temp file created in different location than requested with systemd - php

Question

I tried creating a trace file with fopen("name","a") and writing to it, but although the fopen succeeds, and returns a handle, and all of the fwrite calls succeed, I cannot find the file at the location specified by the file name parameter to fopen.
$trace = fopen("/var/tmp/billing.log", "a");
if ($trace)
echo "opened /var/tmp/billing.log<br>\n";
else
echo "unable to open /var/tmp/billing.log<br>\n";
...
fwrite($trace, __FILE__ . ': ' . __LINE__ . "ce(" .
"\$aid=$aid, " .
"\$level=$level, " .
"\$id=$id, " .
"\$mode=$mode, " .
"\$nocache=$nocache)\n");
echo __FILE__ . ': ' . __LINE__ . "ce(" .
"\$aid=$aid, " .
"\$level=$level, " .
"\$id=$id, " .
"\$mode=$mode, " .
"\$nocache=$nocache)<br>\n";
The echo indicating the handle has been returned displays, as do the echos immediately after each call to fwrite, but the requested file /var/tmp/billing.log does not exist. As usual /var/tmp is:
drwxrwxrwt. 35 root root 4096 Jul 15 20:06 tmp
so writing to the directory is permitted. I tested this by manually creating a file by:
cat > /var/tmp/test
This appears to be a poorly documented or explained feature of PHP on systems using systemd. Instead of creating the file where I specified PHP instead creates the file a further layer down in a private directory!
$ cd /var/tmp
$ sudo find -name *.log
[sudo] password for jcobban:
./systemd-private-DGOkBT/tmp/recur-billing.log
./systemd-private-DGOkBT/tmp/process-registration.log
./systemd-private-uEF6lO/tmp/recur-billing.log
./systemd-private-uEF6lO/tmp/billing.log
./systemd-private-uEF6lO/tmp/process-registration.log
$ ls systemd-private-uEF6lO/
ls: cannot open directory systemd-private-uEF6lO/: Permission denied
Observe that I have to use sudo to even see the file, because I am not running with the userid of the PHP process. This is a consequence of the decision by PHP to use the private temp directory service of systemd. This action does not appear to be described anywhere in the PHP documentation. If you call the function sys_get_temp_dir it even fibs about the location of the temp files. Note that the decision to use a private temp directory does not affect PHP scripts themselves because the temporary file names are always mapped. From the point of view of scripts the files are at the location specified by the program. That is fopen("/var/tmp/billing.log", "r"); will read the file. The problem is only with the external visibility of temporary files. I haven't checked but I suspect that on Windows PHP maps /tmp and /var/tmp to the Windows temporary directory C:\Windows\Temp\ for portability.
This is a pain because in order to make these trace files visible for external analysis I now have to define and manage my own directory to put trace files in, an action which is complicated because on my site 'DOCUMENT_ROOT' is not a valid path so I have to figure out what the real path to the document root is.
$document_root = substr(__FILE__, 0, -strlen($_SERVER['PHP_SELF']));
$trace = fopen($document_root . "/log/billing.log", "a");

Answer 1

Actually this is a systemd security feature: PrivateTmp
You can disable it for your specific httpd service. On Arch Linux:
juergen#samson:/tmp → grep PrivateTmp /usr/lib/systemd/system/httpd.service
PrivateTmp=true