cos-kernel: Use root disk p12 as EFI partition

If a second boot disk has been mounted, the
/dev/disk/bypartlabel/EFI-SYSTEM link can be overwritten. On COS,
it's more reliable to use partition 12 of the root disk instead,
which should always be the actual EFI partition.

BUG=b/567952125
TEST=presubmit, unlock and deploy kernel on x86 and arm64

Change-Id: If44102e4f85ca14ef1b1a5667024568b43fe0afe
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/219608
Reviewed-by: Derek Taylor <ddtaylor@google.com>
Cloud-Build: 228075978874@cloudbuild.gserviceaccount.com <228075978874@cloudbuild.gserviceaccount.com>
Tested-by: Kevin Berry <kpberry@google.com>

src/pkg/kernel/scripts/cos-kernel-deploy

diff --git a/src/pkg/kernel/scripts/cos-kernel-deploy b/src/pkg/kernel/scripts/cos-kernel-deploy
index b919e98..30c9553 100755
--- a/src/pkg/kernel/scripts/cos-kernel-deploy
+++ b/src/pkg/kernel/scripts/cos-kernel-deploy
@@ -48,7 +48,31 @@
 # We have to do this before removing the existing modules since we might need
 # to load a file system driver (such as vfat) to mount the EFI partition.
 ESPMOUNT=$(mktemp -d)
-mount /dev/disk/by-label/EFI-SYSTEM "${ESPMOUNT}"
+
+# Get the EFI partition of the root device. The steps are:
+# 1. Get the block devices.
+# 2. Filter to the block dev that has a descendant with "/" as a mount point.
+# 3. Filter to the objects that have a device maj:min number.
+# 4. Filter to the leaf partitions (no children).
+# 5. Take the partition name from the 12th partition.
+#
+# This avoids several edge cases:
+# 1. The /dev/disk/by-partlabel/EFI-SYSTEM symlink can be overwritten if a second disk is
+#    mounted.
+# 2. There may be virtual mounts for the root partition, so it might not be at the top level.
+# 3. The device minor number for the 12th partition on the root device might not be 12 on
+#    NVME since NVME devices share a device major number and increment the minor number for
+#    each partition.
+readonly ESPNAME=$(
+    lsblk --json |
+        jq '.blockdevices.[]' |
+        jq 'select(.. | (.mountpoints? // []).[] == "/")' |
+        jq '.. | select(."maj:min"? // "" != "")' |
+        jq 'select(.children == null)' |
+        jq -sr '.[11].name'
+)
+readonly ESPDEV="/dev/${ESPNAME}"
+mount "${ESPDEV}" "${ESPMOUNT}"
 
 if [ -d "/lib/modules/${VERSION}" ]; then
     rm -rf "/lib/modules/${VERSION}"

src/pkg/kernel/scripts/cos-set-dev-mode

diff --git a/src/pkg/kernel/scripts/cos-set-dev-mode b/src/pkg/kernel/scripts/cos-set-dev-mode
index cdde8cf..2c7cb99 100755
--- a/src/pkg/kernel/scripts/cos-set-dev-mode
+++ b/src/pkg/kernel/scripts/cos-set-dev-mode
@@ -29,7 +29,31 @@
   exit 1
 fi
 
-readonly ESPDEV="/dev/disk/by-partlabel/EFI-SYSTEM"
+
+# Get the EFI partition of the root device. The steps are:
+# 1. Get the block devices.
+# 2. Filter to the block dev that has a descendant with "/" as a mount point.
+# 3. Filter to the objects that have a device maj:min number.
+# 4. Filter to the leaf partitions (no children).
+# 5. Take the partition name from the 12th partition.
+#
+# This avoids several edge cases:
+# 1. The /dev/disk/by-partlabel/EFI-SYSTEM symlink can be overwritten if a second disk is
+#    mounted.
+# 2. There may be virtual mounts for the root partition, so it might not be at the top level.
+# 3. The device minor number for the 12th partition on the root device might not be 12 on
+#    NVME since NVME devices share a device major number and increment the minor number for
+#    each partition.
+readonly ESPNAME=$(
+    lsblk --json |
+        jq '.blockdevices.[]' |
+        jq 'select(.. | (.mountpoints? // []).[] == "/")' |
+        jq '.. | select(."maj:min"? // "" != "")' |
+        jq 'select(.children == null)' |
+        jq -sr '.[11].name'
+)
+ESPDEV="/dev/${ESPNAME}"
+
 readonly GRUBCFG="/efi/boot/grub.cfg"
 
 CONFIRMED="no"