Merge lp:~mwhudson/debian-cd/half-the-grubs into lp:~ubuntu-cdimage/debian-cd/ubun3

[Michael Hudson-Doyle]

go back to two copies of grub on the ISO, not four

Staring hard at the previous command line and some experimentation got me there
in the end. Read the comments for more explanation.

[Michael Hudson-Doyle (mwhudson)]

ping

[Paride Legovini (paride)]

Two inline comments, of which one I think it's an actual thing to fix.

review: Needs Fixing

[Paride Legovini (paride)]

The static validation of the Groovy images is currently failing because UTAH is unable to find the EFI files, but the images themselves work. I'd prefer to see this MP land before working on UTAH to avoid fixing it twice :)

The ISO testing jobs are currently only testing the images booting them in cdrom mode (both bios and uefi). I'd like to add jobs to also test the "boot as disk" path at some point, as that's certainly the most common scenario nowadays.

[lp:~mwhudson/debian-cd/half-the-grubs]

2082. By Michael Hudson-Doyle on 2020-09-10

review feedback

[Michael Hudson-Doyle (mwhudson)]

Thanks for the review! I'd like this to land soon too :/

[Steve Langasek (vorlon)]

We'll have to subject this to testing to figure out in the end if it's correct.

review: Approve

=== modified file 'tools/boot/groovy/boot-amd64'
--- tools/boot/groovy/boot-amd64	2020-09-10 23:09:52 +0000
+++ tools/boot/groovy/boot-amd64	2020-09-10 23:09:53 +0000
@@ -36,14 +36,11 @@
 mkdir cd-boot-images
 $BASEDIR/tools/apt-selection download cd-boot-images-amd64
 dpkg --fsys-tarfile cd-boot-images-amd64*_all.deb | tar xf - -C cd-boot-images
-
-# --efi-boot -efi-boot-part --efi-boot-image seem to generate broken
-# .iso, specifically despite -partition_offset 16, the first partition
-# is mountable, but results in I/O errors without anything
-# readable. Thus use just append_partition syntax to add ESP & -e
-# "more rawly" way to specify ESP.
-#
-# TODO test if --efi-boot-image can be used instead of -e
+# we should add isohdpfx.bin to cd-boot-images and then the next three lines
+# will not be needed.
+mkdir syslinux
+$BASEDIR/tools/apt-selection download isolinux
+dpkg --fsys-tarfile isolinux*_all.deb | tar xf - -C syslinux ./usr/lib
 
 mkisofs_opts ()
 {
@@ -77,11 +74,15 @@
 
 # The bootloader to write to the MBR for legacy-disk boot.
 #
-# Another approach is to use the very small isohdpfx.bin from isolinux
-# and -isohybrid-mbr to boot from a disk (isohdpfx.bin just jumps to
-# the legacy el torito boot block set up above) but this didn't work
-# when mwhudson tested it in July 2020.
-mkisofs_opts --grub2-mbr cd-boot-images/usr/share/cd-boot-images-amd64/tree/boot/grub/i386-pc/boot_hybrid.img
+# We use the very small isohdpfx.bin from isolinux and -isohybrid-mbr
+# to boot from a disk which just jumps to the legacy el torito boot
+# block set up above.
+#
+# We also needs -isohybrid-mbr to have the -isohybrid-gpt-basdat below
+# to have any effect. Newer xorriso supports a "-part_like_isohybrid"
+# option which makes -isohybrid-gpt-basdat work without -isohybrid-mbr
+# but it's not in the xorriso in Xenial.
+mkisofs_opts -isohybrid-mbr syslinux/usr/lib/ISOLINUX/isohdpfx.bin
 
 ## Set up the mkisofs options for UEFI boot.
 
@@ -99,18 +100,14 @@
 
 # Add an ESP for uefi-disk boot.
 #
-# This is a very by hand way of doing it: it is possible to create a
-# partition table that points to the FAT filesystem above that will be
-# used by the uefi-cdrom boot. But it seems this leads to xorriso
-# creating a GPT that does not work; instead of creating a GPT with
-# overlapping partitions (which is not allowed by the spec) like so:
-#   | partition 1 / p1                                 |
-#             | ESP |
-# it splits the "main" partition so the partitions look like this:
-#   | p1      | ESP | p3                               |
-# which doesn't work at all when we mount p1 (it doesn't contain all
-# the files!)
-mkisofs_opts -append_partition 2 0xef cd-boot-images/usr/share/cd-boot-images-amd64/tree/boot/grub/efi.img
+# This isn't what this command is documented to do (which is "Mark El
+# Torito boot image as Basic Data in GPT") but the effect of it is to
+# create an invalid (!) GPT and a MBR with two partitions, one of type
+# 0 covering the whole ISO and one of type 0xef (i.e. ESP) pointing at
+# the boot/grub/efi.img specified above, which perhaps more by luck
+# than judgement allows the ISO to boot when used as a disk in an UEFI
+# system.
+mkisofs_opts -isohybrid-gpt-basdat
 
 ## Create a partition table entry that covers the iso9660 filesystem.