Merge ~mwhudson/casper:lp-1926137 into casper:main

[Michael Hudson-Doyle]

only use a partition from the blockdev holding the livefs for auto logs

https://bugs.launchpad.net/ubuntu/+source/casper/+bug/1926137 describes
a situation where a partition called "writable" on the disk the user
wanted to install to was mounted and then caused the install to fail.
To avoid this, change things to only write logs to a partition on the
same block device as that holding the livefs by default.

I would really really really like to be able to rewrite casper in an actually good programming language.

[Steve Langasek (vorlon)]

review: Needs Fixing

[Steve Langasek (vorlon)]

review: Approve

diff --git a/debian/changelog b/debian/changelog
index 3758744..67e8f12 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,10 @@
+casper (1.464) UNRELEASED; urgency=medium
+
+  * Only use a partition from the blockdev holding the livefs for auto logs.
+    (LP: #1926137)
+
+ -- Michael Hudson-Doyle <[e-mail]>  Mon, 19 Jul 2021 12:39:36 +1200
+
 casper (1.463) impish; urgency=medium
 
   * Allow LAYERFS_PATH to be set in configuration files.
diff --git a/scripts/casper b/scripts/casper
index 307a2c7..6438a13 100644
--- a/scripts/casper
+++ b/scripts/casper
@@ -842,6 +842,59 @@ find_livefs() {
     return 1
 }
 
+setup_auto_log_persistence() {
+    # If there is a writable (or casper-rw) filesystem on a partition
+    # of the same block device we found the live filesystem on and
+    # we're not already using it for persistence, use it for logs and
+    # crash reports.
+    #
+    # The goal here is to do this in the common case where the image
+    # has been dd-ed onto a USB stick that has some extra space that
+    # can be used for logs. Any kind of deviation from this scenario
+    # and we just abort to avoid situations like
+    # https://bugs.launchpad.net/ubuntu/+source/casper/+bug/1926137
+    # where a partition called "writable" on the disk the user wanted
+    # to install to was mounted and then caused the install to fail.
+    if [ -n "$PERSISTENT" ] || [ -n "$NOPERSISTENT" ]; then
+        return
+    fi
+    if [ ! -e "/dev/disk/by-label/$(root_persistence_label)" ]; then
+        return
+    fi
+    livefs_src_dev=$(what_is_mounted $mountpoint)
+    if [ -z "$livefs_src_dev" ]; then
+        return
+    fi
+    livefs_src_sysfs=$(readlink -f "/sys/class/block/$(basename $livefs_src_dev)")
+    if [ ! -f $livefs_src_sysfs/partition ]; then
+        return
+    fi
+    livefs_dev=$(cat $livefs_src_sysfs/../dev)
+    pers_dev=$(readlink -f "/dev/disk/by-label/$(root_persistence_label)")
+    pers_sysfs=$(readlink -f "/sys/class/block/$(basename $pers_dev)")
+    if [ ! -f $pers_src_sysfs/partition ]; then
+        return
+    fi
+    pers_dev=$(cat $pers_src_sysfs/../dev)
+    if [ $livefs_dev != $pers_dev ]; then
+        return
+    fi
+    mkdir /log-persistence
+    if mount -w "/dev/disk/by-label/$(root_persistence_label)" /log-persistence; then
+        local i=0 d=$(date -uIdate) pdir
+        while :; do
+            pdir=/log-persistence/install-logs-$d.$i
+            if [ ! -e $pdir ]; then
+                mkdir -p $pdir/log $pdir/crash
+                mount --bind $pdir/log "${rootmnt}/var/log"
+                mount --bind $pdir/crash "${rootmnt}/var/crash"
+                break
+            fi
+            i=$((i+1))
+        done
+    fi
+}
+
 mountroot() {
     exec 6>&1
     exec 7>&2
@@ -922,24 +975,7 @@ mountroot() {
 
     mount_images_in_directory "${livefs_root}" "${rootmnt}"
 
-    if [ -z "$PERSISTENT" ] && [ -z "$NOPERSISTENT" ] && [ -e "/dev/disk/by-label/$(root_persistence_label)" ]; then
-        # If there is a casper-rw filesystem and we're not using it
-        # for persistence, use it for logs and crash reports.
-        mkdir /log-persistence
-        if mount -w "/dev/disk/by-label/$(root_persistence_label)" /log-persistence; then
-            local i=0 d=$(date -uIdate) pdir
-            while :; do
-                pdir=/log-persistence/install-logs-$d.$i
-                if [ ! -e $pdir ]; then
-                    mkdir -p $pdir/log $pdir/crash
-                    mount --bind $pdir/log "${rootmnt}/var/log"
-                    mount --bind $pdir/crash "${rootmnt}/var/crash"
-                    break
-                fi
-                i=$((i+1))
-            done
-        fi
-    fi
+    setup_auto_log_persistence
 
     # initialize the /var/crash directory in overlayfs so that inotify for
     # /var/crash works and update-notifier will notify of crashes
diff --git a/scripts/casper-helpers b/scripts/casper-helpers
index 183587d..6fed1f9 100644
--- a/scripts/casper-helpers
+++ b/scripts/casper-helpers
@@ -65,6 +65,10 @@ where_is_mounted() {
     return 1
 }
 
+what_is_mounted() {
+    cat /proc/mounts | awk -v P=$1 '$2 == P { print $1 }'
+}
+
 lastline() {
     while read lines ; do
         line=${lines}