Re: [PATCH] core/partmap: Add El Torito boot catalog parsing

[Andrei Borzenkov <arvidjaar@gmail.com>]

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В Mon, 8 Jun 2015 18:31:06 +0100
Ross Lagerwall <rosslagerwall@gmail.com> пишет:

> On Mon, Jun 08, 2015 at 07:51:44PM +0300, Andrei Borzenkov wrote:
> > В Sun,  7 Jun 2015 11:24:46 +0100
> > Ross Lagerwall <rosslagerwall@gmail.com> пишет:
> > 
> > > Add a module, part_eltorito, to allow parsing of the El Torito boot
> > > catalog into partitions. This follows the El Torito Bootable CD-ROM
> > > Format Specification Version 1.0 and the UEFI Specification 2.5.
> > > In cases where the specification is unclear, the code follows the UEFI
> > > reference implementation.
> > > 
> > > This is useful when booting CDs in UEFI mode. Before, GRUB would not be
> > > able to use the embedded ESP from which it was executed, so it would
> > > have a root and prefix set to the top level of the CD. This could result
> > > in subtle configuration bugs, because the same ISO booted from a USB
> > > disk (using isohybrid) would have its root and prefix set to the
> > > embedded ESP because it can find it through the MBR.
> > 
> > You can already access ESP on hybrid image using e.g. (cd0,msdos1); why
> > is it not enough?
> 
> This is about booting a CD in UEFI mode; it may not be a hybrid disc in
> which case it doesn't have an MBR.
> 

So you need to either add partition label or add additional driver to
grub.efi. The former works already and does not require any new code.

> Also, in the case of a hybrid image booted using a CD, the mismatch between
> the 2048 byte sectors of the CD and the 512 byte sector values used in the
> MBR means that GRUB is not able to access the correctly ESP anyway.
> 

It may depend on how you create partition table. Neither openSUSE DVD
nor images created by grub-mkrescue have problems. Did you actually try
it?

As example

bor@opensuse:~/src/grub> xorriso --indev /datastore/iso/openSUSE-13.2-DVD-x86_64.iso --report-el-torito plain --report-system-area plain
xorriso 1.3.8 : RockRidge filesystem manipulator, libburnia project.

xorriso : NOTE : Loading ISO image tree from LBA 0
xorriso : UPDATE : 6102 nodes read in 1 seconds
xorriso : NOTE : Detected El-Torito boot information which currently is set to be discarded
Drive current: -indev '/datastore/iso/openSUSE-13.2-DVD-x86_64.iso'
Media current: stdio file, overwriteable
Media status : is written , is appendable
Boot record  : El Torito , MBR isohybrid cyl-align-off
Media summary: 1 session, 2284287 data blocks, 4461m data, 31.5g free
Volume id    : 'openSUSE-13.2-DVD-x86_640051'
El Torito catalog  : 20  1
El Torito images   :   N  Pltf  B   Emul  Ld_seg  Hdpt  Ldsiz         LBA
El Torito boot img :   1  BIOS  y   none  0x0000  0x00      4        5826
El Torito boot img :   2  UEFI  y   none  0x0000  0x00      1         896
El Torito img path :   1  /boot/x86_64/loader/isolinux.bin
El Torito img opts :   1  boot-info-table isohybrid-suitable
El Torito img path :   2  /boot/x86_64/efi
System area options: 0x00000202
System area summary: MBR isohybrid cyl-align-off
ISO image size/512 : 9137148
Partition offset   : 0
MBR heads per cyl  : 0
MBR secs per head  : 0
MBR partition table:   N Status  Type        Start       Blocks
MBR partition      :   1   0x00  0xef         3584         8064
MBR partition      :   2   0x80  0x17        11648      9126528
MBR partition path :   1  /boot/x86_64/efi
bor@opensuse:~/src/grub> 

You may consider using xorriso to create image, it does quite good job
with hybrid images.

> For these reasons, I think it is useful to parse the El Torito boot
> catalog.
> 

I'm not entirely opposed to it, I just do not see compelling reasons to
do it :)

I actually think that viewing ESP as opaque binary executable and using
ISO as primary data location is more generic and useful. This way you
can have any amount of different platform binaries all sharing common
/boot/grub on ISO with common configuration. Also information in El
Torito boot image (as grub.cfg) is not as easily accessible as
information on ISO. Making it hard to understand what image actually
does.


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