msdosfs rename report by kib

2021q3/msdosfs_rename.adoc

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+== Fixes for msdosfs_rename VOP
+
+Contact: Konstantin Belousov <kib@FreeBSD.org>  
+Contact: Peter Holm <pho@FreeBSD.org>  
+
+Our msdosfs(5) implementation is old code, and it has a relatively
+large legacy cost.  In particular, even though it got fine-grained
+locking and miscellaneous bugfixes over time, sometimes a big bug spot
+is found in it.
+
+Recently trasz@ found that msdosfs rename can be easily deadlocked.
+Further code examination revealed a lot of issues with locking,
+potential use after free, and filesystem structure corruption.
+
+During this work, all known issues were fixed and msdosfs passes
+enhanced stress2 suite of tests.
+
+In particular, locking in the msdosfs rename code was reworked: we
+need to lock up to four vnodes, and check one path to ensure that
+rename does not create cicrular parent relations between directories.
+For that, the locking procedure was copied from UFS rename, where all
+vnodes except the first are locked in try-mode.  Lockless relockup was
+added to msdosfs and the directory path checker changed to non-blocking
+mode.
+
+Sponsor: The FreeBSD Foundation (kib' part of the work)