pid1,vconsole-setup: lock /dev/console instead of the tty device

As requested in https://github.com/systemd/systemd/pull/27867#pullrequestreview-1567161854.
/dev/console, /dev/tty0, and /dev/ttyN are "different" device nodes
that may point to a single underlying device. We want to use a single
lock so that we don't get a race if different writers are using a different
device path, so let's just always lock around /dev/console.
This effectively makes the locking less granular.

Fixup for a0043bfa51281c2374878e2a98cf2a3ee10fd92c.
Fixes https://github.com/systemd/systemd/issues/28721.
Maybe fixes https://github.com/systemd/systemd/issues/28778 and
https://github.com/systemd/systemd/issues/28634.

1 files changed, 18 insertions, 0 deletions

diff --git a/src/shared/dev-setup.c b/src/shared/dev-setup.c
index 7dca6ad7d4..f7ed16193f 100644
--- a/src/shared/dev-setup.c
+++ b/src/shared/dev-setup.c
@@ -6,14 +6,32 @@
 
 #include "alloc-util.h"
 #include "dev-setup.h"
+#include "fd-util.h"
 #include "label-util.h"
+#include "lock-util.h"
 #include "log.h"
 #include "mkdir-label.h"
 #include "nulstr-util.h"
 #include "path-util.h"
+#include "terminal-util.h"
 #include "umask-util.h"
 #include "user-util.h"
 
+int lock_dev_console(void) {
+        _cleanup_close_ int fd = -EBADF;
+        int r;
+
+        fd = open_terminal("/dev/console", O_RDONLY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW);
+        if (fd < 0)
+                return fd;
+
+        r = lock_generic(fd, LOCK_BSD, LOCK_EX);
+        if (r < 0)
+                return log_error_errno(r, "Failed to lock /dev/console: %m");
+
+        return TAKE_FD(fd);
+}
+
 int dev_setup(const char *prefix, uid_t uid, gid_t gid) {
         static const char symlinks[] =
                 "-/proc/kcore\0"     "/dev/core\0"