Performing verification for Focal.

I will first reproduce the problem with glib2.0 2.64.6-1~ubuntu20.04.3 from -security with the libglib2.0-0 libglib2.0-bin libglib2.0-data packages.

I deleted all existing schemas from /usr/share/glib-2.0/schemas and replaced them with a set of schemas which reproduce the problem easily from my customer.

$ cd /usr/share/glib-2.0/schemas/
$ sudo rm *
$ sudo cp ~/schemas/* .

The gsettings.compiled from the customer has been corrupted, and when I reboot, gdm fails to start and I get a blank screen with a blinking insertion pointer.

The sha256 of the customers corrupted gsettings.compiled is:

$ sudo openssl sha256 /usr/share/glib-2.0/schemas/gschemas.compiled
SHA256(/usr/share/glib-2.0/schemas/gschemas.compiled)= 2c98dc9a7fdbac858a8d5ca7e4dd813f16058a46dba2c54b5239cd8cdba5bb3e

When I ssh back in, and recompile the file:

$ sudo glib-compile-schemas /usr/share/glib-2.0/schemas
Error parsing key “logout” in schema “org.gnome.settings-daemon.plugins.media-keys” as specified in override file “/usr/share/glib-2.0/schemas/50_vmware_viewagent.gschema.override”: 0-22:can not parse as value of type 'as'. Ignoring override for this key.
$ sudo openssl sha256 /usr/share/glib-2.0/schemas/gschemas.compiled
SHA256(/usr/share/glib-2.0/schemas/gschemas.compiled)= 78163b5f0000efbd6320ce0d355c9531bf657a4f4dc15f057d95ef144323cd56

The sha256 has changed. Doing a bindiff, I see:

$ sudo cmp -l /home/ubuntu/schemas/gschemas.compiled /usr/share/glib-2.0/schemas/gschemas.compiled | gawk '{printf "%08X %02X %02X\n", $1, strtonum(0$2), strtonum(0$3)}'
0000376F E3 4F
00003771 A4 C2

We see two bytes different. These bytes are the uninitialised memory this bug is about. When I reboot, gdm starts fine, but that is because this time I got lucky and the parser for the gschema.compiled file thinks 4F and C2 are okay. But there are combinations which aren't okay, and will end up with a corrupted gschema.compiled file.

Re-compiling the file again:

$ sudo glib-compile-schemas /usr/share/glib-2.0/schemas
Error parsing key “logout” in schema “org.gnome.settings-daemon.plugins.media-keys” as specified in override file “/usr/share/glib-2.0/schemas/50_vmware_viewagent.gschema.override”: 0-22:can not parse as value of type 'as'. Ignoring override for this key.
$ sudo openssl sha256 /usr/share/glib-2.0/schemas/gschemas.compiled
SHA256(/usr/share/glib-2.0/schemas/gschemas.compiled)= 460c70faca7afc26fa88a0e5918d312478e15f20ad84f4afaa5d17627a823e01

The sha256 changed, and if we bindiff, the bytes have changed:

$ sudo cmp -l /home/ubuntu/schemas/gschemas.compiled /usr/share/glib-2.0/schemas/gschemas.compiled | gawk '{printf "%08X %02X %02X\n", $1, strtonum(0$2), strtonum(0$3)}'
0000376F E3 A6
00003771 A4 A1

If we run glib-compile-schemas through valgrind, it reports that we are writing to uninitialised memory:

https://paste.ubuntu.com/p/sxrQtbswpw/

I then enabled -proposed and installed libglib2.0-0 libglib2.0-bin libglib2.0-data version 2.64.6-1~ubuntu20.04.4.

Now, when I re-compile the gschemas.compiled file, the sha256 matches every time, meaning no more non-deterministic behaviour caused by writing unitialised memory to disk:

$ sudo glib-compile-schemas /usr/share/glib-2.0/schemas
Error parsing key “logout” in schema “org.gnome.settings-daemon.plugins.media-keys” as specified in override file “/usr/share/glib-2.0/schemas/50_vmware_viewagent.gschema.override”: 0-22:can not parse as value of type 'as'. Ignoring override for this key.
$ sudo openssl sha256 /usr/share/glib-2.0/schemas/gschemas.compiled
SHA256(/usr/share/glib-2.0/schemas/gschemas.compiled)= cd9132d18b596a304251cd1eb50b64aa6fd7511a312906f9a49e1975a319fbf1

$ sudo glib-compile-schemas /usr/share/glib-2.0/schemas
Error parsing key “logout” in schema “org.gnome.settings-daemon.plugins.media-keys” as specified in override file “/usr/share/glib-2.0/schemas/50_vmware_viewagent.gschema.override”: 0-22:can not parse as value of type 'as'. Ignoring override for this key.
$ sudo openssl sha256 /usr/share/glib-2.0/schemas/gschemas.compiled
SHA256(/usr/share/glib-2.0/schemas/gschemas.compiled)= cd9132d18b596a304251cd1eb50b64aa6fd7511a312906f9a49e1975a319fbf1

Doing a bindiff, I see the changed bytes from before are now all zeros, which is what the patch initialises the buffer to:

$ sudo cmp -l /home/ubuntu/schemas/gschemas.compiled /usr/share/glib-2.0/schemas/gschemas.compiled | gawk '{printf "%08X %02X %02X\n", $1, strtonum(0$2), strtonum(0$3)}'
0000376F E3 00
00003771 A4 00
00003772 55 00

Doing a run through valgrind, we no longer get a report about writing to uninitialised memory:

https://paste.ubuntu.com/p/z52DGZcdz3/

Rebooting, the VM comes up and GDM starts properly, so glib can parse the gsettings.compiled file without any issues.

Wonderful. The problem is fixed by the package in -proposed, happy to mark as verified.