linux-kernel - Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE

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Message-ID: <20181110013611.GA199560@google.com>
Date:   Fri, 9 Nov 2018 17:36:11 -0800
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Daniel Colascione <dancol@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        John Reck <jreck@...gle.com>,
        John Stultz <john.stultz@...aro.org>,
        Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bruce Fields <bfields@...ldses.org>,
        Jeff Layton <jlayton@...nel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>, Lei.Yang@...driver.com,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>, marcandre.lureau@...hat.com,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Minchan Kim <minchan@...nel.org>,
        Shuah Khan <shuah@...nel.org>, valdis.kletnieks@...edu,
        Hugh Dickins <hughd@...gle.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to
 memfd

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