linux-kernel - Re: [PATCH v2] x86: Remove compat vdso support

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Date:	Wed, 12 Mar 2014 14:45:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Stefani Seibold <stefani@...bold.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Dave Jones <davej@...hat.com>,
	Martin Runge <Martin.Runge@...de-schwarz.com>,
	Andreas Brief <Andreas.Brief@...de-schwarz.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] x86: Remove compat vdso support

On Wed, Mar 12, 2014 at 2:37 PM, H. Peter Anvin <hpa@...ux.intel.com> wrote:
> On 03/12/2014 12:41 PM, Linus Torvalds wrote:
>>
>> So my reaction was "don't do that".
>>
>> But people pointing out that we can't do what x86-64 does made me
>> think: we could avoid the whole "nasty code for a legacy case" by
>> making it the *non*-legacy case. We could get rid of the fixmap
>> HPET/VVAR entirely - on x86-64 (which can use those addresses) a
>> PC-relative addressing is probably actually better anyway, so mapping
>> them together with the vdso code shouldn't hurt.
>>
>
> How would that deal with the legacy vsyscall case for x86-64?  Just rely
> on the "legacy vsyscall emulation" (which seems to have its own class of
> problems...)?

The emulated 64-bit vsyscall is completely independent of the vdso and
any user-accessible data whatsoever -- it's just a self-contained bit
of code that runs in kernel space.  There is no user-executable code
involved at all.  (There is code that *looks* executable to a dynamic
recompiler, but that code consists of ordinary system calls.)  As far
as I know, all of the problems with it (other than the fact that it
exists at all) were solved a couple years ago.

--Andy
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