`libglycin` image loader breaks GTK in `base: core26` snaps due to sandboxing

[Saviq]

26.04 introduces glycin, an image loader that sandboxes each image parser to improve the security stance in case of malicious images.

This doesn't work within snaps b/c it doesn't have privileges to create nested sandboxes for the loaders. This ends up killing any app trying to load images:

Gtk:ERROR:../../../gtk/gtkiconhelper.c:495:ensure_surface_for_gicon: assertion failed (error == NULL): Failed to load /snap/ubuntu-frame-osk/1409/usr/share/icons/Adwaita/scalable/status/image-missing.svg: Loader process exited early with status '1'Command:
 "bwrap" "--unshare-all" "--die-with-parent" "--chdir" "/" "--ro-bind" "/usr" "/usr" "--dev" "/dev" "--ro-bind-try" "/etc/ld.so.cache" "/etc/ld.so.cache" "--ro-bind-try" "/nix/store" "/nix/store" "--tmpfs" "/tmp-home" "--tmpfs" "/tmp-run" "--clearenv" "--setenv" "HOME" "/tmp-home" "--setenv" "XDG_RUNTIME_DIR" "/tmp-run" "--setenv" "XDG_RUNTIME_DIR" "/run/user/1000/snap.ubuntu-frame-osk" "--symlink" "/usr/lib64" "/lib64" "--symlink" "/usr/lib32" "/lib32" "--symlink" "/usr/lib" "/lib" "--ro-bind-try" "/home/michal/snap/ubuntu-frame-osk/1409/.cache/fontconfig" "/home/michal/snap/ubuntu-frame-osk/1409/.cache/fontconfig" "--ro-bind-try" "/snap/ubuntu-frame-osk/1409/etc/fonts/conf.d" "/snap/ubuntu-frame-osk/1409/etc/fonts/conf.d" "--ro-bind-try" "/snap/ubuntu-frame-osk/1409/etc/fonts/fonts.conf" "/snap/ubuntu-frame-osk/1409/etc/fonts/fonts.conf" "--ro-bind-try" "/snap/ubuntu-frame-osk/1409/usr/share/fonts" "/snap/ubuntu-frame-osk/1409/usr/share/fonts" "--ro-bind-try" "/var/cache/fontconfig" "/var/cache/fontconfig" "--bind-try" "/home/michal/snap/ubuntu-frame-osk/1409/.cache/glycin/usr/libexec/glycin-loaders/2+/glycin-svg" "/home/michal/snap/ubuntu-frame-osk/1409/.cache/glycin/usr/libexec/glycin-loaders/2+/glycin-svg" "--setenv" "XDG_CACHE_HOME" "/home/michal/snap/ubuntu-frame-osk/1409/.cache/glycin/usr/libexec/glycin-loaders/2+/glycin-svg" "--seccomp" "18" "/usr/libexec/glycin-loaders/2+/glycin-svg" "--dbus-fd" "17" (gdk-pixbuf-error-quark, 0)

Glycin would need to be patched (in the archive? in GNOME SDK?) to work around this when in a snapped environment. It's trivial to make it skip the sandboxing:

https://gitlab.gnome.org/GNOME/glycin/-/blob/main/glycin-core/src/sandbox.rs#L706-765

But that's ill-advised, and even then not enough, as the environment isn't preserved so e.g. LD_LIBRARY_PATH is stripped, making the loaders miss .so dependencies.

[3v1n0]

Glycin has already the ability to run without its own sandbox (bubblewrap) when running inside another sandbox, and there is code for this.

The detection mechanism was quite fragile in the past due to bubblewrap way of errors handling, but this should be fixed now.

However there may still some scenarios in which Glycin is not handling this in the right way, so I think we should figure out the root cause and propose an upstream fix

[aleasto]

The fallback from glycin doesn't work within snap because:

1. bwrap fails earlier, with a different error message ("bwrap: Can't read /proc/sys/kernel/overflowuid: Permission denied")
2. Even if we manage to fallback, glycin still clears the environment of the loader subprocess, which makes LD_LIBRARY_PATH unset and some necessary libraries missing