Re: [RFC PATCH] fs: allow open(dir, O_TMPFILE|..., 0) with mode 0

[Jeremy Allison <jra@samba.org>]

On Mon, Nov 03, 2014 at 10:49:24AM -0800, Eric Rannaud wrote:
> On Mon, Nov 3, 2014 at 9:06 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> >> That doesn't help because we explicitly reject O_RDONLY when combined
> >> with O_TMPFILE.
> >
> > I think I'm missing something.  How is an O_RDONLY temporary file
> > useful?  Wouldn't you want an O_RDWR tempfile with mode 0400 or
> > something like that?
> 
> Isn't it because they are essentially emulating an atomic open()
> capable of creating a file with inherited ACLs, according to
> relatively complex rules? open *can* be used with O_CREAT|O_RDONLY
> (touch(1) might do that), which would naively translate into:
> 
>         fd = open(dir, O_TMPFILE|O_RDONLY, 0600)
>         fsetxattr(fd, "...")
>         fsetxattr(fd, "...")
>         linkat(AT_FDCWD, "/proc/self/fd/...", ..., AT_SYMLINK_FOLLOW)
>         return fd;
> 
> Now this would be happening on the server, and the only reason why it
> would be important to ensure that fd is O_RDONLY, is that smbd does
> not do its own bookkeeping of how each file handle was opened, and
> would rather have the kernel enforce O_RDONLY?
> 
> With O_TMPFILE as implemented now, smbd would have to do open(dir,
> O_TMPFILE|O_RDWR, 0600), but internally keep track that O_RDONLY was
> requested by the client on that fd, and block any writes to fd itself.

Which we already do, actually..

Although the atomic open emulation is
a very interesting idea for us. That's
something we currently don't do correctly
across different protocols (although we
do it between smbd's themselves).

Jeremy.