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From: Joel Fernandes <joel@joelfernandes.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Daniel Colascione <dancol@google.com>,
	Jann Horn <jannh@google.com>,
	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 17:36:11 -0800
Message-ID: <20181110013611.GA199560@google.com> (raw)
In-Reply-To: <F8A6A5DC-3BA0-43BD-B7EC-EDE199B33A02@amacapital.net>

On Fri, Nov 09, 2018 at 03:14:02PM -0800, Andy Lutomirski wrote:
> >>>> 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 don't think there is a way to fix this in /proc/pid/fd. At the proc
level, the /proc/pid/fd/N files are just soft symlinks that follow through to
the actual file. The open is actually done on that inode/file. I think
changing it the way being discussed here means changing the way symlinks work
in Linux.

I think the right way to fix this is at the memfd inode level. I am working
on a follow up patch on top of this patch, and will send that out in a few
days (along with the man page updates).

thanks!

 - Joel

  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
2018-11-10  1:36           ` Joel Fernandes [this message]
2018-11-09 23:46   ` Joel Fernandes

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