linux-kernel - Re: Pondering per-process vsyscall disablement

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Message-ID: <CALCETrX9s7xJRddB26ZiyjMEGbupbDj2qDHhio=80XSQ+staDA@mail.gmail.com>
Date:	Fri, 30 May 2014 13:11:03 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Marian Marinov <mm@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Linux API <linux-api@...r.kernel.org>
Subject: Re: Pondering per-process vsyscall disablement

On Fri, May 30, 2014 at 1:05 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 05/30/2014 01:00 PM, Andy Lutomirski wrote:
>>
>> Do the flags go in the ELF loader or in the executable we're running?
>> Or both (and, if both, do we and them or or them)?
>>
>> I think the interpreter makes a little more sense in general: for the
>> most part, use of vsyscalls is a property of the runtime environment,
>> not of the program being run.  But maybe this is naive.
>>
>
> They go into each object which becomes part of the running program, i.e.
> executable, dynamic libraries, and dynamic linker.

Well, sure, but the kernel is not about to start reading ELF headers
in dynamic libraries.  So we need to make a decision based on the
interpreter and the executable.  The conservative approach is to
require both to have the flag set *and* to offer a prctl to twiddle
the flags.  Then userspace loaders can do whatever they want, and
distros get to rebuild the world :)

--Andy
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