Bug#983423: Bug#856877: schroot: Please mount a new instance of /dev/pts
On Sun, 05 Mar 2017 at 19:23:40 +0000, Simon McVittie wrote:
> the preferred way to
> set up /dev/pts inside containers in recent kernels is to mount a new
> instance of the devpts filesystem on /dev/pts
...
> In particular, this would make the chroots created by debootstrap
> versions 1.0.76 to 1.0.88 (inclusive) work as expected.
...
> A nice side-effect of this change, which I discovered while testing a
> cut-down version of the same code in a new debootstrap autopkgtest, is
> that it makes script(1) work inside "schroot --sbuild" inside a LXC
> container on a Debian jessie kernel. Previously, that would have failed.
On Tue, 23 Feb 2021 at 23:55:07 +0000, Simon McVittie wrote:
> schroot: Default profile doesn't provide a working /dev/ptmx inside lxc >= 3
...
> The same patch I proposed in 2017 for #856877 resolves this, setting
> up a working /dev/ptmx. Under lxc 2 it's a symlink to /dev/ptx/ptmx,
> and under lxc >= 3 it's a device node.
>¯
> Under lxc 4, that patch also provides a working /dev/console.
Here's the patch I proposed in 2017, with an updated commit message.
Also available as a merge request:
https://salsa.debian.org/debian/schroot/-/merge_requests/2
smcv
From: Simon McVittie <smcv@debian.org>
Date: Mon, 20 Feb 2017 10:43:24 +0000
Subject: Mount a new instance of /dev/pts in the chroot
This avoids various failure modes when schroot is run inside some other
container manager, such as lxc, most commonly manifesting as inability
to run programs that create pseudo-terminals such as script(1).
Mounting a new instance of devpts is considered to be
best-practice for container managers in Linux >= v2.6.29 with
CONFIG_DEVPTS_MULTIPLE_INSTANCES=y. That config option was made
unconditional in v4.7.
This has some assumptions, which cannot be avoided if we are going to
mount /dev/pts using schroot's fstab:
* If the kernel is older than v4.7, it is assumed to be v2.6.29 or
later with CONFIG_DEVPTS_MULTIPLE_INSTANCES=y. Users of older kernels,
or intermediate versions with CONFIG_DEVPTS_MULTIPLE_INSTANCES=n,
can revert this change via /etc.
* gid 5 must be the right owner for ptys. This is correct for Debian
(it's the hard-coded tty group specified in base-passwd) and probably
many other distributions (it's systemd's configure-time default) but
not necessarily correct everywhere. However, if the host system and the
chroot disagree on the right gid, schroot's previous behaviour would
have been wrong anyway, because it bind-mounted the host's /dev/pts.
* /dev/ptmx inside the chroot must be either a real device node (as
created by debootstrap < 1.0.76, and debootstrap >= 1.0.89 if possible)
or a symlink to pts/ptmx (as created by debootstrap between 1.0.76 and
1.0.88 inclusive, and by debootstrap >= 1.0.89 if run in a container
whose seccomp rules do not allow it to create the device node, such
as systemd-nspawn).
Bind-mounting /dev/pts/ptmx over /dev/ptmx, so that we get the
new instance's /dev/ptmx equivalent instead of the host's, can only
be done from code, so I have done it in the 10mount hook instead of
in the fstab.
To keep the host system terminal on which we were invoked (which might
itself be a pty, from a different instance of /dev/pts) available to
the chroot, bind-mount it onto /dev/console. This is the same trick
used in the lxc and systemd-nspawn Linux container managers.
Bug-Debian: https://bugs.debian.org/856877
Bug-Debian: https://bugs.debian.org/983423
Signed-off-by: Simon McVittie <smcv@debian.org>
---
etc/profile-templates/buildd/linux/fstab | 2 +-
etc/profile-templates/default/linux/fstab | 2 +-
etc/profile-templates/desktop/linux/fstab | 2 +-
etc/profile-templates/sbuild/linux/fstab | 2 +-
etc/setup.d/10mount | 27 +++++++++++++++++++++++++++
5 files changed, 31 insertions(+), 4 deletions(-)
diff --git a/etc/profile-templates/buildd/linux/fstab b/etc/profile-templates/buildd/linux/fstab
index 26efe88..f2f6136 100644
--- a/etc/profile-templates/buildd/linux/fstab
+++ b/etc/profile-templates/buildd/linux/fstab
@@ -1,4 +1,4 @@
-/dev/pts /dev/pts none rw,bind 0 0
+/dev/pts /dev/pts devpts rw,newinstance,ptmxmode=666,mode=620,gid=5 0 0
tmpfs /dev/shm tmpfs defaults 0 0
# Mount a large scratch space for the build, so we don't use up
# space on an LVM snapshot of the chroot itself.
diff --git a/etc/profile-templates/default/linux/fstab b/etc/profile-templates/default/linux/fstab
index 777f0ed..181ed80 100644
--- a/etc/profile-templates/default/linux/fstab
+++ b/etc/profile-templates/default/linux/fstab
@@ -1,5 +1,5 @@
/dev /dev none rw,bind 0 0
-/dev/pts /dev/pts none rw,bind 0 0
+/dev/pts /dev/pts devpts rw,newinstance,ptmxmode=666,mode=620,gid=5 0 0
/home /home none rw,bind 0 0
/tmp /tmp none rw,bind 0 0
diff --git a/etc/profile-templates/desktop/linux/fstab b/etc/profile-templates/desktop/linux/fstab
index 7f61d8d..b0dae37 100644
--- a/etc/profile-templates/desktop/linux/fstab
+++ b/etc/profile-templates/desktop/linux/fstab
@@ -1,5 +1,5 @@
/dev /dev none rw,bind 0 0
-/dev/pts /dev/pts none rw,bind 0 0
+/dev/pts /dev/pts devpts rw,newinstance,ptmxmode=666,mode=620,gid=5 0 0
/home /home none rw,bind 0 0
/tmp /tmp none rw,bind 0 0
diff --git a/etc/profile-templates/sbuild/linux/fstab b/etc/profile-templates/sbuild/linux/fstab
index 26efe88..f2f6136 100644
--- a/etc/profile-templates/sbuild/linux/fstab
+++ b/etc/profile-templates/sbuild/linux/fstab
@@ -1,4 +1,4 @@
-/dev/pts /dev/pts none rw,bind 0 0
+/dev/pts /dev/pts devpts rw,newinstance,ptmxmode=666,mode=620,gid=5 0 0
tmpfs /dev/shm tmpfs defaults 0 0
# Mount a large scratch space for the build, so we don't use up
# space on an LVM snapshot of the chroot itself.
diff --git a/etc/setup.d/10mount b/etc/setup.d/10mount
index fb1c87b..d78b5d6 100755
--- a/etc/setup.d/10mount
+++ b/etc/setup.d/10mount
@@ -275,3 +275,30 @@ if [ $STAGE = "setup-start" ] || [ $STAGE = "setup-recover" ]; then
fi
fi
+
+if [ $STAGE = "setup-start" ] || [ $STAGE = "setup-recover" ]; then
+ if [ "$(uname -s)" = "Linux" ]; then
+ # Depending on how /dev was set up, /dev/ptmx might either be
+ # character device (5,2), or a symbolic link to pts/ptmx.
+ # Either way we want it to be equivalent to /dev/pts/ptmx, assuming
+ # both exist.
+ if [ -e "$CHROOT_PATH/dev/pts/ptmx" ] && \
+ [ -e "$CHROOT_PATH/dev/ptmx" ] && \
+ ! [ "$CHROOT_PATH/dev/pts/ptmx" -ef "$CHROOT_PATH/dev/ptmx" ]; then
+ mount --bind "$CHROOT_PATH/dev/pts/ptmx" "$CHROOT_PATH/dev/ptmx"
+ fi
+
+ # If schroot was invoked from a terminal, we still want to be able to
+ # access that terminal. lxc and systemd-nspawn achieve this by
+ # binding it onto /dev/console; so can we.
+ if stdin_tty="$(tty)"; then
+ if [ ! -e "$CHROOT_PATH/dev/console" ]; then
+ # We need something to mount onto, and it might as well be
+ # the correctly-numbered device node.
+ mknod -m700 "$CHROOT_PATH/dev/console" c 5 1
+ fi
+
+ mount --bind "$stdin_tty" "$CHROOT_PATH/dev/console"
+ fi
+ fi
+fi
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