[OE-core] [PATCH v3] boot-directdisk: mount root by MBR disk signature for Linux 3.8+

[Darren Hart]

On Wed, 2013-07-10 at 00:12 +1000, Jonathan Liu wrote:
> The root device is currently set as /dev/hda2. However, this is
> only correct if it's the first IDE drive. If booting off the first SATA
> drive instead, it would be /dev/sda2. It's not the first drive, neither
> /dev/hda2 or /dev/sda2 would be correct.
> 
> The solution to this has typically been to use the filesystem UUID to
> specify the root device but this requires extra support in the initrd.
> Linux 3.8 introduces the ability to specify the root device using the
> MBR disk signature and the partition number which is much simpler to
> use and avoids the extra overhead of an initrd.
> 
> This change uses the MBR disk signature to specify the root device when
> using Linux 3.8+ and CONFIG_BLOCK (required for root=PARTUUID=)
> is enabled in the kernel.
> 
> This has been tested with QEMU x86 and Intel Desktop Board D2500HN using
> an image recipe inheriting boot-directdisk and core-image.
> 
> Signed-off-by: Jonathan Liu <net147 at gmail.com>
> ---
>  meta/classes/boot-directdisk.bbclass | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/meta/classes/boot-directdisk.bbclass b/meta/classes/boot-directdisk.bbclass
> index efeadab..f97060f 100644
> --- a/meta/classes/boot-directdisk.bbclass
> +++ b/meta/classes/boot-directdisk.bbclass
> @@ -35,7 +35,9 @@ BOOTDD_EXTRA_SPACE ?= "16384"
>  # Get the build_syslinux_cfg() function from the syslinux class
>  
>  AUTO_SYSLINUXCFG = "1"
> -SYSLINUX_ROOT ?= "root=/dev/sda2"
> +SYSLINUX_FIXED_ROOT = "root=/dev/sda2"
> +SYSLINUX_UUID_ROOT = "root=PARTUUID=${DISK_SIGNATURE}-02"

Why  '=' and not '?=' for the above? Shouldn't the user be able to
override these?

> +SYSLINUX_ROOT ?= "${@'${SYSLINUX_UUID_ROOT}' if has_kernel_mbr_partuuid_support(d) else '${SYSLINUX_FIXED_ROOT}'}"
>  SYSLINUX_TIMEOUT ?= "10"
>  
>  inherit syslinux
> @@ -98,6 +100,10 @@ build_boot_dd() {
>  	parted $IMAGE set 1 boot on 
>  	parted $IMAGE print
>  
> +	# Disk signature generated by parted isn't really random, so use our own generated signature
> +	echo -ne "$(echo ${DISK_SIGNATURE} | fold -w 2 | tac | paste -sd '' | sed 's/\(..\)/\\x&/g')" | \
> +		dd of=$IMAGE bs=1 seek=440 conv=notrunc

Oi! So, can "fold -w 2 | tac | paste -sd '' be replaced with "rev" from
the standard util-linux package?

And the sed command... just insert "\x" every two characters? That
doesn't seem to add any more randomness.... maybe I'm missing the point
here?


> +
>  	OFFSET=`expr $END2 / 512`
>  	dd if=${STAGING_DATADIR}/syslinux/mbr.bin of=$IMAGE conv=notrunc
>  	dd if=$HDDIMG of=$IMAGE conv=notrunc seek=1 bs=512
> @@ -113,4 +119,26 @@ python do_bootdirectdisk() {
>      bb.build.exec_func('build_boot_dd', d)
>  }
>  
> +def generate_disk_signature():
> +    import uuid
> +    return str(uuid.uuid4())[:8]
> +
> +def get_kernel_version(d):
> +    import subprocess
> +    version_cmd = r"grep '^VERSION\s*=' '%s/Makefile' | grep -o '[0-9]*$'" % (d.getVar("STAGING_KERNEL_DIR", True))
> +    version = int(subprocess.Popen(version_cmd, shell=True, stdout=subprocess.PIPE).communicate()[0])
> +    patchlevel_cmd = r"grep '^PATCHLEVEL\s*=' '%s/Makefile' | grep -o '[0-9]*$'" % (d.getVar("STAGING_KERNEL_DIR", True))
> +    patchlevel = int(subprocess.Popen(patchlevel_cmd, shell=True, stdout=subprocess.PIPE).communicate()[0])
> +    return (version, patchlevel)
> +
> +def has_kernel_config_option(option, d):
> +    import subprocess
> +    grep_cmd = r"grep '^CONFIG_%s=y$' '%s/.config'" % (option, d.getVar("STAGING_KERNEL_DIR", True))
> +    return subprocess.call(grep_cmd, shell=True, stdout=subprocess.PIPE) == 0

Is there a specific kernel option to test for the MBR disk signature
support or is that inherent in the block drivers now? It would be nice
if we could skip the kernel version check, that code looks dangerously
fragile (no fault of yours).

> +
> +def has_kernel_mbr_partuuid_support(d):
> +    return get_kernel_version(d) >= (3, 8) and has_kernel_config_option("BLOCK", d)
> +
> +DISK_SIGNATURE := "${@generate_disk_signature()}"
> +
>  addtask bootdirectdisk before do_build

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Technical Lead - Linux Kernel