Shell Script mktemp, what's the best method to create temporary named pipe?

Question

I'm aware its best to create temporary files with mktemp, but what about named pipes?

I prefer things to be as POSIX compliant as possible, but Linux only is acceptable. Avoiding Bashisms is my only hard criteria, as I write in dash.

Solution 1

tmppipe=$(mktemp -u)
mkfifo -m 600 "$tmppipe"

Unlike regular file creation, which is prone to being hijacked by an existing file or a symbolic link, the creation of a name pipe through mkfifo or the underlying function either creates a new file in the specified place or fails. Something like : >foo is unsafe because if the attacker can predict the output of mktemp then the attacker can create the target file for himself. But mkfifo foo would fail in such a scenario.

If you need full POSIX portability, mkfifo -m 600 /tmp/myfifo is safe against hijacking but prone to a denial of service; without access to a strong random file name generator, you would need to manage retry attempts.

If you don't care for the subtle security problems around temporary files, you can follow a simple rule: create a private directory, and keep everything in there.

tmpdir=
cleanup () {
  trap - EXIT
  if [ -n "$tmpdir" ] ; then rm -rf "$tmpdir"; fi
  if [ -n "$1" ]; then trap - $1; kill -$1 $$; fi
}
tmpdir=$(mktemp -d)
trap 'cleanup' EXIT
trap 'cleanup HUP' HUP
trap 'cleanup TERM' TERM
trap 'cleanup INT' INT
mkfifo "$tmpdir/pipe"

Solution 2

Use the "dry-run" option:

mkfifo $(mktemp -ut pipe.XXX)

Solution 3

You can use mktemp to create a temporary file, then delete it and create a named pipe with the same name.

For example:

TMPPIPE=$(mktemp -t pipe.XXX) && {
    rm -f $TMPPIPE
    mkfifo $TMPPIPE
}

Comments

[Nikhil Mulley]

oh ok, best is to create them under /tmp, they are temporary files by definition and will clear off once the system reboots. Or better yet, have a shell function which will create the namedpipe out of the mktemp result itself (ofcourse by deleting the temp file first and then running mkfifo on the same). mktemp can also be used to create a temporary directory, try with -t -d switch.

[J. M. Becker]

Does deleting the $TMPPIPE before the mkfifo avoid the 'unsafe' problem associated from doing TMPPIPE=`mktemp -u` ; mkfifo $TMPPIPE ?

[CurtainDog]

According to the man pages, the use of the -u option "is not encouraged".

[polemon]

@dogbane So is the use of -t, but as long as it works reliably, I'd go with it.

[CurtainDog]

sorry, where does it say that -t is discouraged?

[polemon]

@dogbane If it is in any way critical, I'd make a tiny C application, calling the mkstemp() function (linux.die.net/man/3/mkstemp). The -t switch isn't discouraged, it is -p, my bad.

[Gilles 'SO- stop being evil']

@TechZilla mkfifo is actually safe, unlike usual regular file creation from the shell. If it wasn't, creating a file then deleting it would not help at all (in fact it would greatly facilitate the attacker's job by not requiring him to guess the file name). So dogbane's answer works, but the intermediate file creation is a useless complication.

[J. M. Becker]

@Gilles, Do you know in what regard the mktemp man page called the -u option 'unsafe'?

[J. M. Becker]

@dogbane, I voted up on your answer, and your comments which I felt were solid. Although, I already completed this yesterday.. I was just waiting to see when someone posted either my solution, or hopefully a better one. So even though I used mktemp -d you still got some points for your input. Thank you for all your help, I really appreciate it!

[J. M. Becker]

I actually already chose this as my solution earlier yesterday, I was waiting to see if someone posted it, or a better one. I also felt the dir was the way to go, in my case. Regardless... Thanks for the help, I appreciate it.

[Gilles 'SO- stop being evil']

@TechZilla mktemp -u is unsafe when creating a regular file, because it provides the protection against denial of service (if the name it generates is sufficiently unpredictable) but does not prevent an attacker from creating the file under the program's nose. Creating a fifo instead of a regular file is a rare use case that the man page doesn't address.

[Chris Wolfe]

Shouldn't the trap command be trap "rm -rf '$tempdir'" EXIT HUP INT TERM? Can trap do it's own variable expansion?

[Gilles 'SO- stop being evil']

@Six Your command would expand $tempdir at the time the trap command is evaluated, not at the time the trap is triggered. In this case, it doesn't make any difference, except that your code breaks horribly if the value of tempdir contains a single quote, whereas my code always works.

[Chris Wolfe]

@Gilles That's a nice feature. I had tried your single quote example without luck, so I thought trap was unable to evaluate variables at runtime. It turned out that my $tempdir was declared locally and it must be defined globally for trap to have access to it. Makes sense now...

[MestreLion]

@Gilles ... as long as $tempdir value does not change either, which is acceptable for basically all purposes. Just be careful not to use the same name to create multiple temp files/dirs...

[Sukima]

@gilles Why do you trap INT HUP TERM? Doesn't EXIT cover all of those?

[Robin A. Meade]

Would readonly tmpdir=$(mktemp -d) protect against the problem MestreLion mentioned, namely that tmpdir might be accidentally redefined later in the script?

[Gilles 'SO- stop being evil']

@RobinA.Meade Yes, if the shell has readonly. But if that's an issue, you have bigger problems. If your script is so complex that you can't keep track of all the variables that it uses, you should probably use a different language.