linux-kernel - Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

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Date:   Wed, 11 Jul 2018 02:15:20 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        David Howells <dhowells@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for
 superblock creation [ver #9]

On Tue, Jul 10, 2018 at 06:05:49PM -0700, Linus Torvalds wrote:
> Yeah, Andy is right that we should *not* make "write()" have side effects.
> 
> Use it to queue things by all means, but not "do" things. Not unless
> there's a very sane security model.
> 
> On Tue, Jul 10, 2018 at 4:59 PM Andy Lutomirski <luto@...capital.net> wrote:
> >
> > I think the right solution is one of:
> >
> > (a) Pass a netlink-formatted blob to fsopen() and do the whole thing in one syscall. I don’t mean using netlink sockets — just the nlattr format.  Or you could use a different format. The part that matters is using just one syscall to do the whole thing.
> 
> Please no. Not another nasty marshalling thing.
> 
> > (b) Keep the current structure but use a new syscall instead of write().
> >
> > (c) Keep using write() but literally just buffer the data. Then have a new syscall to commit it.  In other words, replace “x” with a syscall and call all the fs_context_operations helpers in that context instead of from write().
> 
> But yeah, b-or-c sounds fine.

Umm...  How about "use credentials of opener for everything"?