kern/57010: ffs: mounting unclean non-root fs read-only causes spurious write to superblock

[campbell+netbsd%mumble.net@localhost]

>Number:         57010
>Category:       kern
>Synopsis:       ffs: mounting unclean non-root fs read-only causes spurious write to superblock
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Sep 14 08:40:00 +0000 2022
>Originator:     Taylor R Campbell
>Release:        current
>Organization:
Ffe FfetBSD Ffoundation
>Environment:
NetBSD singbulli 9.99.99 NetBSD 9.99.99 (GENERIC) #174: Thu Sep  8 01:10:04 UTC 2022  root@singbulli:/home/riastradh/netbsd/current/obj.amd64/sys/arch/amd64/compile/GENERIC amd64
>Description:
The following logic in ffs_mount causes a write to the medium in ffs_cgupdate even if it is mounted read-only:

	if (fs->fs_fmod != 0) {	/* XXX */
		int err;

		fs->fs_fmod = 0;
		if (fs->fs_clean & FS_WASCLEAN)
			fs->fs_time = time_second;
		else {
			printf("%s: file system not clean (fs_clean=%#x); "
			    "please fsck(8)\n", mp->mnt_stat.f_mntfromname,
			    fs->fs_clean);
			printf("%s: lost blocks %" PRId64 " files %d\n",
			    mp->mnt_stat.f_mntfromname, fs->fs_pendingblocks,
			    fs->fs_pendinginodes);
		}
		err = UFS_WAPBL_BEGIN(mp);
		if (err == 0) {
			(void) ffs_cgupdate(ump, MNT_WAIT);
			UFS_WAPBL_END(mp);
		}
	}

This triggers the warning in the specfs spec_strategy function in spec_vnops.c (to which I added db_stacktrace, for attribution of blame):

[ 13476.731716] /altroot/altroot blk 5728 written while ro!
[ 13476.731716] VOP_STRATEGY() at netbsd:VOP_STRATEGY+0x3c
[ 13476.731716] bwrite() at netbsd:bwrite+0x158
[ 13476.731716] ffs_cgupdate() at netbsd:ffs_cgupdate+0x126
[ 13476.731716] ffs_mount() at netbsd:ffs_mount+0x281
[ 13476.741712] VFS_MOUNT() at netbsd:VFS_MOUNT+0x3c
[ 13476.741712] mount_domount() at netbsd:mount_domount+0xd0
[ 13476.741712] do_sys_mount() at netbsd:do_sys_mount+0x26a
[ 13476.741712] sys___mount50() at netbsd:sys___mount50+0x33
[ 13476.741712] syscall() at netbsd:syscall+0x196
[ 13476.741712] --- syscall (number 410) ---

>How-To-Repeat:
1. Interrupt use of a non-root file system during operation so fs->fs_fmod is set.
2. Mount the file system read-only.
>Fix:
Yes, please!