linux-kernel - Re: Thoughts on credential switching

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Date:	Thu, 27 Mar 2014 11:46:39 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jeremy Allison <jra@...ba.org>
Cc:	Jeff Layton <jlayton@...hat.com>,
	Florian Weimer <fweimer@...hat.com>,
	Jim Lieb <jlieb@...asas.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	"Serge E. Hallyn" <serge@...onical.com>,
	Kees Cook <keescook@...omium.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	bfields@...hat.com
Subject: Re: Thoughts on credential switching

On Thu, Mar 27, 2014 at 11:26 AM, Jeremy Allison <jra@...ba.org> wrote:
>
> Amen to that :-).
>
> However, after talking with Jeff and Jim at CollabSummit,
> I was 'encouraged' to make my opinions known on the list.
>
> To me, calling the creds handle a file descriptor just
> feels wrong. IT *isn't* an fd, you can't read/write/poll
> on it, and it's only done as a convenience to get the
> close-on-exec semantics and the fact that the creds are
> already hung off the fd's in kernel space.

Windows calls these things "handles."  Linux has "file descriptors,"
and there's plenty of precedent for things that aren't files.

>
> I'd rather any creads call use a different type, even if
> it's a typedef of 'int -> creds_handle_t', just to make
> it really clear it's *NOT* an fd.
>
> That way we can also make it clear this thing only has
> meaning to a thread group, and SHOULD NOT (and indeed
> preferably CAN NOT) be passed between processes.
>

If you want those semantics, then stick a struct pid * in there for
the tgid of the cretor and make sure that current's tgid matches when
you try to use it.

I think they'd be more useful without that check, though.

BTW, what do you want to have happen on fork?  I think they should keep working.

> Cheers,
>
>         Jeremy.



-- 
Andy Lutomirski
AMA Capital Management, LLC
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