From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 58E7C4C6EEE; Mon, 12 Oct 2026 18:07:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791828476; cv=none; b=J4+bA1Kt0WDqIZJgGwy/+fxsTbb/38vf821vuook5LLREADkT08DfyjKwWSPLquf59cqMOb1enE2FGn/9eCRgmVo0gKQMIAhqf+4HXYmmavyXFQ2E9IGlfOWDmUh74897mKsTHYmb3UaIWRNTEpAmkUjWR4/aV6ILotoXPYjihU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791828476; c=relaxed/simple; bh=QZ16+1BqTvxVlZnvetZ5guF0tnfS/81HjExGSkHLxxo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GpcwcvGFwdNbgAFyEl0MuRYya2iot4xtE/pmBG16py7gFVFSQYfMdnGfSj+Nn/lcSa1OX6ur2uHw5wt/FFQeZF+xG4051lrnOajqfxusLfoUCXJLTiZWg4pEBMUAqJwzh+y6fbCprjQBIiSRmXDfp8QblvD9ZvnU9TYX9piZP0U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=jwx+5rDY; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="jwx+5rDY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 58BFE1F0089B; Mon, 12 Oct 2026 18:07:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791828474; bh=5a6IsUlcXJHOADvAwqphif4IshvHpUsfPxJJpTYlkEY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=jwx+5rDYQjOIUBj08GLlEX9Web1Y8AehnEyD+NKJ+6JGLiDzQ2ykGnZMGinpwVxEm t43H8LUI4C36o21aCrvggHyN9RyNzw2SGdD/OyPLFBA80sFmKNnm84NtWUvo2kYU8N cYNbAPpDSSO2uHexKXokGU+9jMdh9g5QSHt0uWmtCrNqLoFovjiA4CtdU2HJfp322U 9aldNEnM+7h1H81AjZw6XqSBXIUYwZUU/Fk7ap4BPata9FLOdT63qFOxwbma7qOQ8U Nq/+WfROCztx3KWcWvYYP+Ey80vw0kj1tlhrwaXghvM9DOVKk2/vrEDswVQZvybKaC EY265kFEoC1fg== Message-ID: <5a7597e2-e4f6-46ef-a4b4-2f0746b5009a@kernel.org> Date: Mon, 12 Oct 2026 20:07:14 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 15/38] mm/vma: only allow mmap to clear VMA_MAYWRITE_BIT if not mm-managed To: "Lorenzo Stoakes (ARM)" , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Greg Kroah-Hartman , Dennis Dalessandro , Jason Gunthorpe , Leon Romanovsky , Paul Moore , Stephen Smalley , Jaroslav Kysela , Takashi Iwai , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Doug Gilbert , "James E.J. Bottomley" , "Martin K. Petersen" , Simona Vetter , Helge Deller , Sebastian Reichel , John Hubbard , Peter Xu , Masami Hiramatsu , Oleg Nesterov , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Rik van Riel , Harry Yoo , Juri Lelli , Vincent Guittot , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Arnd Bergmann , Muchun Song , Oscar Salvador , "Matthew Wilcox (Oracle)" , Jan Kara , Marc Zyngier , Oliver Upton , Catalin Marinas , Madhavan Srinivasan , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , "David S. Miller" , Andreas Larsson , Alexander Viro , Christian Brauner , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Youngjun Park , Johannes Weiner , Qi Zheng , Shakeel Butt , Axel Rasmussen , Yuanchu Xie , Wei Xu , Chengming Zhou , Michal Hocko , Miklos Szeredi , Xu Xin Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-usb@vger.kernel.org, linux-rdma@vger.kernel.org, selinux@vger.kernel.org, linux-sound@vger.kernel.org, bpf@vger.kernel.org, linux-scsi@vger.kernel.org, linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-arch@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, 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; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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