From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 69F70ECDFBB for ; Wed, 18 Jul 2018 18:13:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 231A42084E for ; Wed, 18 Jul 2018 18:13:02 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 231A42084E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ZenIV.linux.org.uk Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727657AbeGRSwD (ORCPT ); Wed, 18 Jul 2018 14:52:03 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:37382 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726633AbeGRSwD (ORCPT ); Wed, 18 Jul 2018 14:52:03 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.87 #1 (Red Hat Linux)) id 1ffqwW-0001Q5-Gg; Wed, 18 Jul 2018 18:12:52 +0000 Date: Wed, 18 Jul 2018 19:12:52 +0100 From: Al Viro To: Miklos Szeredi Cc: Stephen Rothwell , linux-fsdevel , Linux Kernel Mailing List , Linus Torvalds Subject: [RFC] call_with_creds() Message-ID: <20180718181252.GU30522@ZenIV.linux.org.uk> References: <20180711161540.GS30522@ZenIV.linux.org.uk> <20180712124326.GA19272@ZenIV.linux.org.uk> <20180712155337.GU30522@ZenIV.linux.org.uk> <20180718025636.GA26175@ZenIV.linux.org.uk> <20180718132955.2bf185b7@canb.auug.org.au> <20180718124340.GS30522@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 18, 2018 at 05:46:09PM +0200, Miklos Szeredi wrote: > I like the call_with_creds() idea. I didn't realize that > override_creds()/revert_creds() can be quite heavyweight due to doing > (unnecessary in this case) refcounting. Could use call_with_creds() > in overlayfs too, since we hold ref on ofs->creator_cred for the > lifetime of the filesystem. OK... /* expr is non-void here */ #define __call_with_creds(__cred, expr) ({ \ const struct cred *____old = current->cred; \ const struct cred *____new = __cred; \ __typeof__(expr) __res; \ rcu_assign_pointer(current->cred, ____new); \ __res = (expr); \ BUG_ON(current->cred != ____new); \ rcu_assign_pointer(current->cred, ____old); \ __res; }) /* expr for non-void, expr,0 for void */ #define __wrap_void(expr) __builtin_choose_expr( \ __builtin_types_compatible_p(__typeof__(expr), void), \ ((expr),0), (expr)) /* * Evaluate an expression with current->cred temporary set to __cred. * NOTE: expr must not result in changed ->cred - any changes during * its evaluation must be undone. */ #define call_with_creds(__cred, expr) \ ((__typeof__(expr))__call_with_creds(__cred, __wrap_void(expr))) Linus, David - do you have any objections to the above? I would like to do some of the file method calls via that - e.g. have callers of ->write() (both __vfs_write() and do_loop_readv_writev()) use nr = call_with_creds(file->f_cred, file->f_op->write(file, iovec.iov_base, iovec.iov_len, ppos)); instead of nr = file->f_op->write(file, iovec.iov_base, iovec.iov_len, ppos)); Ditto for call of open() in do_dentry_open(), etc.