How can I get hostapd_cli to work under sudo on debian stretch?

I have a bash script that runs hostapd_cli all_sta, and the script executes successfully from the command line under both jessie and stretch. The script also works when run under sudo on jessie but not on stretch. On stretch the command times out with the error 'STA-FIRST' command timed out. When I invoke hostapd_cli under strace I see that it opens a socket file under /tmp: bind(3, {sa_family=AF_UNIX, sun_path="/tmp/wpa_ctrl_13552-1"}, 110) = 0 connect(3, {sa_family=AF_UNIX, sun_path="/var/run/hostapd/wlan1"}, 110) = 0 As a test I temporarily modified the script and added a line:

echo "this is a test" >/tmp/test 2>/root/error

When the modified script runs under sudo, the file in /tmp is not created and no error is written to /tmp/error. On my system, /tmp is not a tmpfs, just a plain old directory under / on an ext3 filesystem. So root is unable to create a file under /tmp and there is ample space. # df -h /tmp Filesystem Size Used Avail Use% Mounted on /dev/sdb2 6.7G 5.1G 1.4G 80% / And an ls -ld /tmp gives: # ls -ld /tmp drwxrwxrwt 9 root root 4096 Jul 27 23:50 /tmp/ If I can figure out why /tmp can't be written to, I believe the hostapd_cli command will work. What could be happening here?