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

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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
This commit is contained in:
Johannes Schauer Marin Rodrigues 2022-06-14 08:26:48 +02:00
parent 679f6cb2fc
commit 009089ee8a
Signed by: josch
GPG key ID: F2CBA5C78FBD83E1
3 changed files with 191 additions and 5 deletions
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4
coverage.txt
View file

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

41
mmdebstrap
View file

@ -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";
}

145
tests/dev-ptmx Normal file
View file

@ -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