how does mktemp make temporary files in a directory without write permissions in ubuntu?

Question

I noticed that in ubuntu, mktemp allows a user to make temporary files in a directory even if write permissions are not allowed for that user. I thus expected the permissions on mktemp to read something like this, giving the user temporary root privileges while running it:

-rwsr-xr-x 1 root root   35392 Nov 19  2012 mktemp

but instead they read something like this:

-rwxr-xr-x 1 root root   35392 Nov 19  2012 mktemp

Where I thought there was an 's', there is an 'x', meaning mktemp does not run with root privileges. How is mktemp able to create temporary files in a directory without having the permission to do so?

Answer 1

mktemp, by default, creates a temporary directory under /tmp, which by default is writable by anyone; if you pass the --tmpdir option to mktemp (or set the TMPDIR environment variable), it will attempt to create a temporary directory in the directory given by the option's argument, and that will fail unless you have write permission in the given directory. For example:

[me@box] $ mktemp
/tmp/tmp.sL1g7rRGQv
[me@box] $ mktemp --tmpdir=/root
mktemp: failed to create file via template `/tmp.XXXXXXXXXXX': Permission denied

If you're doing, e.g., mktemp --tmpdir=/root as a non-root user and it doesn't fail, then something very weird is going on. Are you sure that's what you're seeing?