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Date:   Mon, 7 Dec 2020 22:32:23 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>, criu@openvz.org,
        bpf <bpf@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Jann Horn <jann@thejh.net>, Kees Cook <keescook@chromium.org>,
        Daniel P =?iso-8859-1?Q?=2E_Berrang=E9?= <berrange@redhat.com>,
        Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Matthew Wilcox <willy@infradead.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        Chris Wright <chrisw@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>
Subject: Re: [PATCH v2 02/24] exec: Simplify unshare_files
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On Mon, Nov 23, 2020 at 10:25:13AM -0800, Linus Torvalds wrote:
> On Mon, Nov 23, 2020 at 9:52 AM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > Can anyone explain why does do_coredump() need unshare_files() at all?
> 
> Hmm. It goes back to 2012, and it's placed just before calling
> "->core_dump()", so I assume some core dumping function messed with
> the file table back when..
> 
> I can't see anything like that currently.
> 
> The alternative is that core-dumping just keeps the file table around
> for a long while, and thus files don't actually close in a timely
> manner. So it might not be a "correctness" issue as much as a latency
> issue.

IIRC, it was "weird architecture hooks might be playing silly buggers
with some per-descriptor information they want in coredumps, better
make sure it can't change under them"; it doesn't cost much and
it reduced the analysis surface nicely.

Had been a while ago, so the memories might be faulty...  Anyway, that
reasoning seems to be applicable right now - rather than keeping an
eye on coredump logics on random architectures that might be looking
at descriptor table in unsafe way, just make sure they have a stable
private table and be done with that.

How much is simplified by not doing it there, anyway?