Re: [PATCH 2.5.30+] Fourth attempt at a shared credentials patch

[torvalds@transmeta.com (Linus Torvalds)]

In article <55560000.1028921049@baldur.austin.ibm.com>,
Dave McCracken  <dmccr@us.ibm.com> wrote:
>
>--On Thursday, August 08, 2002 11:55:05 PM +0200 Trond Myklebust
><trond.myklebust@fys.uio.no> wrote:
>
>> What if one thread is doing an RPC call while the other is changing
>> the 'groups' entry?
>
>Gah.  Good point.  Ok, I've added locking to the cred structure to handle
>this.

Please don't do this with locking, I really think the right thing to do
is to have a "duplicate()" function, and when you pass credentials off
to something, you dup them at that point.

If you start off as non-root, and then execve suid into root, a pending
NFS request should _not_ suddently have the credentials changed under
it. Yet clearly that kind of thing can't just be locked either.

Along with copy-on-write semantics, this should perform perfectly well
(ie "duplicate()" would only increment a count, and then setuid() would
have to have code soemthing like

	if (cred->count > 1) {
		newcred = alloc_cred();
		copy_cred(newcred, cred);
		for_each_cred_group(p) {
			p->cred = newcred;
			atomic_inc(&newcred->count);
			putcred(cred);
		}
	}

instead.

		Linus