something filters out TMPDIR, even for classic snaps

[Michael Hudson-Doyle]

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.

[Michael Hudson-Doyle (mwhudson)]

This turns out to be the program loader, it removes certain environment variables when AT_SECURE is set (in this case because snap-confine is setuid). The list of variables is in https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=sysdeps/generic/unsecvars.h;hb=HEAD, maybe snap run could shuffle TMPDIR (or HOSTALIASES) into SNAP_TMDIR (or SNAP_HOSTALIASES) and snap-exec could shuffle them back again.

[Michael Hudson-Doyle (mwhudson)]

I had a go at fixing this: https://github.com/snapcore/snapd/pull/3872