Mount a new instance of /dev/pts in the chroot

Before, we bind-mounted /dev/ptmx and /dev/pts from the host into the
chroot. This will make posix_openpt() fail with 'No such file or
directory'.  The ability to create pseudo terminals is important for apt
(which will throw a warning otherwise) or running script(1) or source
package testsuites like for src:util-linux. This functionality is
restored by mounting a new devpts instance to /dev/pts and making
/dev/ptmx a symlink to /dev/pts/ptmx. Mounting with ptmxmode=666 is
required such that also non-root users in unshare mode are able to
create pseudo terminals. See also:

https://www.kernel.org/doc/Documentation/filesystems/devpts.txt
https://salsa.debian.org/debian/schroot/-/merge_requests/2
https://bugs.debian.org/856877
https://bugs.debian.org/817236

coverage.txt

@@ -318,3 +318,7 @@ Skip-If:
 
 Test: no-sbin-in-path
 Modes: fakechroot
+
+Test: dev-ptmx
+Modes: root unshare
+Needs-QEMU: true

mmdebstrap

@@ -1091,6 +1091,20 @@ sub run_chroot {
                             );
                             next;
                         }
+                        if ($fname eq "ptmx") {
+                            # We must not bind-mount ptmx from the outside or
+                            # otherwise posix_openpt() will fail. Instead
+                            # /dev/ptmx must refer to /dev/pts/ptmx either by
+                            # symlink or by bind-mounting. We choose a symlink.
+                            symlink '/dev/pts/ptmx',
+                              "$options->{root}/dev/ptmx"
+                              or error "cannot create /dev/pts/ptmx symlink";
+                            push @cleanup_tasks, sub {
+                                unlink "$options->{root}/dev/ptmx"
+                                  or error "unlink /dev/ptmx";
+                            };
+                            next;
+                        }
                         if (!-e "/dev/$fname") {
                             warning("skipping creation of ./dev/$fname because"
                                   . " /dev/$fname does not exist"
@@ -1133,13 +1147,13 @@ sub run_chroot {
                               . " /dev directory is missing in the target");
                         next;
                     }
-                    if (!-e "/dev/$fname") {
+                    if (!-e "/dev/$fname" && $fname ne "pts/") {
                         warning("skipping creation of ./dev/$fname because"
                               . " /dev/$fname does not exist"
                               . " on the outside");
                         next;
                     }
-                    if (!-d "/dev/$fname") {
+                    if (!-d "/dev/$fname" && $fname ne "pts/") {
                         warning("skipping creation of ./dev/$fname because"
                               . " /dev/$fname on the outside is not a"
                               . " directory");
@@ -1187,9 +1201,32 @@ sub run_chroot {
                             "$options->{root}/dev/$fname")
                           or warning("umount ./dev/$fname failed: $?");
                     };
+                    if ($fname eq "pts/") {
+                        # We cannot just bind-mount /dev/pts from the host as
+                        # doing so will make posix_openpt() fail. Instead, we
+                        # need to mount a new devpts.
+                        # We need ptmxmode=666 because /dev/ptmx is a symlink
+                        # to /dev/pts/ptmx and without it posix_openpt() will
+                        # fail if we are not the root user.
+                        # See also:
+                        # kernel.org/doc/Documentation/filesystems/devpts.txt
+                        # salsa.debian.org/debian/schroot/-/merge_requests/2
+                        # https://bugs.debian.org/856877
+                        # https://bugs.debian.org/817236
+                        0 == system(
+                            'mount',
+                            '-t',
+                            'devpts',
+                            'none',
+                            "$options->{root}/dev/pts",
+                            '-o',
+                            'noexec,nosuid,uid=5,mode=620,ptmxmode=666'
+                        ) or error "mount /dev/pts failed";
+                    } else {
                         0 == system('mount', '-o', 'bind', "/dev/$fname",
                             "$options->{root}/dev/$fname")
                           or error "mount ./dev/$fname failed: $?";
+                    }
                 } else {
                     error "unsupported type: $type";
                 }

tests/dev-ptmx

@@ -0,0 +1,145 @@
+#!/bin/sh
+set -eu
+export LC_ALL=C.UTF-8
+
+if [ {{ MODE }} != unshare ] && [ {{ MODE }} != root ]; then
+	echo "test requires root or unshare mode" >&2
+	exit 1
+fi
+
+if [ ! -e /mmdebstrap-testenv ]; then
+	echo "this test modifies the system and should only be run inside a container" >&2
+	exit 1
+fi
+if [ "$(id -u)" -eq 0 ] && ! id -u user > /dev/null 2>&1; then
+	adduser --gecos user --disabled-password user
+fi
+prefix=
+[ "$(id -u)" -eq 0 ] && [ "{{ MODE }}" != "root" ] && prefix="runuser -u user --"
+
+# this mimics what apt does in apt-pkg/deb/dpkgpm.cc/pkgDPkgPM::StartPtyMagic()
+cat > /tmp/test.c << 'END'
+#define _GNU_SOURCE
+
+#include <stdlib.h>
+#include <fcntl.h>
+#include <termios.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <sys/ioctl.h>
+#include <signal.h>
+
+int main() {
+	int ret;
+	int fd = posix_openpt(O_RDWR | O_NOCTTY);
+	if (fd < 0) {
+		perror("posix_openpt");
+		return 1;
+	}
+	char buf[64]; // 64 is used by apt
+	ret = ptsname_r(fd, buf, sizeof(buf));
+	if (ret != 0) {
+		perror("ptsname_r");
+		return 1;
+	}
+	ret = grantpt(fd);
+	if (ret == -1) {
+		perror("grantpt");
+		return 1;
+	}
+	struct termios origtt;
+	ret = tcgetattr(STDIN_FILENO, &origtt);
+	if (ret != 0) {
+		perror("tcgetattr1");
+		return 1;
+	}
+	struct termios tt;
+	ret = tcgetattr(STDOUT_FILENO, &tt);
+	if (ret != 0) {
+		perror("tcgetattr2");
+		return 1;
+	}
+	struct winsize win;
+	ret = ioctl(STDOUT_FILENO, TIOCGWINSZ, &win);
+	if (ret < 0) {
+		perror("ioctl stdout TIOCGWINSZ");
+		return 1;
+	}
+	ret = ioctl(fd, TIOCSWINSZ, &win);
+	if (ret < 0) {
+		perror("ioctl fd TIOCGWINSZ");
+		return 1;
+	}
+	ret = tcsetattr(fd, TCSANOW, &tt);
+	if (ret != 0) {
+		perror("tcsetattr1");
+		return 1;
+	}
+	cfmakeraw(&tt);
+	tt.c_lflag &= ~ECHO;
+	tt.c_lflag |= ISIG;
+	sigset_t sigmask;
+	sigset_t sigmask_old;
+	ret = sigemptyset(&sigmask);
+	if (ret != 0) {
+		perror("sigemptyset");
+		return 1;
+	}
+	ret = sigaddset(&sigmask, SIGTTOU);
+	if (ret != 0) {
+		perror("sigaddset");
+		return 1;
+	}
+	ret = sigprocmask(SIG_BLOCK,&sigmask, &sigmask_old);
+	if (ret != 0) {
+		perror("sigprocmask1");
+		return 1;
+	}
+	ret = tcsetattr(STDIN_FILENO, TCSAFLUSH, &tt);
+	if (ret != 0) {
+		perror("tcsetattr2");
+		return 1;
+	}
+	ret = sigprocmask(SIG_BLOCK,&sigmask_old, NULL);
+	if (ret != 0) {
+		perror("sigprocmask2");
+		return 1;
+	}
+	ret = tcsetattr(STDIN_FILENO, TCSAFLUSH, &origtt);
+	if (ret != 0) {
+		perror("tcsetattr3");
+		return 1;
+	}
+	return 0;
+}
+END
+
+# use script to create a fake tty
+# run all tests as root and as a normal user (the latter requires ptmxmode=666)
+script -qfc "$prefix {{ CMD }} --mode={{ MODE }} --variant=apt \
+	--include=gcc,libc6-dev,python3 \
+	--customize-hook='chroot \"\$1\" adduser --gecos user --disabled-password user' \
+	--customize-hook='chroot \"\$1\" python3 -c \"import pty; print(pty.openpty())\"' \
+	--customize-hook='chroot \"\$1\" runuser -u user -- python3 -c \"import pty; print(pty.openpty())\"' \
+	--customize-hook='chroot \"\$1\" script -c \"echo foobar\"' \
+	--customize-hook='chroot \"\$1\" runuser -u user -- env --chdir=/home/user script -c \"echo foobar\"' \
+	--customize-hook='chroot \"\$1\" apt-get install --yes doc-debian 2>&1 | tee /tmp/log' \
+	--customize-hook=\"copy-in /tmp/test.c /tmp\" \
+	--customize-hook='chroot \"\$1\" gcc /tmp/test.c -o /tmp/test' \
+	--customize-hook='chroot \"\$1\" /tmp/test' \
+	--customize-hook='chroot \"\$1\" runuser -u user -- /tmp/test' \
+	--customize-hook='rm \"\$1\"/tmp/test \"\$1\"/tmp/test.c' \
+	{{ DIST }} /dev/null {{ MIRROR }}" /dev/null
+
+fail=0
+grep '^E:' /tmp/log && fail=1
+grep 'Can not write log' /tmp/log && fail=1
+grep 'posix_openpt' /tmp/log && fail=1
+grep 'No such file or directory' /tmp/log && fail=1
+if [ $fail -eq 1 ]; then
+	echo "apt failed to write log:" >&2
+	cat /tmp/log >&2
+	exit 1
+fi
+
+rm /tmp/test.c /tmp/log