snapd's mkfs.MakeWithContent() uses the host fakeroot instead of the snap-bundled one in ubuntu-image

[Łukasz Zemczak]

We noticed a regression when trying to run our latest 2.1 ubuntu-image snap on jammy. The image builds fail on the mkfs.ext4 stage with the following error:

Error: Error running mkfs: mkfs.ext4: /snap/core20/current/lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /usr/lib/x86_64-linux-gnu/libfakeroot/libfakeroot-sysv.so)

William dug a bit deeper and found that the reason for that is that the wrong fakeroot is used during the mkfs stage. Since ubuntu-image has been re-written in Go and using snap directly, we use the mkfs.MakeWithContent() instead of calling mkfs ourselves (to avoid any incompatibilities). But this results in using the host fakeroot instead of the bundled one. Overriding the environment with LD_LIBRARY_PATH etc. via snapcraft.yaml does not work as the environment does not seem to be passed to the mkfs call.

We suppose this could be fixed by doing a `cmd.Env = os.Environ()` right after preparing the fakeroot call in https://github.com/snapcore/snapd/blob/master/osutil/mkfs/mkfs.go#L108 , but we're not sure about the implications of that.

How should we proceed? How can we use mkfs.MakeWithContent() and have guarantee that the right fakeroot is used?

[Maciej Borzecki (maciek-borzecki)]

I belive related snapd changes are: https://github.com/snapcore/snapd/pull/11320 https://github.com/snapcore/snapd/pull/11284