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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: 5ogzfbydc178kgcdh5ootu71hs88xwou X-Rspamd-Queue-Id: 4CCF51C000F X-HE-Tag: 1682280144-272342 X-HE-Meta: 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 RsZpHOnT D4ImiFSRYN59ukAWgX6wsFxJ/S5h0nPAXUwiMshxIMDoU4VusPOBCxm6AQbInvZhQisRSTFvDmxltINhAF/Gk85j9ACWjc/lgXsJxjznxtJD+3sDqFQfALjT4m3Yvlaszdy9s6c8G3xcP7IOn1SB7DhlxcssGZ0J1ULw+eMqSWVQTTq8NyYU2DEf6JD0/32xUUgGrpnmCz7JoYJgOZGw4ALCDyRIToXbp4FiyZXRcNTzlATe0zJlLiuhjoATlKrXHcfI7xxWaFl6hTR9gakggL9IUjCc0/LMLmK5Z48l3NpC1dmxR/i7t7QkHniPv8As7AIuP+b0xFkc2UmDsQuFX4P4Ux/7RHiOea6LQIY7HTLXK3N4= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sun, Apr 23, 2023 at 08:40:53PM +0100, Lorenzo Stoakes wrote: > On Sun, Apr 23, 2023 at 09:32:01PM +0200, Simon Horman wrote: > > On Sat, Apr 22, 2023 at 02:37:05PM +0100, Lorenzo Stoakes wrote: > > > It isn't safe to write to file-backed mappings as GUP does not ensure that > > > the semantics associated with such a write are performed correctly, for > > > instance filesystems which rely upon write-notify will not be correctly > > > notified. > > > > > > There are exceptions to this - shmem and hugetlb mappings are (in effect) > > > anonymous mappings by other names so we do permit this operation in these > > > cases. > > > > > > In addition, if no pinning takes place (neither FOLL_GET nor FOLL_PIN is > > > specified and neither flags gets implicitly set) then no writing can occur > > > so we do not perform the check in this instance. > > > > > > This is an important exception, as populate_vma_page_range() invokes > > > __get_user_pages() in this way (and thus so does __mm_populate(), used by > > > MAP_POPULATE mmap() and mlock() invocations). > > > > > > There are GUP users within the kernel that do nevertheless rely upon this > > > behaviour, so we introduce the FOLL_ALLOW_BROKEN_FILE_MAPPING flag to > > > explicitly permit this kind of GUP access. > > > > > > This is required in order to not break userspace in instances where the > > > uAPI might permit file-mapped addresses - a number of RDMA users require > > > this for instance, as do the process_vm_[read/write]v() system calls, > > > /proc/$pid/mem, ptrace and SDT uprobes. Each of these callers have been > > > updated to use this flag. > > > > > > Making this change is an important step towards a more reliable GUP, and > > > explicitly indicates which callers might encouter issues moving forward. > > > > nit: s/encouter/encounter/ > > > > Ack, I always seem to leave at least one or two easter egg spelling > mistakes in :) :) > Will fix up on next respin (in unlikely event of no further review, > hopefully Andrew would pick up!) >