linux-kernel - Re: [RFC] [PATCH] drop

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Date:	Wed, 27 Apr 2011 11:57:10 +0200
From:	Andrea Righi <righi.andrea@...il.com>
To:	Mike Frysinger <vapier@...too.org>
Cc:	Andrea Righi <andrea@...terlinux.com>,
	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>, linux-fsdevel@...r.kernel.org,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] drop_pagecache syscall

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 to immediately drop
> >> >> > pages that for sure will not be reused in the near future, without calling
> >> >> > posix_fadvise() for all the files they've touched), or to provide a more fine
> >> >> > grained debugging feature usable by the filesystem benchmarks.
> >> >> >
> >> >> > The system call does not require root privileges and it can be called by any
> >> >> > unprivileged application. For example, we can write a userspace tool to run
> >> >> > something like this:
> >> >> >
> >> >> >   $ 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 execute
> >> > this syscall.
> >>
> >> if /proc/sys/vm/drop_caches has any checks other than file permission
> >> checks (i.e. UID==0), it'd probably be better to copy those rather
> >> than picking something different.
> >
> > ok, what about checking current_euid() == 0?
> 
> that's not what i meant.  if the drop_caches file already has certain
> cap checks/whatever in place, let's use those.  if it doesnt, then
> picking a cap level as you proposed makes sense.

mmh, drop_caches has a file ownership (root:root) and a permission mask
(0644), how to apply the same checks to a system call? The most similar
thing seems to check the current euid. Am I missing something?

-Andrea
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