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Date: Sun, 29 May 2011 18:40:36 +0200
Message-ID: <BANLkTinAawq1SRXO0aK5GiGBVcbJk9sy4Q@mail.gmail.com>
Subject: Re: [GIT pull] x86 vdso updates
From: richard -rw- weinberger <richard.weinberger@gmail.com>
To: Andrew Lutomirski <luto@mit.edu>
Cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>
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On Sun, May 29, 2011 at 5:28 PM, Andrew Lutomirski <luto@mit.edu> wrote:
> On Sun, May 29, 2011 at 11:10 AM, richard -rw- weinberger
> <richard.weinberger@gmail.com> wrote:
>> On Sun, May 29, 2011 at 4:57 PM, Andrew Lutomirski <luto@mit.edu> wrote:
>>> On Sun, May 29, 2011 at 5:51 AM, richard -rw- weinberger
>>> <richard.weinberger@gmail.com> wrote:
>>>> Yesterday I had a closer look at 64bit UML.
>>>> Glibc is always using vsyscalls because 64bit UML does not support the vDSO.
>>>>
>>>> On 32bit UML simply scans the ELF auxiliary vector provided by the host to
>>>> get the address of the vDSO.
>>>> How can I get this address on a 64bit host?
>>>
>>> I believe it's exactly the same. ?There's an auxv entry that points to the vDSO.
>>
>> I don't think so.
>> See:
>> http://www.win.tue.nl/~aeb/linux/lk/lk-4.html
>> Section "Address space randomization".
>> The demo program finds the vDSO only on x86.
>>
>> UML uses quite the same method to find it.
>> arch/um/os-Linux/elf_aux.c
>
> The attached program works for me.

Shouldn't it print the addresses of both AT_SYSINFO and AT_SYSINFO_EHDR?
On my x86_64 system (2.6.37) it seems to find only AT_SYSINFO_EHDR.

> I don't know what this is supposed to mean, though:
>
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?/* See if the page is under TASK_SIZE */
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?if (vsyscall_ehdr < (unsigned long) envp)
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?vsyscall_ehdr = 0;

See commits a502a359 and 14251809.

>
> First, envp != TASK_SIZE.
>
> Second, the vDSO can be wherever it wants. ?On current kernels at
> least it is *always* mapped below TASK_SIZE (in the unsigned sense)
> because it's mapped into user address space.

Hmm, this would explain why UML cannot find it. :-\

-- 
Thanks,
//richard
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