On macOS, GuestWriteWatchCollectFresh reported nothing and returned true, so
MarkWritten took pages no collection had armed yet for clean, and they stayed
writable without being tracked. GuestWriteWatchCollectArmed was a full collect,
which also took those pages.
Pages now carry a fresh bit from registration until a collection takes them, as
the Linux watch keeps its fresh ranges: CollectFresh reports and arms only fresh
pages, CollectArmed skips them, and Collect takes both. Protection changes, host
writes and faults keep the bit.
The write_watch fresh and armed tests check both collections.
Register copied the protection it had recorded for pages it already
watched over the protection the kernel had just reported. A guest that
reserves address space, which registers it as PROT_NONE, then maps
direct memory over it read-write and registers it again kept the stale
PROT_NONE. Collect only write-protects pages with PROT_WRITE, so writes
to that memory were reported once and never again. With PPSA12544 on
the macOS port, the title menu text blinked and glyphs went missing.
Register now keeps a recorded protection only where its own write
protection explains what the kernel reports, and takes the kernel's
protection otherwise. A protected page that was mapped again also loses
its protected mark, so erase no longer restores the old protection over
the new mapping. The kernel is read under the lock, so a concurrent
Collect cannot protect a page between the read and the merge.
Test write_watch_mapped: a reservation, a mapping over it and a second
registration keep reporting writes, and a protected page reserved again
stays PROT_NONE.
AI-assisted: yes
macOS has no counterpart to userfaultfd write-protect or
MEM_WRITE_WATCH, so GuestWriteWatch had no backend there and the driver
re-read every guest buffer on each use.
The macOS backend write-protects watched pages. The first write to a
page faults, the SIGBUS/SIGSEGV handler records the page and restores
its protection, and Collect reports the recorded pages and protects
them again. A page is protected before its record is cleared, under the
lock the handler takes, so a write that lands in between is recorded.
The lock hands out turns in order: an unfair one starved writers under
Rosetta.
The protection is real, which adds three entry points:
- GuestWriteWatchBeginHostWrite/EndHostWrite lift it around a system
call that writes guest memory, which would otherwise fail with
EFAULT. GuestArena::HostWrite calls them, as it calls the Windows
equivalent.
- GuestWriteWatchProtectionChanged tells the watch the protection given
to watched pages by mprotect or a new mapping.
- GuestWriteWatchProtection reports that protection, since a watched
page is read-only between a collection and the next write.
Register keeps the recorded protection of a page it already watches:
read from the kernel it would be the watch's own read-only protection,
and the page would never be watched again.
The tests build without the prx libraries and run natively and through
Rosetta.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Utku Halis
fablevoid
MarcZX100
Elijah De Guzman