kern/59076: File is silently removed when renaming on msdos filesystem when mounted with -l

[triaxx%netbsd.org@localhost]

>Number:         59076
>Category:       kern
>Synopsis:       File is silently removed when renaming on msdos filesystem when mounted with -l
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Feb 14 22:05:00 +0000 2025
>Originator:     Frédéric Fauberteau
>Release:        NetBSD 10.1
>Organization:
>Environment:
System: NetBSD goliath.triaxx.org 10.99.12 NetBSD 10.99.12 (GENERIC) #0: Mon Feb 10 00:52:25 CET 2025  triaxx%goliath.triaxx.org@localhost:/usr/obj/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:
I have a msdos filesystem mounted with:
    # mount -tmsdos -orw,-U,-l,-M755,-m644

I try to rename a file from uppercase to lowercase:
    # cd /mnt/boot/EFI/NETBSD
    # mv BOOT.CFG boot.cfg

The file BOOT.CFG is silently removed.
>How-To-Repeat:
    # touch BOOT.CFG && mv BOOT.CFG boot.cfg
>Fix: