From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Frysinger Subject: Re: [RFC] [PATCH] drop_pagecache syscall Date: Wed, 27 Apr 2011 11:25:47 -0400 Message-ID: References: <1303853727-21444-1-git-send-email-andrea@betterlinux.com> <20110427001453.GD12436@dastard> <20110427085910.GA1749@linux.betterlinux.com> <20110427094717.GB1749@linux.betterlinux.com> <20110427095709.GA1687@linux.develer.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20110427095709.GA1687-fxUVXftIFDlZdMzt4l2sLQC/G2K4zDHf@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Andrea Righi Cc: Andrea Righi , Dave Chinner , Andrew Morton , Al Viro , Arnd Bergmann , linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-api@vger.kernel.org On Wed, Apr 27, 2011 at 05:57, Andrea Righi wrote: > On Wed, Apr 27, 2011 at 05:50:04AM -0400, Mike Frysinger wrote: >> On Wed, Apr 27, 2011 at 05:47, Andrea Righi wrote: >> > On Wed, Apr 27, 2011 at 05:10:41AM -0400, Mike Frysinger wrote: >> >> On Wed, Apr 27, 2011 at 05:01, Andrea Righi wrote: >> >> > On Wed, Apr 27, 2011 at 10:14:53AM +1000, Dave Chinner wrote: >> >> >> On Tue, Apr 26, 2011 at 11:35:27PM +0200, Andrea Righi wrote: >> >> >> > This functionality can be used by all the applications that = want to have a >> >> >> > better control over the page cache management (for example t= o immediately drop >> >> >> > pages that for sure will not be reused in the near future, w= ithout calling >> >> >> > posix_fadvise() for all the files they've touched), or to pr= ovide a more fine >> >> >> > grained debugging feature usable by the filesystem benchmark= s. >> >> >> > >> >> >> > The system call does not require root privileges and it can = be called by any >> >> >> > unprivileged application. For example, we can write a usersp= ace tool to run >> >> >> > something like this: >> >> >> > >> >> >> > =C2=A0 $ drop-pagecache /path/file_or_dir >> >> >> >> >> >> That's a potential DOS vector, I think. Drop the pagecache in = a hard >> >> >> loop on the root fs of a busy server and watch it crawl... >> >> > >> >> > Yes, probably we could allow only the CAP_SYS_ADMIN tasks to ex= ecute >> >> > this syscall. >> >> >> >> if /proc/sys/vm/drop_caches has any checks other than file permis= sion >> >> checks (i.e. UID=3D=3D0), it'd probably be better to copy those r= ather >> >> than picking something different. >> > >> > ok, what about checking current_euid() =3D=3D 0? >> >> that's not what i meant. =C2=A0if the drop_caches file already has c= ertain >> cap checks/whatever in place, let's use those. =C2=A0if it doesnt, t= hen >> picking a cap level as you proposed makes sense. > > mmh, drop_caches has a file ownership (root:root) and a permission ma= sk > (0644), how to apply the same checks to a system call? The most simil= ar > thing seems to check the current euid. Am I missing something? my (limited) understanding is that you should be using cap checks, not = UID -mike