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

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Message-ID: <BANLkTi=+E+WzUgqEQTnRPmL1g5yPsXu8Bw@mail.gmail.com>
Date:	Wed, 27 Apr 2011 11:25:47 -0400
From:	Mike Frysinger <vapier@...too.org>
To:	Andrea Righi <righi.andrea@...il.com>
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 05:57, 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 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?

my (limited) understanding is that you should be using cap checks, not UID
-mike
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