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Date:   Sun, 23 Apr 2023 22:02:11 +0200
From:   Simon Horman <horms@kernel.org>
To:     Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     Simon Horman <simon.horman@corigine.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jason Gunthorpe <jgg@ziepe.ca>, Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
        Leon Romanovsky <leon@kernel.org>,
        Christian Benvenuti <benve@cisco.com>,
        Nelson Escobar <neescoba@cisco.com>,
        Bernard Metzler <bmt@zurich.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Bjorn Topel <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
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        Alexei Starovoitov <ast@kernel.org>,
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        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        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: <ZEWOwz/82AKHGrXW@kernel.org>
References: <f86dc089b460c80805e321747b0898fd1efe93d7.1682168199.git.lstoakes@gmail.com>
 <ZEWHsbxhQlrSqnSC@corigine.com>
 <a6ceb82a-1766-4229-ba33-b6f25e111561@lucifer.local>
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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!)
>