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From: Andy Lutomirski <luto@amacapital.net>
To: Daniel Colascione <dancol@google.com>
Cc: Jann Horn <jannh@google.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	John Reck <jreck@google.com>,
	John Stultz <john.stultz@linaro.org>,
	Todd Kjos <tkjos@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Christoph Hellwig <hch@infradead.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Bruce Fields <bfields@fieldses.org>,
	Jeff Layton <jlayton@kernel.org>,
	Khalid Aziz <khalid.aziz@oracle.com>,
	Lei.Yang@windriver.com, linux-fsdevel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
	marcandre.lureau@redhat.com,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Minchan Kim <minchan@kernel.org>, Shuah Khan <shuah@kernel.org>,
	valdis.kletnieks@vt.edu, Hugh Dickins <hughd@google.com>,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
Date: Fri, 9 Nov 2018 15:14:02 -0800
Message-ID: <F8A6A5DC-3BA0-43BD-B7EC-EDE199B33A02@amacapital.net> (raw)
In-Reply-To: <CAKOZuetZrL10zWwn4Jzzg0QL2nd3Fm0JxGtzC79SZAfOK525Ag@mail.gmail.com>



> On Nov 9, 2018, at 2:42 PM, Daniel Colascione <dancol@google.com> wrote:
> 
> On Fri, Nov 9, 2018 at 2:37 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> Another, more general fix might be to prevent /proc/pid/fd/N opens
>>> from "upgrading" access modes. But that'd be a bigger ABI break.
>> 
>> I think we should fix that, too.  I consider it a bug fix, not an ABI break, personally.
> 
> Someone, somewhere is probably relying on it though, and that means
> that we probably can't change it unless it's actually causing
> problems.
> 
> <mumble>spacebar heating</mumble>

I think it has caused problems in the past. It’s certainly extremely surprising behavior.  I’d say it should be fixed and, if needed, a sysctl to unfix it might be okay.

> 
>>>> That aside: I wonder whether a better API would be something that
>>>> allows you to create a new readonly file descriptor, instead of
>>>> fiddling with the writability of an existing fd.
>>> 
>>> That doesn't work, unfortunately. The ashmem API we're replacing with
>>> memfd requires file descriptor continuity. I also looked into opening
>>> a new FD and dup2(2)ing atop the old one, but this approach doesn't
>>> work in the case that the old FD has already leaked to some other
>>> context (e.g., another dup, SCM_RIGHTS). See
>>> https://developer.android.com/ndk/reference/group/memory. We can't
>>> break ASharedMemory_setProt.
>> 
>> 
>> Hmm.  If we fix the general reopen bug, a way to drop write access from an existing struct file would do what Android needs, right?  I don’t know if there are general VFS issues with that.
> 
> I also proposed that. :-) Maybe it'd work best as a special case of
> the perennial revoke(2) that people keep proposing. You'd be able to
> selectively revoke all access or just write access.

Sounds good to me, modulo possible races, but that shouldn’t be too hard to deal with.

  reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-08  4:15 Joel Fernandes (Google)
2018-11-08  4:15 ` [PATCH v3 resend 2/2] selftests/memfd: Add tests for F_SEAL_FUTURE_WRITE seal Joel Fernandes (Google)
2018-11-09  8:49 ` [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd Joel Fernandes
2018-11-09 20:36 ` Andrew Morton
2018-11-10  3:54   ` Joel Fernandes
2018-11-09 21:06 ` Jann Horn
2018-11-09 21:19   ` Jann Horn
2018-11-10  3:20     ` Joel Fernandes
2018-11-10  6:05       ` Andy Lutomirski
2018-11-10 18:24         ` Joel Fernandes
2018-11-10 18:45           ` Daniel Colascione
2018-11-10 19:11             ` Daniel Colascione
2018-11-10 19:55               ` Andy Lutomirski
2018-11-10 22:09               ` Joel Fernandes
2018-11-10 22:18                 ` Andy Lutomirski
2018-11-11  2:38                   ` Joel Fernandes
2018-11-11  3:40                     ` Andy Lutomirski
2018-11-11  4:01                       ` Joel Fernandes
2018-11-11  8:09                       ` Joel Fernandes
2018-11-11  8:30                         ` Daniel Colascione
2018-11-11 15:14                           ` Andy Lutomirski
2018-11-11 17:36                             ` Joel Fernandes
     [not found]       ` <CAKOZuethC7+YrRyyGciUCfhSSa9cCcAFJ8g_qEw9uh3TBbyOcg@mail.gmail.com>
2018-11-10 17:10         ` Joel Fernandes
2018-11-09 21:40   ` Andy Lutomirski
2018-11-09 20:02     ` Michael Tirado
2018-11-10  1:49       ` Joel Fernandes
2018-11-09 22:20   ` Daniel Colascione
2018-11-09 22:37     ` Andy Lutomirski
2018-11-09 22:42       ` Daniel Colascione
2018-11-09 23:14         ` Andy Lutomirski [this message]
2018-11-10  1:36           ` Joel Fernandes
2018-11-09 23:46   ` Joel Fernandes

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