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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 24 Jan 2019 13:20:37 +1300
Message-ID: <CAHk-=whVyE2TL4NpEgsSnx=w0Pf-vNBidJY9HEeOVLO-m=Mx+g@mail.gmail.com>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged
To:     Jiri Kosina <jikos@kernel.org>
Cc:     Dominique Martinet <asmadeus@codewreck.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Josh Snyder <joshs@netflix.com>,
        Dave Chinner <david@fromorbit.com>,
        Matthew Wilcox <willy@infradead.org>,
        Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>, Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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On Thu, Jan 24, 2019 at 12:12 PM Jiri Kosina <jikos@kernel.org> wrote:
>
> >
> > I think the "test vm_file" thing may be unnecessary, because a
> > non-anonymous mapping should always have a file pointer and an inode.
> > But I could  imagine some odd case (vdso mapping, anyone?) that
> > doesn't have a vm_file, but also isn't anonymous.
>
> Hmm, good point.
>
> So dropping the 'vma->vm_file' test and checking whether given vma is
> special mapping should hopefully provide the desired semantics, shouldn't
> it?

Maybe. But on the whole I think it would  be simpler and more
straightforward to just instead add a vm_file test for the
inode_permission() case. That way you at least know that you aren't
following a NULL pointer.

If the file then turns out to be some special thing, it doesn't really
_matter_, I think. It won't have anything in the page cache etc, but
the code should "work".

             Linus