From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josh Triplett Subject: Re: [PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation Date: Wed, 8 Apr 2020 20:17:08 -0700 Message-ID: <20200409031708.GC6149@localhost> References: <90bf6fd43343ca862e7f61b0834baf2bdbd0e24c.1586321767.git.josh@joshtriplett.org> <20200408120040.mtkqmymfazrv3lqk@yavin.dot.cyphar.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200408120040.mtkqmymfazrv3lqk@yavin.dot.cyphar.com> Sender: linux-kernel-owner@vger.kernel.org To: Aleksa Sarai Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, io-uring@vger.kernel.org, linux-arch@vger.kernel.org, Alexander Viro , Arnd Bergmann , Jens Axboe List-Id: linux-arch.vger.kernel.org On Wed, Apr 08, 2020 at 10:00:40PM +1000, Aleksa Sarai wrote: > On 2020-04-07, Josh Triplett wrote: > > Some applications want to prevent the usual "lowest available fd" > > allocation from allocating certain file descriptors. For instance, they > > may want to prevent allocation of a closed fd 0, 1, or 2 other than via > > dup2/dup3, or reserve some low file descriptors for other purposes. > > > > Add a prctl to increase the minimum fd and return the previous minimum. > > > > System calls that allocate a specific file descriptor, such as > > dup2/dup3, ignore this minimum. > > > > exec resets the minimum fd, to prevent one program from interfering with > > another program's expectations about fd allocation. > > Why is it implemented as an "increase the value" interface? It feels > like this is meant to avoid some kind of security trap (with a library > reducing the value) but it means that if you want to temporarily raise > the minimum fd number it's not possible (without re-exec()ing yourself, > which is hardly a fun thing to do). > > Then again, this might've been discussed before and I missed it... It was: the previous version was a "get" and "set" interface. That interface didn't allow for the possibility that something else in the process had already set a minimum. This new atomic increase interface (which also serves as a "get" interface if you pass 0) makes it possible for a userspace library to reserve a range. (You have no guarantee about previously allocated descriptors in that range, but you know that no *new* automatically allocated descriptors will appear in that range, which suffices; userspace can do the rest.) - Josh Triplett