From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jim Lieb <jlieb@panasas.com>
Subject: Re: Re: Re: Re: Re: [PATCH 1/3] switch_creds: Syscall to switch creds for file server ops
Date: Fri, 1 Nov 2013 08:49:12 -0700
Message-ID: <3320616.dLyWU2ChR5@jlieb-e6410>
References: <1381960919-4542-1-git-send-email-jlieb@panasas.com> <1397707.n4PsIJcfRu@jlieb-e6410> <201311012224.CBG18210.StOOMLHVJOFQFF@I-love.SAKURA.ne.jp>
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Cc: <luto@amacapital.net>, <linux-fsdevel@vger.kernel.org>,
	<ebiederm@xmission.com>, <viro@zeniv.linux.org.uk>,
	<tytso@mit.edu>, <linux-kernel@vger.kernel.org>,
	<bfields@redhat.com>, <jlayton@redhat.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <201311012224.CBG18210.StOOMLHVJOFQFF@I-love.SAKURA.ne.jp>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

On Friday, November 01, 2013 22:24:12 Tetsuo Handa wrote:
> Jim Lieb wrote:
> > Subsequent uses look like:
> > 	use_creds(cached fd);
> >=20
> > followed by
> >=20
> > 	open/creat/mknod/write
> >=20
> > followed by
> >=20
> > 	use_creds(-1);
>=20
> Are you aware that calling commit_creds() is prohibitted between
> override_creds() and revert_creds() ?
>=20
> If the caller does some operation that calls commit_creds() (like
> example below), the kernel triggers BUG().

Yes, I do.  I caught this in an early pass.  I only use override_creds(=
) and=20
revert_creds(). =20
>=20
> ---------- example module start ----------
> #include <linux/module.h>
> #include <linux/cred.h>
> #include <linux/fs.h>
> #include <linux/file.h>
>=20
> static int __init test_init(void)
> {
>         { /* switch_creds() syscall */
>                 struct fd f =3D fdget(0);
>                 if (!f.file)
>                         return -EBADF;
>                 put_cred(override_creds(f.file->f_cred));
>                 fdput(f);
>         }
>         { /* something that calls commit_creds() */
>                 struct cred *cred =3D prepare_creds();
>                 if (cred)
>                         commit_creds(cred);
>         }
>         return 0;
> }
>=20
> static void test_exit(void)
> {
> }
>=20
> module_init(test_init);
> module_exit(test_exit);
> MODULE_LICENSE("GPL");
> ---------- example module end ----------
>=20
> Since nobody can guarantee that the caller of switch_creds() never do=
es
> some operation that calls commit_creds(), I don't think switch_creds(=
)
> based on override_creds() will work.

--=20
Jim Lieb
Linux Systems Engineer
Panasas Inc.

"If ease of use was the only requirement, we would all be riding tricyc=
les"
- Douglas Engelbart 1925=E2=80=932013