Re: [PATCH] expand-on-first-boot: Allow expanding extended MBR partitions

[Harald Seiler <hws@denx.de>]

Hello Jan,

On Wed, 2020-06-10 at 14:50 +0200, Jan Kiszka wrote:
> On 10.06.20 10:02, Harald Seiler wrote:
> > Extended partitions cannot be resized like primary partitions because
> > both the last logical partition and the EBR primary partition containing
> > it need to be expanded.
> > 
> > To do this, add a second SED directive for removing the parition size of
> > the EBR partition.  This part is detected by having either type `f` (Win
> > 95 Ext') or `5` (Extended).
> > 
> > Signed-off-by: Harald Seiler <hws@denx.de>
> > ---
> > 
> > Notes:
> >     I have tested this in an ongoing project and it seems to work
> >     reliably.  For some reason WIC creates an extended partition of
> >     type `f` but this can't be kept because while fdisk can read it,
> >     it is only happy about writing the EBR with type `5`.
> 
> Wikipedia says about 0x5:
> 
> "Extended partition with CHS addressing. It must reside within the first
> physical 8 GB of disk, else use 0Fh instead"
> 
> So we likely need to account for that case as well.
> 

Hm, this is getting quite complicated ... And I suppose this is only one
of many corner cases.  I'm not sure how much effort is worth here: This
change is only relevant to legacy systems that (have to) use MBR *and*
deal with more than 4 partitions which is most likely not too many.

What do you think?  Is it likely that other projects would benefit from
a general solution or would it make more sense to keep this a) project
specific or b) detect one supported case and only do EBR expansion for
that one.

> >     
> >     While probably a very uncommon layout, I am not sure how this
> >     would behave when the EBR is not the last partition, but
> >     somewhere in between.  I am a bit worried it would silently wreak
> >     havoc ...
> 
> We should probably throw some test patterns at that. Could be done
> against a disk image file as well, I suppose.
> 
> >  .../expand-on-first-boot/files/expand-last-partition.sh      | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh b/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> > index 08c69db30529..ddf1a089e87d 100755
> > --- a/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> > +++ b/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> > @@ -17,12 +17,13 @@ if [ "${ROOT_DEV}" = "${BOOT_DEV}" ]; then
> >  	exit 1
> >  fi
> >  
> > -LAST_PART="$(sfdisk -d ${BOOT_DEV} 2>/dev/null | tail -1 | cut -d ' ' -f 1)"
> > +LAST_PART="$(sfdisk -d "${BOOT_DEV}" 2>/dev/null | tail -1 | cut -d ' ' -f 1)"
> >  
> >  # Remove all hints to the current medium (last-lba) and last partition size,
> >  # then ask sfdisk to recreate the partitioning
> 
> The comment should likely be updated as well.
> 
> >  sfdisk -d "${BOOT_DEV}" 2>/dev/null | grep -v last-lba | \
> > -	sed 's|\('"${LAST_PART}"' .*, \)size=[^,]*, |\1|' | \
> > +	sed 's|^\(.*, \)size=[^,]*, type=[f5]$|\1type=5|' | \
> > +	sed 's|^\('"${LAST_PART}"' .*, \)size=[^,]*, |\1|' | \
> 
> Would it make things worse or better to combine both sed expressions
> into the same sed call?
> 

I don't think either option would make this prettier.  I think the most
reasonable way forward is documenting what exactly is done with those
expressions.

> >  	sfdisk --force "${BOOT_DEV}"
> >  
> >  # Inform the kernel about the partitioning change
> > 
> 
> Jan
> 
> -- 
> Siemens AG, Corporate Technology, CT RDA IOT SES-DE
> Corporate Competence Center Embedded Linux
> 
-- 
Harald

DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
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