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From: Terry Lambert <tlambert@primenet.com>
Message-Id: <200011300628.XAA06955@usr08.primenet.com>
Subject: Re: HEADSUP user struct ucred -> xucred (Was: Re: serious problem with mutexs and userland visibility?)
To: bright@wintelcom.net (Alfred Perlstein)
Date: Thu, 30 Nov 2000 06:28:12 +0000 (GMT)
Cc: arch@FreeBSD.ORG
In-Reply-To: <20001129174905.S8051@fw.wintelcom.net> from "Alfred Perlstein" at Nov 29, 2000 05:49:05 PM
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> > I recently locked down struct ucred, not a big deal, basically just
> > a mutex in each struct to protect the refcount.
> > 
> > Unfortunetly struct ucred is used by some userland utils and
> > sys/ucred is included in sys/mount.h as well as sys/user.h, this
> > creates somewhat of a problem, forcing all users of sys/ucred.h to
> > include sys/mutex.g.
> > 
> > I have a patch here that sort of takes care of this problem, the
> > problem is that I had to add sys/mutex.h includes to both sys/mount.h
> > and sys/user.h, this doesn't make me very happy.
> 
> After a short discussion it has been determined that there will be
> a xucred exported to userland following the concention of xsocket
> and the various other xfoo structs exported to the kernel.
> 
> Struct ucred will no longer be visible outside the kernel.
> 
> Any userland things using struct ucred will need to use xucred.
> 
> This will be the convention used to resolve mutex (or other MD 
> fields) in kernel exported structures in the future.

This is a really gross way to handle this.  The ucred structure
is used by a lot of user space programs.

You should do what several UNIX vendors have already done, and
implement a MUTEX() declaration macro that differes in user and
kernel space, and forces an alignment; then when you copy out,
copy out everything _BUT_ the mutex portion to the user space,
and no user space source or object code will need to change.

So:

	#ifdef _KERNEL
	#define	MUTEX(x)	mutex_t	x;
	#define	UREF(x,y)	(void *)&((x)->y)
	#else
	#define	MUTEX(x)	/* user space = no mutex*/
	#define UREF(x,y)	(void *)(x)
	#endif

	struct foo {
		MUTEX(save_foo_from_bad_programmers)
		int	normal_foo_item_1;
		char	normal_foo_item_2;
		...
	};

	...

	struct foo *foop;

	...

	copyout( UREF(foop, normal_foo_item_1), user_space_foo);

It is much better to not impact user space code at all.


					Terry Lambert
					terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.


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