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Terminals and Standard IO

Note that the default configuration of runc (foreground, new terminal) is generally the best option for most users. This document exists to help explain what the purpose of the different modes is, and to try to steer users away from common mistakes and misunderstandings.

In general, most processes on Unix (and Unix-like) operating systems have 3 standard file descriptors provided at the start, collectively referred to as "standard IO" (stdio):

  • 0: standard-in (stdin), the input stream into the process
  • 1: standard-out (stdout), the output stream from the process
  • 2: standard-error (stderr), the error stream from the process

When creating and running a container via runc, it is important to take care to structure the stdio the new container's process receives. In some ways containers are just regular processes, while in other ways they're an isolated sub-partition of your machine (in a similar sense to a VM). This means that the structure of IO is not as simple as with ordinary programs (which generally just use the file descriptors you give them).

Other File Descriptors

Before we continue, it is important to note that processes can have more file descriptors than just stdio. By default in runc no other file descriptors will be passed to the spawned container process. If you wish to explicitly pass file descriptors to the container you have to use the --preserve-fds option. These ancillary file descriptors don't have any of the strange semantics discussed further in this document (those only apply to stdio) -- they are passed untouched by runc.

It should be noted that --preserve-fds does not take individual file descriptors to preserve. Instead, it takes how many file descriptors (not including stdio or LISTEN_FDS) should be passed to the container. In the following example:

% runc run --preserve-fds 5 <container>

runc will pass the first 5 file descriptors (3, 4, 5, 6, and 7 -- assuming that LISTEN_FDS has not been configured) to the container.

In addition to --preserve-fds, LISTEN_FDS file descriptors are passed automatically to allow for systemd-style socket activation. To extend the above example:

% LISTEN_PID=$pid_of_runc LISTEN_FDS=3 runc run --preserve-fds 5 <container>

runc will now pass the first 8 file descriptors (and it will also pass LISTEN_FDS=3 and LISTEN_PID=1 to the container). The first 3 (3, 4, and 5) were passed due to LISTEN_FDS and the other 5 (6, 7, 8, 9, and 10) were passed due to --preserve-fds. You should keep this in mind if you use runc directly in something like a systemd unit file. To disable this LISTEN_FDS-style passing just unset LISTEN_FDS.

Be very careful when passing file descriptors to a container process. Due to some Linux kernel (mis)features, a container with access to certain types of file descriptors (such as O_PATH descriptors) outside of the container's root file system can use these to break out of the container's pivoted mount namespace. This has resulted in CVEs in the past.

Terminal Modes

runc supports two distinct methods for passing stdio to the container's primary process:

When first using runc these two modes will look incredibly similar, but this can be quite deceptive as these different modes have quite different characteristics.

By default, runc spec will create a configuration that will create a new terminal (terminal: true). However, if the terminal: ... line is not present in config.json then pass-through is the default.

In general we recommend using new terminal, because it means that tools like sudo will work inside your container. But pass-through can be useful if you know what you're doing, or if you're using runc as part of a non-interactive pipeline.

New Terminal

In new terminal mode, runc will create a brand-new "console" (or more precisely, a new pseudo-terminal using the container's namespaced /dev/pts/ptmx) for your contained process to use as its stdio.

When you start a process in new terminal mode, runc will do the following:

  1. Create a new pseudo-terminal.
  2. Pass the slave end to the container's primary process as its stdio.
  3. Send the master end to a process to interact with the stdio for the container's primary process (details below).

It should be noted that since a new pseudo-terminal is being used for communication with the container, some strange properties of pseudo-terminals might surprise you. For instance, by default, all new pseudo-terminals translate the byte '\n' to the sequence '\r\n' on both stdout and stderr. In addition there are a whole range of ioctls(2) that can only interact with pseudo-terminal stdio.

NOTE: In new terminal mode, all three stdio file descriptors are the same underlying file. The reason for this is to match how a shell's stdio looks to a process (as well as remove race condition issues with having to deal with multiple master pseudo-terminal file descriptors). However this means that it is not really possible to uniquely distinguish between stdout and stderr from the caller's perspective.

Issues

If you see an error like

open /dev/tty: no such device or address

from runc, it means it can't open a terminal (because there isn't one). This can happen when stdin (and possibly also stdout and stderr) are redirected, or in some environments that lack a tty (such as GitHub Actions runners).

The solution to this is to not use a terminal for the container, i.e. have terminal: false in config.json. If the container really needs a terminal (some programs require one), you can provide one, using one of the following methods.

One way is to use ssh with the -tt flag. The second t forces a terminal allocation even if there's no local one -- and so it is required when stdin is not a terminal (some ssh implementations only look for a terminal on stdin).

Another way is to run runc under the script utility, like this

$ script -e -c 'runc run <container>'

Pass-Through

If you have already set up some file handles that you wish your contained process to use as its stdio, then you can ask runc to pass them through to the contained process (this is not necessarily the same as --preserve-fds's passing of file descriptors -- details below). As an example (assuming that terminal: false is set in config.json):

% echo input | runc run some_container > /tmp/log.out 2> /tmp/log.err

Here the container's various stdio file descriptors will be substituted with the following:

  • stdin will be sourced from the echo input pipeline.
  • stdout will be output into /tmp/log.out on the host.
  • stderr will be output into /tmp/log.err on the host.

It should be noted that the actual file handles seen inside the container may be different based on the mode runc is being used in (for instance, the file referenced by 1 could be /tmp/log.out directly or a pipe which runc is using to buffer output, based on the mode). However the net result will be the same in either case. In principle you could use the new terminal mode in a pipeline, but the difference will become more clear when you are introduced to runc's detached mode.

runc Modes

runc itself runs in two modes:

You can use either terminal mode with either runc mode. However, there are considerations that may indicate preference for one mode over another. It should be noted that while two types of modes (terminal and runc) are conceptually independent from each other, you should be aware of the intricacies of which combination you are using.

In general we recommend using foreground because it's the most straight-forward to use, with the only downside being that you will have a long-running runc process. Detached mode is difficult to get right and generally requires having your own stdio management.

Foreground

The default (and most straight-forward) mode of runc. In this mode, your runc command remains in the foreground with the container process as a child. All stdio is buffered through the foreground runc process (irrespective of which terminal mode you are using). This is conceptually quite similar to running a normal process interactively in a shell (and if you are using runc in a shell interactively, this is what you should use).

Because the stdio will be buffered in this mode, some very important peculiarities of this mode should be kept in mind:

  • With new terminal mode, the container will see a pseudo-terminal as its stdio (as you might expect). However, the stdio of the foreground runc process will remain the stdio that the process was started with -- and runc will copy all stdio between its stdio and the container's stdio. This means that while a new pseudo-terminal has been created, the foreground runc process manages it over the lifetime of the container.

  • With pass-through mode, the foreground runc's stdio is not passed to the container. Instead, the container's stdio is a set of pipes which are used to copy data between runc's stdio and the container's stdio. This means that the container never has direct access to host file descriptors (aside from the pipes created by the container runtime, but that shouldn't be an issue).

The main drawback of the foreground mode of operation is that it requires a long-running foreground runc process. If you kill the foreground runc process then you will no longer have access to the stdio of the container (and in most cases this will result in the container dying abnormally due to SIGPIPE or some other error). By extension this means that any bug in the long-running foreground runc process (such as a memory leak) or a stray OOM-kill sweep could result in your container being killed through no fault of the user. In addition, there is no way in foreground mode of passing a file descriptor directly to the container process as its stdio (like --preserve-fds does).

These shortcomings are obviously sub-optimal and are the reason that runc has an additional mode called "detached mode".

Detached

In contrast to foreground mode, in detached mode there is no long-running foreground runc process once the container has started. In fact, there is no long-running runc process at all. However, this means that it is up to the caller to handle the stdio after runc has set it up for you. In a shell this means that the runc command will exit and control will return to the shell, after the container has been set up.

You can run runc in detached mode in one of the following ways:

  • runc run -d ... which operates similar to runc run but is detached.
  • runc create followed by runc start which is the standard container lifecycle defined by the OCI runtime specification (runc create sets up the container completely, waiting for runc start to begin execution of user code).

The main use-case of detached mode is for higher-level tools that want to be wrappers around runc. By running runc in detached mode, those tools have far more control over the container's stdio without runc getting in the way (most wrappers around runc like cri-o or containerd use detached mode for this reason).

Unfortunately using detached mode is a bit more complicated and requires more care than the foreground mode -- mainly because it is now up to the caller to handle the stdio of the container.

Another complication is that the parent process is responsible for acting as the subreaper for the container. In short, you need to call prctl(PR_SET_CHILD_SUBREAPER, 1, ...) in the parent process and correctly handle the implications of being a subreaper. Failing to do so may result in zombie processes being accumulated on your host.

These tasks are usually performed by a dedicated (and minimal) monitor process per-container. For the sake of comparison, other runtimes such as LXC do not have an equivalent detached mode and instead integrate this monitor process into the container runtime itself -- this has several tradeoffs, and runc has opted to support delegating the monitoring responsibility to the parent process through this detached mode.

Detached Pass-Through

In detached mode, pass-through actually does what it says on the tin -- the stdio file descriptors of the runc process are passed through (untouched) to the container's stdio. The purpose of this option is to allow a user to set up stdio for a container themselves and then force runc to just use their pre-prepared stdio (without any pseudo-terminal funny business). If you don't see why this would be useful, don't use this option.

You must be incredibly careful when using detached pass-through (especially in a shell). The reason for this is that by using detached pass-through you are passing host file descriptors to the container. In the case of a shell, usually your stdio is going to be a pseudo-terminal (on your host). A malicious container could take advantage of TTY-specific ioctls like TIOCSTI to fake input into the host shell (remember that in detached mode, control is returned to your shell and so the terminal you've given the container is being read by a shell prompt).

There are also several other issues with running non-malicious containers in a shell with detached pass-through (where you pass your shell's stdio to the container):

  • Output from the container will be interleaved with output from your shell (in a non-deterministic way), without any real way of distinguishing from where a particular piece of output came from.

  • Any input to stdin will be non-deterministically split and given to either the container or the shell (because both are blocked on a read(2) of the same FIFO-style file descriptor).

They are all related to the fact that there is going to be a race when either your host or the container tries to read from (or write to) stdio. This problem is especially obvious when in a shell, where usually the terminal has been put into raw mode (where each individual key-press should cause read(2) to return).

NOTE: There is also currently a known problem where using detached pass-through will result in the container hanging if the stdout or stderr is a pipe (though this should be a temporary issue).

Detached New Terminal

When creating a new pseudo-terminal in detached mode, and fairly obvious problem appears -- how do we use the new terminal that runc created? Unlike in pass-through, runc has created a new set of file descriptors that need to be used by something in order for container communication to work.

The way this problem is resolved is through the use of Unix domain sockets. There is a feature of Unix sockets called SCM_RIGHTS which allows a file descriptor to be sent through a Unix socket to a completely separate process (which can then use that file descriptor as though they opened it). When using runc in detached new terminal mode, this is how a user gets access to the pseudo-terminal's master file descriptor.

To this end, there is a new option (which is required if you want to use runc in detached new terminal mode): --console-socket. This option takes the path to a Unix domain socket which runc will connect to and send the pseudo-terminal master file descriptor down. The general process for getting the pseudo-terminal master is as follows:

  1. Create a Unix domain socket at some path, $socket_path.
  2. Call runc run or runc create with the argument --console-socket $socket_path.
  3. Using recvmsg(2) retrieve the file descriptor sent using SCM_RIGHTS by runc.
  4. Now the manager can interact with the stdio of the container, using the retrieved pseudo-terminal master.

After runc exits, the only process with a copy of the pseudo-terminal master file descriptor is whoever read the file descriptor from the socket.

NOTE: Currently runc doesn't support abstract socket addresses (due to it not being possible to pass an argv with a null-byte as the first character). In the future this may change, but currently you must use a valid path name.

In order to help users make use of detached new terminal mode, we have provided a Go implementation in the go-runc bindings, as well as a simple client.