block: partition: msdos: provide UUIDs for partitions

The MSDOS/MBR partition table includes a 32-bit unique ID, often referred
to as the NT disk signature.  When combined with a partition number within
the table, this can form a unique ID similar in concept to EFI/GPT's
partition UUID.  Constructing and recording this value in struct
partition_meta_info allows MSDOS partitions to be referred to on the
kernel command-line using the following syntax:

root=PARTUUID=0002dd75-01

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Will Drewry <wad@chromium.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/partitions/msdos.c

@@ -94,6 +94,17 @@ static int aix_magic_present(struct parsed_partitions *state, unsigned char *p)
 	return ret;
 }
 
+static void set_info(struct parsed_partitions *state, int slot,
+		     u32 disksig)
+{
+	struct partition_meta_info *info = &state->parts[slot].info;
+
+	snprintf(info->uuid, sizeof(info->uuid), "%08x-%02x", disksig,
+		 slot);
+	info->volname[0] = 0;
+	state->parts[slot].has_info = true;
+}
+
 /*
  * Create devices for each logical partition in an extended partition.
  * The logical partitions form a linked list, with each entry being
@@ -106,7 +117,8 @@ static int aix_magic_present(struct parsed_partitions *state, unsigned char *p)
  */
 
 static void parse_extended(struct parsed_partitions *state,
-			   sector_t first_sector, sector_t first_size)
+			   sector_t first_sector, sector_t first_size,
+			   u32 disksig)
 {
 	struct partition *p;
 	Sector sect;
@@ -166,6 +178,7 @@ static void parse_extended(struct parsed_partitions *state,
 			}
 
 			put_partition(state, state->next, next, size);
+			set_info(state, state->next, disksig);
 			if (SYS_IND(p) == LINUX_RAID_PARTITION)
 				state->parts[state->next].flags = ADDPART_FLAG_RAID;
 			loopct = 0;
@@ -437,6 +450,7 @@ int msdos_partition(struct parsed_partitions *state)
 	struct partition *p;
 	struct fat_boot_sector *fb;
 	int slot;
+	u32 disksig;
 
 	data = read_part_sector(state, 0, &sect);
 	if (!data)
@@ -491,6 +505,8 @@ int msdos_partition(struct parsed_partitions *state)
 #endif
 	p = (struct partition *) (data + 0x1be);
 
+	disksig = le32_to_cpup((__le32 *)(data + 0x1b8));
+
 	/*
 	 * Look for partitions in two passes:
 	 * First find the primary and DOS-type extended partitions.
@@ -515,11 +531,12 @@ int msdos_partition(struct parsed_partitions *state)
 			put_partition(state, slot, start, n);
 
 			strlcat(state->pp_buf, " <", PAGE_SIZE);
-			parse_extended(state, start, size);
+			parse_extended(state, start, size, disksig);
 			strlcat(state->pp_buf, " >", PAGE_SIZE);
 			continue;
 		}
 		put_partition(state, slot, start, size);
+		set_info(state, slot, disksig);
 		if (SYS_IND(p) == LINUX_RAID_PARTITION)
 			state->parts[slot].flags = ADDPART_FLAG_RAID;
 		if (SYS_IND(p) == DM6_PARTITION)

init/do_mounts.c

@@ -189,6 +189,10 @@ static dev_t devt_from_partuuid(const char *uuid_str)
  *	   used when disk name of partitioned disk ends on a digit.
  *	6) PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF representing the
  *	   unique id of a partition if the partition table provides it.
+ *	   The UUID may be either an EFI/GPT UUID, or refer to an MSDOS
+ *	   partition using the format SSSSSSSS-PP, where SSSSSSSS is a zero-
+ *	   filled hex representation of the 32-bit "NT disk signature", and PP
+ *	   is a zero-filled hex representation of the 1-based partition number.
  *	7) PARTUUID=<UUID>/PARTNROFF=<int> to select a partition in relation to
  *	   a partition with a known unique id.
  *