PKGBUILDs/core/linux-sun4i/0001-Backport-msdos-partition-UUIDs.patch

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2015-01-03 03:10:09 +00:00
From dbef9fc3e1410edcd553989a3ddc1120609a5ad3 Mon Sep 17 00:00:00 2001
From: Kevin Mihelich <kevin@archlinuxarm.org>
Date: Fri, 2 Jan 2015 19:50:37 -0700
Subject: [PATCH] Backport msdos partition UUIDs
Squashed commit of the following:
commit 2819ae1987539215d1ce91d80d831e0350107e1e
Author: Stephen Warren <swarren@nvidia.com>
Date: Thu Nov 8 16:12:28 2012 -0800
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>
commit a4f7d3032fb20f11722d266f678a004ec0f9f0b1
Author: Stephen Warren <swarren@nvidia.com>
Date: Thu Nov 8 16:12:27 2012 -0800
init: reduce PARTUUID min length to 1 from 36
Reduce the minimum length for a root=PARTUUID= parameter to be considered
valid from 36 to 1. EFI/GPT partition UUIDs are always exactly 36
characters long, hence the previous limit. However, the next patch will
support DOS/MBR UUIDs too, which have a different, shorter, format.
Instead of validating any particular length, just ensure that at least
some non-empty value was given by the user.
Also, consider a missing UUID value to be a parsing error, in the same
vein as if /PARTNROFF exists and can't be parsed. As such, make both
error cases print a message and disable rootwait. Convert to pr_err while
we're at it.
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>
commit 0422f1d96772bf35a14bc87f3bb603b9ff825799
Author: Stephen Warren <swarren@nvidia.com>
Date: Thu Nov 8 16:12:25 2012 -0800
block: store partition_meta_info.uuid as a string
This will allow other types of UUID to be stored here, aside from true
UUIDs. This also simplifies code that uses this field, since it's usually
constructed from a, used as a, or compared to other, strings.
Note: A simplistic approach here would be to set uuid_str[36]=0 whenever a
/PARTNROFF option was found to be present. However, this modifies the
input string, and causes subsequent calls to devt_from_partuuid() not to
see the /PARTNROFF option, which causes different results. In order to
avoid misleading future maintainers, this parameter is marked const.
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>
commit 152f0b9a9d1d0ee8c01eaa1dda689bd5d0f9d512
Author: Vivek Goyal <vgoyal@redhat.com>
Date: Wed Aug 1 12:24:18 2012 +0200
block: add partition resize function to blkpg ioctl
Add a new operation code (BLKPG_RESIZE_PARTITION) to the BLKPG ioctl that
allows altering the size of an existing partition, even if it is currently
in use.
This patch converts hd_struct->nr_sects into sequence counter because
One might extend a partition while IO is happening to it and update of
nr_sects can be non-atomic on 32bit machines with 64bit sector_t. This
can lead to issues like reading inconsistent size of a partition. Sequence
counter have been used so that readers don't have to take bdev mutex lock
as we call sector_in_part() very frequently.
Now all the access to hd_struct->nr_sects should happen using sequence
counter read/update helper functions part_nr_sects_read/part_nr_sects_write.
There is one exception though, set_capacity()/get_capacity(). I think
theoritically race should exist there too but this patch does not
modify set_capacity()/get_capacity() due to sheer number of call sites
and I am afraid that change might break something. I have left that as a
TODO item. We can handle it later if need be. This patch does not introduce
any new races as such w.r.t set_capacity()/get_capacity().
v2: Add CONFIG_LBDAF test to UP preempt case as suggested by Phillip.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Phillip Susi <psusi@ubuntu.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
block/genhd.c | 28 +++++++++++---------
block/ioctl.c | 59 +++++++++++++++++++++++++++++++++++++++---
block/partition-generic.c | 4 ++-
block/partitions/efi.c | 7 +----
block/partitions/msdos.c | 21 +++++++++++++--
include/linux/blkpg.h | 1 +
include/linux/genhd.h | 65 +++++++++++++++++++++++++++++++++++++++++++++--
init/do_mounts.c | 61 +++++++++++++++++++++++++++++---------------
8 files changed, 199 insertions(+), 47 deletions(-)
diff --git a/block/genhd.c b/block/genhd.c
index dd44885..3d9d3f59 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -154,7 +154,7 @@ struct hd_struct *disk_part_iter_next(struct disk_part_iter *piter)
part = rcu_dereference(ptbl->part[piter->idx]);
if (!part)
continue;
- if (!part->nr_sects &&
+ if (!part_nr_sects_read(part) &&
!(piter->flags & DISK_PITER_INCL_EMPTY) &&
!(piter->flags & DISK_PITER_INCL_EMPTY_PART0 &&
piter->idx == 0))
@@ -191,7 +191,7 @@ EXPORT_SYMBOL_GPL(disk_part_iter_exit);
static inline int sector_in_part(struct hd_struct *part, sector_t sector)
{
return part->start_sect <= sector &&
- sector < part->start_sect + part->nr_sects;
+ sector < part->start_sect + part_nr_sects_read(part);
}
/**
@@ -744,7 +744,6 @@ void __init printk_all_partitions(void)
struct hd_struct *part;
char name_buf[BDEVNAME_SIZE];
char devt_buf[BDEVT_SIZE];
- char uuid_buf[PARTITION_META_INFO_UUIDLTH * 2 + 5];
/*
* Don't show empty devices or things that have been
@@ -763,16 +762,11 @@ void __init printk_all_partitions(void)
while ((part = disk_part_iter_next(&piter))) {
bool is_part0 = part == &disk->part0;
- uuid_buf[0] = '\0';
- if (part->info)
- snprintf(uuid_buf, sizeof(uuid_buf), "%pU",
- part->info->uuid);
-
printk("%s%s %10llu %s %s", is_part0 ? "" : " ",
bdevt_str(part_devt(part), devt_buf),
- (unsigned long long)part->nr_sects >> 1,
- disk_name(disk, part->partno, name_buf),
- uuid_buf);
+ (unsigned long long)part_nr_sects_read(part) >> 1
+ , disk_name(disk, part->partno, name_buf),
+ part->info ? part->info->uuid : "");
if (is_part0) {
if (disk->driverfs_dev != NULL &&
disk->driverfs_dev->driver != NULL)
@@ -863,7 +857,7 @@ static int show_partition(struct seq_file *seqf, void *v)
while ((part = disk_part_iter_next(&piter)))
seq_printf(seqf, "%4d %7d %10llu %s\n",
MAJOR(part_devt(part)), MINOR(part_devt(part)),
- (unsigned long long)part->nr_sects >> 1,
+ (unsigned long long)part_nr_sects_read(part) >> 1,
disk_name(sgp, part->partno, buf));
disk_part_iter_exit(&piter);
@@ -1286,6 +1280,16 @@ struct gendisk *alloc_disk_node(int minors, int node_id)
}
disk->part_tbl->part[0] = &disk->part0;
+ /*
+ * set_capacity() and get_capacity() currently don't use
+ * seqcounter to read/update the part0->nr_sects. Still init
+ * the counter as we can read the sectors in IO submission
+ * patch using seqence counters.
+ *
+ * TODO: Ideally set_capacity() and get_capacity() should be
+ * converted to make use of bd_mutex and sequence counters.
+ */
+ seqcount_init(&disk->part0.nr_sects_seq);
hd_ref_init(&disk->part0);
disk->minors = minors;
diff --git a/block/ioctl.c b/block/ioctl.c
index ba15b2d..4476e0e8 100644
--- a/block/ioctl.c
+++ b/block/ioctl.c
@@ -13,7 +13,7 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
{
struct block_device *bdevp;
struct gendisk *disk;
- struct hd_struct *part;
+ struct hd_struct *part, *lpart;
struct blkpg_ioctl_arg a;
struct blkpg_partition p;
struct disk_part_iter piter;
@@ -36,8 +36,8 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
case BLKPG_ADD_PARTITION:
start = p.start >> 9;
length = p.length >> 9;
- /* check for fit in a hd_struct */
- if (sizeof(sector_t) == sizeof(long) &&
+ /* check for fit in a hd_struct */
+ if (sizeof(sector_t) == sizeof(long) &&
sizeof(long long) > sizeof(long)) {
long pstart = start, plength = length;
if (pstart != start || plength != length
@@ -92,6 +92,59 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
bdput(bdevp);
return 0;
+ case BLKPG_RESIZE_PARTITION:
+ start = p.start >> 9;
+ /* new length of partition in bytes */
+ length = p.length >> 9;
+ /* check for fit in a hd_struct */
+ if (sizeof(sector_t) == sizeof(long) &&
+ sizeof(long long) > sizeof(long)) {
+ long pstart = start, plength = length;
+ if (pstart != start || plength != length
+ || pstart < 0 || plength < 0)
+ return -EINVAL;
+ }
+ part = disk_get_part(disk, partno);
+ if (!part)
+ return -ENXIO;
+ bdevp = bdget(part_devt(part));
+ if (!bdevp) {
+ disk_put_part(part);
+ return -ENOMEM;
+ }
+ mutex_lock(&bdevp->bd_mutex);
+ mutex_lock_nested(&bdev->bd_mutex, 1);
+ if (start != part->start_sect) {
+ mutex_unlock(&bdevp->bd_mutex);
+ mutex_unlock(&bdev->bd_mutex);
+ bdput(bdevp);
+ disk_put_part(part);
+ return -EINVAL;
+ }
+ /* overlap? */
+ disk_part_iter_init(&piter, disk,
+ DISK_PITER_INCL_EMPTY);
+ while ((lpart = disk_part_iter_next(&piter))) {
+ if (lpart->partno != partno &&
+ !(start + length <= lpart->start_sect ||
+ start >= lpart->start_sect + lpart->nr_sects)
+ ) {
+ disk_part_iter_exit(&piter);
+ mutex_unlock(&bdevp->bd_mutex);
+ mutex_unlock(&bdev->bd_mutex);
+ bdput(bdevp);
+ disk_put_part(part);
+ return -EBUSY;
+ }
+ }
+ disk_part_iter_exit(&piter);
+ part_nr_sects_write(part, (sector_t)length);
+ i_size_write(bdevp->bd_inode, p.length);
+ mutex_unlock(&bdevp->bd_mutex);
+ mutex_unlock(&bdev->bd_mutex);
+ bdput(bdevp);
+ disk_put_part(part);
+ return 0;
default:
return -EINVAL;
}
diff --git a/block/partition-generic.c b/block/partition-generic.c
index 8d9281d..d5f9306 100644
--- a/block/partition-generic.c
+++ b/block/partition-generic.c
@@ -89,7 +89,7 @@ ssize_t part_size_show(struct device *dev,
struct device_attribute *attr, char *buf)
{
struct hd_struct *p = dev_to_part(dev);
- return sprintf(buf, "%llu\n",(unsigned long long)p->nr_sects);
+ return sprintf(buf, "%llu\n",(unsigned long long)part_nr_sects_read(p));
}
static ssize_t part_ro_show(struct device *dev,
@@ -310,6 +310,8 @@ struct hd_struct *add_partition(struct gendisk *disk, int partno,
err = -ENOMEM;
goto out_free;
}
+
+ seqcount_init(&p->nr_sects_seq);
pdev = part_to_dev(p);
p->start_sect = start;
diff --git a/block/partitions/efi.c b/block/partitions/efi.c
index 6296b40..b62fb88 100644
--- a/block/partitions/efi.c
+++ b/block/partitions/efi.c
@@ -620,7 +620,6 @@ int efi_partition(struct parsed_partitions *state)
gpt_entry *ptes = NULL;
u32 i;
unsigned ssz = bdev_logical_block_size(state->bdev) / 512;
- u8 unparsed_guid[37];
if (!find_valid_gpt(state, &gpt, &ptes) || !gpt || !ptes) {
kfree(gpt);
@@ -649,11 +648,7 @@ int efi_partition(struct parsed_partitions *state)
state->parts[i + 1].flags = ADDPART_FLAG_RAID;
info = &state->parts[i + 1].info;
- /* Instead of doing a manual swap to big endian, reuse the
- * common ASCII hex format as the interim.
- */
- efi_guid_unparse(&ptes[i].unique_partition_guid, unparsed_guid);
- part_pack_uuid(unparsed_guid, info->uuid);
+ efi_guid_unparse(&ptes[i].unique_partition_guid, info->uuid);
/* Naively convert UTF16-LE to 7 bits. */
label_max = min(sizeof(info->volname) - 1,
diff --git a/block/partitions/msdos.c b/block/partitions/msdos.c
index 5f79a66..8752a5d 100644
--- a/block/partitions/msdos.c
+++ b/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)
diff --git a/include/linux/blkpg.h b/include/linux/blkpg.h
index faf8a45..a851944 100644
--- a/include/linux/blkpg.h
+++ b/include/linux/blkpg.h
@@ -40,6 +40,7 @@ struct blkpg_ioctl_arg {
/* The subfunctions (for the op field) */
#define BLKPG_ADD_PARTITION 1
#define BLKPG_DEL_PARTITION 2
+#define BLKPG_RESIZE_PARTITION 3
/* Sizes of name fields. Unused at present. */
#define BLKPG_DEVNAMELTH 64
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 017a7fb..9747efc 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -89,16 +89,26 @@ struct disk_stats {
};
#define PARTITION_META_INFO_VOLNAMELTH 64
-#define PARTITION_META_INFO_UUIDLTH 16
+/*
+ * Enough for the string representation of any kind of UUID plus NULL.
+ * EFI UUID is 36 characters. MSDOS UUID is 11 characters.
+ */
+#define PARTITION_META_INFO_UUIDLTH 37
struct partition_meta_info {
- u8 uuid[PARTITION_META_INFO_UUIDLTH]; /* always big endian */
+ char uuid[PARTITION_META_INFO_UUIDLTH];
u8 volname[PARTITION_META_INFO_VOLNAMELTH];
};
struct hd_struct {
sector_t start_sect;
+ /*
+ * nr_sects is protected by sequence counter. One might extend a
+ * partition while IO is happening to it and update of nr_sects
+ * can be non-atomic on 32bit machines with 64bit sector_t.
+ */
sector_t nr_sects;
+ seqcount_t nr_sects_seq;
sector_t alignment_offset;
unsigned int discard_alignment;
struct device __dev;
@@ -648,6 +658,57 @@ static inline void hd_struct_put(struct hd_struct *part)
__delete_partition(part);
}
+/*
+ * Any access of part->nr_sects which is not protected by partition
+ * bd_mutex or gendisk bdev bd_mutex, should be done using this
+ * accessor function.
+ *
+ * Code written along the lines of i_size_read() and i_size_write().
+ * CONFIG_PREEMPT case optimizes the case of UP kernel with preemption
+ * on.
+ */
+static inline sector_t part_nr_sects_read(struct hd_struct *part)
+{
+#if BITS_PER_LONG==32 && defined(CONFIG_LBDAF) && defined(CONFIG_SMP)
+ sector_t nr_sects;
+ unsigned seq;
+ do {
+ seq = read_seqcount_begin(&part->nr_sects_seq);
+ nr_sects = part->nr_sects;
+ } while (read_seqcount_retry(&part->nr_sects_seq, seq));
+ return nr_sects;
+#elif BITS_PER_LONG==32 && defined(CONFIG_LBDAF) && defined(CONFIG_PREEMPT)
+ sector_t nr_sects;
+
+ preempt_disable();
+ nr_sects = part->nr_sects;
+ preempt_enable();
+ return nr_sects;
+#else
+ return part->nr_sects;
+#endif
+}
+
+/*
+ * Should be called with mutex lock held (typically bd_mutex) of partition
+ * to provide mutual exlusion among writers otherwise seqcount might be
+ * left in wrong state leaving the readers spinning infinitely.
+ */
+static inline void part_nr_sects_write(struct hd_struct *part, sector_t size)
+{
+#if BITS_PER_LONG==32 && defined(CONFIG_LBDAF) && defined(CONFIG_SMP)
+ write_seqcount_begin(&part->nr_sects_seq);
+ part->nr_sects = size;
+ write_seqcount_end(&part->nr_sects_seq);
+#elif BITS_PER_LONG==32 && defined(CONFIG_LBDAF) && defined(CONFIG_PREEMPT)
+ preempt_disable();
+ part->nr_sects = size;
+ preempt_enable();
+#else
+ part->nr_sects = size;
+#endif
+}
+
#else /* CONFIG_BLOCK */
static inline void printk_all_partitions(void) { }
diff --git a/init/do_mounts.c b/init/do_mounts.c
index 7b81405..97fb6f0 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -59,23 +59,28 @@ __setup("ro", readonly);
__setup("rw", readwrite);
#ifdef CONFIG_BLOCK
+struct uuidcmp {
+ const char *uuid;
+ int len;
+};
+
/**
* match_dev_by_uuid - callback for finding a partition using its uuid
* @dev: device passed in by the caller
- * @data: opaque pointer to a 36 byte char array with a UUID
+ * @data: opaque pointer to the desired struct uuidcmp to match
*
* Returns 1 if the device matches, and 0 otherwise.
*/
static int match_dev_by_uuid(struct device *dev, void *data)
{
- u8 *uuid = data;
+ struct uuidcmp *cmp = data;
struct hd_struct *part = dev_to_part(dev);
if (!part->info)
goto no_match;
- if (memcmp(uuid, part->info->uuid, sizeof(part->info->uuid)))
- goto no_match;
+ if (strncasecmp(cmp->uuid, part->info->uuid, cmp->len))
+ goto no_match;
return 1;
no_match:
@@ -85,7 +90,7 @@ no_match:
/**
* devt_from_partuuid - looks up the dev_t of a partition by its UUID
- * @uuid: min 36 byte char array containing a hex ascii UUID
+ * @uuid: char array containing ascii UUID
*
* The function will return the first partition which contains a matching
* UUID value in its partition_meta_info struct. This does not search
@@ -96,38 +101,41 @@ no_match:
*
* Returns the matching dev_t on success or 0 on failure.
*/
-static dev_t devt_from_partuuid(char *uuid_str)
+static dev_t devt_from_partuuid(const char *uuid_str)
{
dev_t res = 0;
+ struct uuidcmp cmp;
struct device *dev = NULL;
- u8 uuid[16];
struct gendisk *disk;
struct hd_struct *part;
int offset = 0;
+ bool clear_root_wait = false;
+ char *slash;
- if (strlen(uuid_str) < 36)
- goto done;
+ cmp.uuid = uuid_str;
+ slash = strchr(uuid_str, '/');
/* Check for optional partition number offset attributes. */
- if (uuid_str[36]) {
+ if (slash) {
char c = 0;
/* Explicitly fail on poor PARTUUID syntax. */
- if (sscanf(&uuid_str[36],
- "/PARTNROFF=%d%c", &offset, &c) != 1) {
- printk(KERN_ERR "VFS: PARTUUID= is invalid.\n"
- "Expected PARTUUID=<valid-uuid-id>[/PARTNROFF=%%d]\n");
- if (root_wait)
- printk(KERN_ERR
- "Disabling rootwait; root= is invalid.\n");
- root_wait = 0;
+ if (sscanf(slash + 1,
+ "PARTNROFF=%d%c", &offset, &c) != 1) {
+ clear_root_wait = true;
goto done;
}
+ cmp.len = slash - uuid_str;
+ } else {
+ cmp.len = strlen(uuid_str);
}
- /* Pack the requested UUID in the expected format. */
- part_pack_uuid(uuid_str, uuid);
+ if (!cmp.len) {
+ clear_root_wait = true;
+ goto done;
+ }
- dev = class_find_device(&block_class, NULL, uuid, &match_dev_by_uuid);
+ dev = class_find_device(&block_class, NULL, &cmp,
+ &match_dev_by_uuid);
if (!dev)
goto done;
@@ -148,6 +156,13 @@ static dev_t devt_from_partuuid(char *uuid_str)
no_offset:
put_device(dev);
done:
+ if (clear_root_wait) {
+ pr_err("VFS: PARTUUID= is invalid.\n"
+ "Expected PARTUUID=<valid-uuid-id>[/PARTNROFF=%%d]\n");
+ if (root_wait)
+ pr_err("Disabling rootwait; root= is invalid.\n");
+ root_wait = 0;
+ }
return res;
}
#endif
@@ -164,6 +179,10 @@ done:
* 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.
*
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2.2.1