toolchain/60008: MAKEDEV should let caller say whether mount_fdesc is used

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>Number:         60008
>Category:       toolchain
>Synopsis:       MAKEDEV should let caller say whether mount_fdesc is used
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    toolchain-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Feb 15 20:55:00 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, 9, ...
>Organization:
The MetaFD Foundation, Inc.
>Environment:
>Description:

	The MAKEDEV script examines whether the working directory has
	fdescfs mounted, but this doesn't make sense when running it at
	build-time to create images with, e.g.,
	src/embedded/utils/mkimage:

    516 	# Set fdesc_mounted=true if the fdesc file system is mounted
    517 	# on the current directory (typically "/dev").
    518 	# Later, this will be used to suppress creation of device nodes
    519 	# that are supplied by the fdesc file system.
    520 	#
    521 	fdesc_mounted=false
    522 	if [ -d fd ]; then
    523 		# Parse the output from "mount -u -o nosuchoption .".
    524 		# We don't parse the output from df(1) because that's
    525 		# less likely to be available on install media.
    526 		#
    527 		# If the current directory is a mount point for the
    528 		# fdesc file system, then the expected output (whether
    529 		# or not the current user is root) is:
    530 		#	mount_fdesc: -o suchoption: option not supported.
    531 		#
    532 		# If the current directory is not a mount point, then
    533 		# the expected output is:
    534 		#	mount: .: unknown special file or file system.
    535 		#
    536 		# If we are not running on NetBSD, or mount(8) is not
    537 		# found, then we should get some other error message.
    538 		#
    539 		case "$({ LC_ALL=C mount -u -o nosuchoption . ; } 2>&1)" in
    540 		*mount_fdesc*)	fdesc_mounted=true ;;
    541 		esac
    542 	fi

https://nxr.netbsd.org/xref/src/etc/MAKEDEV.tmpl?r=1.238#516

	MAKEDEV does this in order to make some decisions about which
	device nodes to create:

    960 std)
...
    973 	if ! $fdesc_mounted; then
    974 		mkdev	tty	c %ctty_chr% 0		666
    975 		mkdev	stdin	c %filedesc_chr% 0	666
    976 		mkdev	stdout	c %filedesc_chr% 1	666
    977 		mkdev	stderr	c %filedesc_chr% 2	666
    978 	fi

https://nxr.netbsd.org/xref/src/etc/MAKEDEV.tmpl?r=1.238#960

   1158 fd)
   1159 	if ! $fdesc_mounted; then
   1160 		# Create the "fd" subdirectory, and devices "fd/0" to "fd/63"
   1161 		makedir fd 755
   1162 		n=0
   1163 		while [ $n -lt 64 ]
   1164 		do
   1165 			mkdev fd/$n c %filedesc_chr% $n 666
   1166 			n=$(($n + 1))
   1167 		done
   1168 	fi

https://nxr.netbsd.org/xref/src/etc/MAKEDEV.tmpl?r=1.238#1158
>How-To-Repeat:

	Try to build an image with mkimage that is appropriate for
	having fdescfs mounted.

>Fix:

	These decisions should be configurable by a command-line option
	so they can be independent of whatever file systems are mounted
	at build-time.

	While it may not exactly hurt for the extra device nodes to
	exist on disk even if fdescfs will later be mounted over them
	(other than costing a few dozen inodes), the behaviour of
	MAKEDEV as a build-time tool should not depend on mounted file
	systems and should be controllable by command-line options.