null
+++ b/include/machine_id.h
@@ -0,0 +1,16 @@
+#ifndef __MACHINE_ID_H__
+#define __MACHINE_ID_H__
+
+#if IS_ENABLED(CONFIG_MACHINE_ID)
+
+void machine_id_set_hashable(const void *hashable, size_t len);
+
+#else
+
+static inline void machine_id_set_hashable(const void *hashable, size_t len)
+{
+}
+
+#endif /* CONFIG_MACHINE_ID */
+
+#endif  /* __MACHINE_ID_H__ */
-- 
2.20.1


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