rc: Bail if /dev/null is not a device

On startup, check that /dev/null exists and is a character device.
Otherwise, one of two things will happen: either /dev is a writable
directory and we will immediately create /dev/null as a regular file
and dump garbage into it, or it does not and we will spit out a stream
of error messages about failing to create /dev/null.

PR:		295782
MFC after:	1 week
Reviewed by:	jhb, emaste
Differential Revision:	https://reviews.freebsd.org/D57447

2 files changed, 16 insertions, 1 deletions

diff --git a/libexec/rc/rc b/libexec/rc/rc
index db3c3e20ab44..75eef606b7fe 100644
--- a/libexec/rc/rc
+++ b/libexec/rc/rc
@@ -46,6 +46,11 @@ HOME=/
 PATH=/sbin:/bin:/usr/sbin:/usr/bin
 export HOME PATH
 
+if ! [ -c /dev/null ]; then
+	echo "/dev is not populated" >&2
+	exit 1
+fi
+
 if [ "$1" = autoboot ]; then
 	autoboot=yes
 	_boot="faststart"
diff --git a/share/man/man8/rc.8 b/share/man/man8/rc.8
index a68878f0a10a..bfdd65b52f4a 100644
--- a/share/man/man8/rc.8
+++ b/share/man/man8/rc.8
@@ -28,7 +28,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 20, 2024
+.Dd June 4, 2026
 .Dt RC 8
 .Os
 .Sh NAME
@@ -97,6 +97,16 @@ command provides a scripting interface to modify system config files.
 .Ss Operation of Nm
 .Bl -enum
 .It
+Verify that
+.Pa /dev/null
+exists and is a character device.
+If that is not the case,
+.Nm
+prints an error message and terminates.
+This is normally caused by forgetting to enable
+.Xr devfs 5
+in a jail's configuration.
+.It
 If autobooting, set
 .Va autoboot Ns = Ns Li yes
 and enable a flag