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Date:   Sun, 23 Apr 2023 20:40:53 +0100
From:   Lorenzo Stoakes <lstoakes@gmail.com>
To:     Simon Horman <simon.horman@corigine.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by
 default
Message-ID: <a6ceb82a-1766-4229-ba33-b6f25e111561@lucifer.local>
References: <f86dc089b460c80805e321747b0898fd1efe93d7.1682168199.git.lstoakes@gmail.com>
 <ZEWHsbxhQlrSqnSC@corigine.com>
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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!)