Re: [PATCH 6/6] userfaultfd: build __VMA_UFFD_FLAGS from config-gated masks

[Kiryl Shutsemau <kas@kernel.org>]

On Tue, Jun 02, 2026 at 11:32:54AM +0300, Mike Rapoport wrote:
> On Fri, May 29, 2026 at 06:23:30PM +0100, Kiryl Shutsemau (Meta) wrote:
> > The VMA flags bitmap is a single word today: NUM_VMA_FLAG_BITS is
> > BITS_PER_LONG, so on 32-bit vma_flags_t holds only 32 bits. (The bitmap
> > type exists so this can grow past BITS_PER_LONG later; until it does,
> > anything declared above the first word is out of range on 32-bit.) The bit
> > enum nevertheless declares some bits unconditionally above BITS_PER_LONG --
> > VMA_UFFD_MINOR_BIT is 41, with VM_UFFD_MINOR == VM_NONE on 32-bit so no VMA
> > actually carries the bit.
> > 
> > __VMA_UFFD_FLAGS feeds VMA_UFFD_MINOR_BIT to mk_vma_flags() unconditionally.
> > On 32-bit that becomes __set_bit(41, &one_long), a write one word past the
> > end of the single-word bitmap. The compiler folds the out-of-bounds store
> > with wraparound (1UL << (41 % 32) == bit 9) into the first word; bit 9 is
> > already in __VMA_UFFD_FLAGS so the mask happens to come out right today, but
> > it is an out-of-bounds write all the same, and any high-numbered bit whose
> > mod-BITS_PER_LONG position is otherwise unused would silently OR an extra
> > bit into the mask.
> > 
> > Rather than feed bit numbers that may not exist on the current build to
> > mk_vma_flags(), build the mask from whole per-mode masks that collapse to
> > EMPTY_VMA_FLAGS when their feature is unavailable. Add
> > mk_vma_flags_from_masks() for that, and define VMA_UFFD_MISSING / _WP /
> > _MINOR alongside the VM_UFFD_* flags, gating VMA_UFFD_MINOR on the same
> > config as VM_UFFD_MINOR (which implies 64BIT, where bit 41 fits). An
> > out-of-range bit is then never materialised, on any arch, and the in-range
> > fast path stays a compile-time constant.
> > 
> > Fixes: 9ea35a25d51b ("mm: introduce VMA flags bitmap type")
> > Cc: stable@vger.kernel.org
> > Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
> > Suggested-by: Lorenzo Stoakes <ljs@kernel.org>
> > Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> > Assisted-by: Claude:claude-opus-4-8
> 
> Can you ask claude to produce more concise changelogs and better split it
> to paragraphs?

Will do!

-- 
  Kiryl Shutsemau / Kirill A. Shutemov