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Wed, 3 Jun 2026 05:17:11 -0400 (EDT) Date: Wed, 3 Jun 2026 10:17:10 +0100 From: Kiryl Shutsemau To: Mike Rapoport Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lorenzo Stoakes , David Hildenbrand , stable@vger.kernel.org, Sashiko AI review , "Liam R. Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Peter Xu , Pedro Falcato , Alice Ryhl Subject: Re: [PATCH 6/6] userfaultfd: build __VMA_UFFD_FLAGS from config-gated masks Message-ID: References: <20260529172331.356655-1-kas@kernel.org> <20260529172331.356655-7-kas@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 0425A140005 X-Stat-Signature: bku7moktwc14to8sp8bnzs81hz5m1fxi X-Rspam-User: X-HE-Tag: 1780478233-856018 X-HE-Meta: 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 a533aHPy 50buBEJ7XItlKC2o+NTbnaDIZVuHuWYEV6xofoQyK1S41DRYqu2KUgX4PUYxTwurwaPAVqe18FM+7MY4lz5e28woIu/ZVQr2o4YYldYMmAxXtSbLChQAZIL1lghxQxvLtBVqmPrbTgBR/+xOM+oszU+ka/VsEPRNijUpFCZKaXy77dUBO1cuL4QkOZOD9dDoFLCGQTvjknJ8l3btIqkG6PhZpL/+e8IAv1YL644uUTjZ000vxNnppgUPZ9ql6EmH271uZGJE1tXmS+etzJul73qS7bQFF9ZNxGBQaKRmbcKmBQYSwHvA13nthNgCjgpjf8QzxnuUEozM3CKC0Xv3o/AUuHXatSQLwWZPz Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > > Suggested-by: Lorenzo Stoakes > > Signed-off-by: Kiryl Shutsemau > > 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