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linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, fuse-devel@lists.linux.dev, Ihor Solodrai , Bert Karwatzki References: <20261009-b4-mmap-prepare-vma-flag-sanify-v5-0-923b70125b1e@kernel.org> <20261009-b4-mmap-prepare-vma-flag-sanify-v5-15-923b70125b1e@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv 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B/5CLJyKFBCXqmIW3sfZ9hhBWoE7L1pnUkjhU4PRrMo2jM1qkCMBlcYrZ1VhpZkwT2gbq9UXgg5I7x84Vt4sMDRBbl8ETAQ8DNviR8nYlGydhxows5VdcyYJl6F/vOC1ogsVBWAqZzIyqEmfRAkmk2E6/mIHZy7rcTg+Euz/GHx9D0WUiqynEQAsvcI4CoXjqaoIkjOWIBtx1R40LThFNQ5yR25v1e0GrU4ZO2aG/1DXI82yVUq0FZNNIFRAljn6oIeGtuxDLP8wH4YMjYORxFWV8KY7OF+OH9RnALYq0M0Cq3S6MQ7VqDoAUqKMuDDBl38aejWb1w3Bi9X4Ep15BQxBbD8NA3Y1gO+wEctOxJ1uxeCVGHGUsSibxUTgecBfp2mZtIWKqY/g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/9/26 20:04, Lorenzo Stoakes (ARM) wrote: > For ordinary files the only way the VMA_MAYWRITE_BIT flag is cleared is if > the underlying file is itself read-only. > > This means that mprotect() cannot mark a shared mapping of a read-only file > as read/write, as doing so would violate the read only attribute, and > permit writes. > > In general, we do not want file systems to be able to do this for > read/write files. > > Doing so would violate fundamental user expectation of file attributes and > likely break userspace. > > However, drivers pose a tricky problem here - the /dev/xxx file may be > read/write but provide access to a resource which is fundamentally > read-only. > > Therefore we must allow drivers to be able to clear VMA_MAYWRITE_BIT. > > To achieve both of these things, forbid this for mm-managed mappings as > identified by vma_flags_is_mm_managed(). > > This constrains this ability to drivers which own the mapping's contents, > whether memory-mapped I/O, kernel-allocated pages, or ordinary pages they > map themselves, and so define its semantics. > > Every in-tree mmap hook which clears VMA_MAYWRITE_BIT, some twenty sites > across drivers, filesystems and bpf, establishes a mapping which is not > mm-managed, > with usbmon and the ALSA PCM status page converted earlier in this series > to do so. > > Note that drivers may, if they do not gate on VMA_SHARED_BIT, be able to > disable MAP_PRIVATE-file-backed mapping CoW semantics. > > This is perhaps not always intended, but we retain this capacity to > maintain existing behaviour. > > As no driver which clears VMA_MAYWRITE_BIT establishes an mm-managed > mapping, no functional change is intended. > > Acked-by: Zi Yan > Signed-off-by: Lorenzo Stoakes (ARM) > --- Acked-by: David Hildenbrand (Arm) -- Cheers, David