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Message-ID: <ZEWHsbxhQlrSqnSC@corigine.com> Date: Sun, 23 Apr 2023 21:32:01 +0200 From: Simon Horman <simon.horman@...igine.com> To: Lorenzo Stoakes <lstoakes@...il.com> Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>, Matthew Wilcox <willy@...radead.org>, Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>, Leon Romanovsky <leon@...nel.org>, Christian Benvenuti <benve@...co.com>, Nelson Escobar <neescoba@...co.com>, Bernard Metzler <bmt@...ich.ibm.com>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Bjorn Topel <bjorn@...nel.org>, Magnus Karlsson <magnus.karlsson@...el.com>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Jonathan Lemon <jonathan.lemon@...il.com>, "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Christian Brauner <brauner@...nel.org>, Richard Cochran <richardcochran@...il.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org, netdev@...r.kernel.org, bpf@...r.kernel.org Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default 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/
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