Something filters out TMPDIR, even for classic snaps

snapd
1

As reported here https://bugs.launchpad.net/snapd/+bug/1682308:

After snap install --classic go:

$ cat tmpdir.go
package main

import (
 "fmt"
 "os"
)

func main() {
 fmt.Printf("%q\n", os.Getenv("TMPDIR"))
}
$ TMPDIR=/tmp go run tmpdir.go
"/tmp"
$ TMPDIR=/tmp /snap/bin/go run tmpdir.go
""

This is surprising.

2

I can see that there is code in snap-confine for setting TMPDIR, but that doesn’t run (afaict) for a classic snap, and anyway certainly doesn’t unset TMPDIR, rather the opposite…

3

So we found that the dynamic linked does unset TMPDIR (but not TEMPDIR) for setuid root executables… We also found a small bug in snap-confine where those two would only be set once (this is now https://github.com/snapcore/snapd/pull/3188).

We could use a hack where snap-run would save those in, say, SNAPD_TMPDIR and snap-confine would restore TMPDIR to $SNAPD_TMPDIR but I’m not sure if this is worth the effort.