Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init_mm page tables

[Andy Lutomirski <luto@amacapital.net>]

> On Nov 2, 2017, at 12:33 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> 
>> On Thu, 2 Nov 2017, Andy Lutomirski wrote:
>>> On Wed, Nov 1, 2017 at 3:20 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>>>> On Wed, 1 Nov 2017, Linus Torvalds wrote:
>>>>> On Wed, Nov 1, 2017 at 2:52 PM, Dave Hansen <dave.hansen@linux.intel.com> wrote:
>>>>>> On 11/01/2017 02:28 PM, Thomas Gleixner wrote:
>>>>>>> On Wed, 1 Nov 2017, Andy Lutomirski wrote:
>>>>>>> The vsyscall page is _PAGE_USER and lives in init_mm via the fixmap.
>>>>>> 
>>>>>> Groan, forgot about that abomination, but still there is no point in having
>>>>>> it marked PAGE_USER in the init_mm at all, kaiser or not.
>>>>> 
>>>>> So shouldn't this patch effectively make the vsyscall page unusable?
>>>>> Any idea why that didn't show up in any of the x86 selftests?
>>>> 
>>>> I actually think there may be two issues here:
>>>> 
>>>> - vsyscall isn't even used much - if any - any more
>>> 
>>> Only legacy user space uses it.
>>> 
>>>> - the vsyscall emulation works fine without _PAGE_USER, since the
>>>> whole point is that we take a fault on it and then emulate.
>>>> 
>>>> We do expose the vsyscall page read-only to user space in the
>>>> emulation case, but I'm not convinced that's even required.
>>> 
>>> I don't see a reason why it needs to be mapped at all for emulation.
>> 
>> At least a couple years ago, the maintainers of some userspace tracing
>> tools complained very loudly at the early versions of the patches.
>> There are programs like pin (semi-open-source IIRC) that parse
>> instructions, make an instrumented copy, and run it.  This means that
>> the vsyscall page needs to contain text that is semantically
>> equivalent to what calling it actually does.
>> 
>> So yes, read access needs to work.  I should add a selftest for this.
>> 
>> This is needed in emulation mode as well as native mode, so removing
>> native mode is totally orthogonal.
> 
> Fair enough. I enabled function tracing with emulate_vsyscall as the filter
> on a couple of machines and so far I have no hit at all. Though I found a
> VM with a real old user space (~2005) and that actually used it.
> 
> So for the problem at hand, I'd suggest we disable the vsyscall stuff if
> CONFIG_KAISER=y and be done with it.

I think that time() on not-so-old glibc uses it.  Even more recent versions of Go use it. :(

> 
> Thanks,
> 
>    tglx
> 
> 
> 
> 
> 

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