From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Layton Subject: Re: [PATCH RESEND v11 7/8] open: openat2(2) syscall Date: Wed, 28 Aug 2019 11:55:47 -0400 Message-ID: <4da231cd52880991d8a038adb8fbb2ef3d724db9.camel@kernel.org> References: <20190820033406.29796-1-cyphar@cyphar.com> <20190820033406.29796-8-cyphar@cyphar.com> <854l2366zp.fsf@catern.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <854l2366zp.fsf@catern.com> Sender: linux-kernel-owner@vger.kernel.org To: sbaugh@catern.com, linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@ozlabs.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org List-Id: linux-api@vger.kernel.org On Mon, 2019-08-26 at 19:50 +0000, sbaugh@catern.com wrote: > Aleksa Sarai writes: > > To this end, we introduce the openat2(2) syscall. It provides all of the > > features of openat(2) through the @how->flags argument, but also > > also provides a new @how->resolve argument which exposes RESOLVE_* flags > > that map to our new LOOKUP_* flags. It also eliminates the long-standing > > ugliness of variadic-open(2) by embedding it in a struct. > > I don't like this usage of a structure in memory to pass arguments that > would fit in registers. This would be quite inconvenient for me as a > userspace developer. > > Others have brought up issues with this: the issue of seccomp, and the > issue of mismatch between the userspace interface and the kernel > interface, are the most important for me. I want to add another, > admittedly somewhat niche, concern. > > This interfaces requires a program to allocate memory (even on the > stack) just to pass arguments to the kernel which could be passed > without allocating that memory. That makes it more difficult and less > efficient to use this syscall in any case where memory is not so easily > allocatable: such as early program startup or assembly, where the stack > may be limited in size or not even available yet, or when injecting a > syscall while ptracing. > > A struct-passing interface was needed for clone, since we ran out of > registers; but we have not run out of registers yet for openat, so it > would be nice to avoid this if we can. We can always expand later... > We can't really expand later like you suggest. Suppose in a couple of years that we need to add some new argument to openat2 that isn't just a new flag. If all these values are passed by individual arguments, you can't add one later without adding yet another syscall. Using a struct for this allows this to be extended later, OTOH. You can extend it, and add a flag that tells the kernel that it can access the new field. No new syscall required. -- Jeff Layton