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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 09F37C76188 for ; Wed, 17 Jul 2019 03:03:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DF1DF20693 for ; Wed, 17 Jul 2019 03:03:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728713AbfGQDDR (ORCPT ); Tue, 16 Jul 2019 23:03:17 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:52610 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728541AbfGQDDQ (ORCPT ); Tue, 16 Jul 2019 23:03:16 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92 #3 (Red Hat Linux)) id 1hnaDa-0008R1-HC; Wed, 17 Jul 2019 03:02:58 +0000 Date: Wed, 17 Jul 2019 04:02:58 +0100 From: Al Viro To: Rich Felker Cc: Palmer Dabbelt , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann , rth@twiddle.net, ink@jurassic.park.msu.ru, mattst88@gmail.com, linux@armlinux.org.uk, catalin.marinas@arm.com, will@kernel.org, tony.luck@intel.com, fenghua.yu@intel.com, geert@linux-m68k.org, monstr@monstr.eu, ralf@linux-mips.org, paul.burton@mips.com, jhogan@kernel.org, James.Bottomley@hansenpartnership.com, deller@gmx.de, benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au, heiko.carstens@de.ibm.com, gor@linux.ibm.com, borntraeger@de.ibm.com, ysato@users.sourceforge.jp, davem@davemloft.net, luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org, peterz@infradead.org, acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, dhowells@redhat.com, firoz.khan@linaro.org, stefan@agner.ch, schwidefsky@de.ibm.com, axboe@kernel.dk, christian@brauner.io, hare@suse.com, deepa.kernel@gmail.com, tycho@tycho.ws, kim.phillips@arm.com, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-arch@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH v2 2/4] Add fchmodat4(), a new syscall Message-ID: <20190717030258.GT17978@ZenIV.linux.org.uk> References: <20190717012719.5524-1-palmer@sifive.com> <20190717012719.5524-3-palmer@sifive.com> <20190717024046.GI1506@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190717024046.GI1506@brightrain.aerifal.cx> User-Agent: Mutt/1.11.3 (2019-02-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 16, 2019 at 10:40:46PM -0400, Rich Felker wrote: > On Tue, Jul 16, 2019 at 06:27:17PM -0700, Palmer Dabbelt wrote: > > man 3p says that fchmodat() takes a flags argument, but the Linux > > syscall does not. There doesn't appear to be a good userspace > > workaround for this issue but the implementation in the kernel is pretty > > straight-forward. The specific use case where the missing flags came up > > was WRT a fuse filesystem implemenation, but the functionality is pretty > > generic so I'm assuming there would be other use cases. > > Note that we do have a workaround in musl libc with O_PATH and > /proc/self/fd, but a syscall that allows a proper fix with the ugly > workaround only in the fallback path for old kernels will be much > appreciated! > > What about also doing a new SYS_faccessat4 with working AT_EACCESS > flag? The workaround we have to do for it is far worse. Umm... That's doable, but getting into the "don't switch creds unless needed" territory. I'll need to play with that a bit and see what gives a tolerable variant... What of this part wrt AT_EACCESS? if (!issecure(SECURE_NO_SETUID_FIXUP)) { /* Clear the capabilities if we switch to a non-root user */ kuid_t root_uid = make_kuid(override_cred->user_ns, 0); if (!uid_eq(override_cred->uid, root_uid)) cap_clear(override_cred->cap_effective); else override_cred->cap_effective = override_cred->cap_permitted; }