[PATCH 18/19] init: discover root by DPS partition type UUID

[Vincent Mailhol]

When the root= cmdline option is absent, try to discover the root block
device using the architecture's DPS root partition type UUID.

DPS limits root discovery to the disk containing the active EFI System
Partition. Read LoaderDevicePartUUID from the Boot Loader Interface and
pass it to early_lookup_bdev_by_type_uuid() so the block lookup only
considers partitions on that disk.

Print a dedicated wait message while waiting for a discoverable root
partition and emit an informational message when discovery succeeds.

If LoaderDevicePartUUID cannot be read or does not contain a valid UUID,
clear root_wait so the kernel does not keep retrying a discovery path
that cannot succeed.

Signed-off-by: Vincent Mailhol <mailhol@xxxxxxxxxx>
---
 init/do_mounts.c | 89 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 87 insertions(+), 2 deletions(-)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 5fb5aeb88da9..20c176945b32 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -10,6 +10,7 @@
 #include <linux/delay.h>
 #include <linux/mount.h>
 #include <linux/device.h>
+#include <linux/efi.h>
 #include <linux/init.h>
 #include <linux/fs.h>
 #include <linux/initrd.h>
@@ -19,6 +20,8 @@
 #include <linux/ramfs.h>
 #include <linux/shmem_fs.h>
 #include <linux/ktime.h>
+#include <linux/ucs2_string.h>
+#include <linux/uuid.h>
 
 #include <linux/nfs_fs.h>
 #include <linux/nfs_fs_sb.h>
@@ -402,9 +405,86 @@ void __init mount_root(char *root_device_name)
 	}
 }
 
+#ifdef CONFIG_DPS_ROOT_AUTO_DISCOVERY
+static char efi_partuuid[EFI_VARIABLE_GUID_LEN + 1] __initdata;
+
+static int __init efi_loader_get_device_part_uuid(char *efi_uuid, size_t size)
+{
+	efi_char16_t efi_uuid_ucs2[EFI_VARIABLE_GUID_LEN + 1] = {};
+	unsigned long efi_uuid_ucs2_size = sizeof(efi_uuid_ucs2);
+	efi_status_t status;
+
+	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE))
+		return -EOPNOTSUPP;
+
+	status = efi.get_variable(L"LoaderDevicePartUUID",
+				  &LINUX_EFI_LOADER_ENTRY_GUID, NULL,
+				  &efi_uuid_ucs2_size, efi_uuid_ucs2);
+	if (status != EFI_SUCCESS)
+		return efi_status_to_err(status);
+
+	if (ucs2_as_utf8((u8 *)efi_uuid, efi_uuid_ucs2, size) != UUID_STRING_LEN ||
+	    !uuid_is_valid(efi_uuid))
+		return -EINVAL;
+
+	return 0;
+}
+
+static int __init lookup_dps_root(dev_t *dev)
+{
+	static const char dps_root_partition_type_uuid[] __initconst =
+		DPS_ROOT_PARTITION_TYPE_UUID;
+	int err;
+
+	err = early_lookup_bdev_by_type_uuid(dps_root_partition_type_uuid,
+					     efi_partuuid, dev);
+	if (!err)
+		pr_info("VFS: Discovered root partition with GPT type UUID %s\n",
+			dps_root_partition_type_uuid);
+
+	return err;
+}
+
+static dev_t __init try_dps_root_discovery(void)
+{
+	dev_t dev;
+	int err;
+
+	err = efi_loader_get_device_part_uuid(efi_partuuid,
+					      sizeof(efi_partuuid));
+	if (err) {
+		pr_err("VFS: Unable to get LoaderDevicePartUUID EFI variable: %pe, skipping root partition discovery\n",
+			ERR_PTR(err));
+		if (root_wait) {
+			pr_err("Disabling rootwait\n");
+			root_wait = 0;
+		}
+		return 0;
+	}
+
+	if (!lookup_dps_root(&dev))
+		return dev;
+
+	return 0;
+}
+#else
+static int __init lookup_dps_root(dev_t *dev)
+{
+	return 0;
+}
+
+static dev_t __init try_dps_root_discovery(void)
+{
+	return 0;
+}
+#endif
+
 static int __init lookup_root_device(char *root_device_name)
 {
-	return early_lookup_bdev(root_device_name, &ROOT_DEV);
+	if (root_device_name[0])
+		return early_lookup_bdev(root_device_name, &ROOT_DEV);
+	else
+		return lookup_dps_root(&ROOT_DEV);
 }
 
 /* wait for any asynchronous scanning to complete */
@@ -415,7 +495,10 @@ static void __init wait_for_root(char *root_device_name)
 	if (ROOT_DEV != 0)
 		return;
 
-	pr_info("Waiting for root device %s...\n", root_device_name);
+	if (root_device_name[0])
+		pr_info("Waiting for root device %s...\n", root_device_name);
+	else if (IS_ENABLED(CONFIG_DPS_ROOT_AUTO_DISCOVERY))
+		pr_info("Waiting for discoverable root partition...\n");
 
 	end = ktime_add_ms(ktime_get_raw(), root_wait);
 
@@ -480,6 +563,8 @@ void __init prepare_namespace(void)
 
 	if (saved_root_name[0])
 		ROOT_DEV = parse_root_device(saved_root_name);
+	else
+		ROOT_DEV = try_dps_root_discovery();
 
 	initrd_load();
 

-- 
2.53.0