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SubjectRe: [PATCH v2] init: add root=PARTUUID=UUID,PARTNROFF=%d support
On Tue,  7 Jun 2011 17:19:33 -0500
Will Drewry <wad@chromium.org> wrote:

> Expand root=PARTUUID=UUID syntax to support selecting a root partition
> by integer offset from a known, unique partition. This approach
> provides similar properties to specifying a device and partition number,
> but using the UUID as the unique path prior to evaluating the offset.
>
> For example,
> root=PARTUUID=99DE9194-FC15-4223-9192-FC243948F88B,PARTNROFF=1
> selects the partition with UUID 99DE.. then select the next
> partition.
>
> This change is motivated by a particular usecase in Chromium OS where
> the bootloader can easily determine what partition it is on (by UUID)
> but doesn't perform general partition table walking.
>
> That said, support for this model provides a direct mechanism for the
> user to modify the root partition to boot without specifically needing
> to extract each UUID or update the bootloader explicitly when the root
> partition UUID is changed (if it is recreated to be larger, for
> instance). Pinning to a /boot-style partition UUID allows the arbitrary
> root partition reconfiguration/modifications with slightly less
> ambiguity than just [dev][partition] and less stringency than the
> specific root partition UUID.
>
> At present, PARTNROFF is tied to PARTUUID use and must follow
> immediately. It is possible to expand support to other values of root=
> or to add other key-value attributes and orderings, but it seems
> unnecessary to add the infrastructure without a concrete need.
>
> ...
>
> init/do_mounts.c | 34 ++++++++++++++++++++++++++++++----
> 1 files changed, 30 insertions(+), 4 deletions(-)

I have bad news.

akpm:/usr/src/linux-3.0-rc2> grep -rl 'root=' Documentation
Documentation/scsi/sym53c8xx_2.txt
Documentation/scsi/ncr53c8xx.txt
Documentation/kernel-parameters.txt
Documentation/virtual/uml/UserModeLinux-HOWTO.txt
Documentation/virtual/lguest/lguest.txt
Documentation/filesystems/nfs/nfsroot.txt
Documentation/filesystems/affs.txt
Documentation/filesystems/ubifs.txt
Documentation/m68k/kernel-options.txt
Documentation/power/swsusp-dmcrypt.txt
Documentation/early-userspace/README
Documentation/kdump/kdump.txt
Documentation/devicetree/booting-without-of.txt
Documentation/security/Smack.txt
Documentation/md.txt
Documentation/ia64/xen.txt
Documentation/intel_txt.txt
Documentation/initrd.txt
Documentation/x86/boot.txt
Documentation/sound/oss/mwave
Documentation/arm/SA1100/Assabet
Documentation/frv/booting.txt
Documentation/networking/cs89x0.txt
Documentation/networking/cxgb.txt
Documentation/init.txt

Please work out which Documentation files need updating?

> index c0851a8..35329b2 100644
> --- a/init/do_mounts.c
> +++ b/init/do_mounts.c
> @@ -85,12 +85,15 @@ no_match:
>
> /**
> * devt_from_partuuid - looks up the dev_t of a partition by its UUID
> - * @uuid: 36 byte char array containing a hex ascii UUID
> + * @uuid: min 36 byte char array containing a hex 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
> * by filesystem UUIDs.
> *
> + * If @uuid is followed by a ",PARTNROFF=%d", then the number will be
> + * extracted and used as an offset from the partition identified by the UUID.
> + *
> * Returns the matching dev_t on success or 0 on failure.
> */
> static dev_t devt_from_partuuid(char *uuid_str)
> @@ -98,6 +101,16 @@ static dev_t devt_from_partuuid(char *uuid_str)
> dev_t res = 0;
> struct device *dev = NULL;
> u8 uuid[16];
> + struct gendisk *disk;
> + struct hd_struct *part;
> + int offset = 0;
> +
> + if (strlen(uuid_str) < 36)
> + goto done;
> +
> + /* Check for optional partition number offset attributes. */
> + if (uuid_str[36])
> + sscanf(&uuid_str[36], ",PARTNROFF=%d", &offset);

Syntax errors here will be silently ignored. I expect that's common in
this area of code, but please take a look, see if we should do
something to help the poor user.

> @@ -143,8 +171,6 @@ dev_t name_to_dev_t(char *name)
> #ifdef CONFIG_BLOCK
> if (strncmp(name, "PARTUUID=", 9) == 0) {
> name += 9;
> - if (strlen(name) != 36)
> - goto fail;
> res = devt_from_partuuid(name);
> if (!res)
> goto fail;

The code changes the behaviour of name_to_dev_t(), so we should
consider all callers. That's hibernation, block2mtd and md. Have you
thought about (or perhaps tested) these code paths? Does it make sense
to use the extended syntax in these cases? Is it possible to do so?
Does it work? etc.

Thanks.


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