2.6.21 strange optimization in svcauth_unix.c

[Frank van Maarseveen <frankvm@frankvm.com>]

While reading the 2.6.21 version of net/sunrpc/svcauth_unix.c it looks
to me that it tries to cache the AUTH_UNIX/AUTH_SYS group list on uid
basis and thus deliberately ignore the group ids supplied by the NFS
client. Fragment of svcauth_unix_accept():

	cred->cr_uid = svc_getnl(argv);		/* uid */
	cred->cr_gid = svc_getnl(argv);		/* gid */
	slen = svc_getnl(argv);			/* gids length */
	if (slen > 16 || (len -= (slen + 2)*4) < 0)
		goto badcred;
=>	if (unix_gid_find(cred->cr_uid, &cred->cr_group_info, rqstp)
	    == -EAGAIN)
		return SVC_DROP;
=>	if (cred->cr_group_info == NULL) {
		cred->cr_group_info = groups_alloc(slen);
		if (cred->cr_group_info == NULL)
			return SVC_DROP;
		for (i = 0; i < slen; i++)
			GROUP_AT(cred->cr_group_info, i) = svc_getnl(argv);
=>	} else {
=>		for (i = 0; i < slen ; i++)
=>			svc_getnl(argv);
=>	}

I guess this caching will break the linux NFS client patch for bypassing
the 16 groups limits maintained by me (http://www.frankvm.com/nfs-ngroups/ )
because in order to bypass the group limitation the group list is
determined dynamically at the NFS client.

Of course I can just disable the above optimization in my own patch but
I wonder what its real purpose is. I don't see a big win so is it an
optimization or is there another reason?

-- 
Frank

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