Re: Forbid deletion of memory mappings

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Thu, 30 Aug 2007, Clemens Kolbitsch wrote:

> On Thursday 30 August 2007 23:50:21 Valdis.Kletnieks@vt.edu wrote:
>> On Thu, 30 Aug 2007 23:41:09 +0200, Clemens Kolbitsch said:
>>> On Thursday 30 August 2007 23:34:52 you wrote:
>>>> On Thu, 30 Aug 2007, Clemens Kolbitsch wrote:
>>>>> is there no way to tell the kernel, that a certain mapping must not
>>>>> be removed, no matter what (except of course an explicit call to
>>>>> sys_unmap, of course)?
>>>>
>>>> I don't seem to get what is the issue here. Your mapping is not
>>>> removed, only the VMAs are merged together into one larger VMA if they
>>>> have neighbouring address ranges and compatible protection bits. See
>>>> vma_merge().
>>>
>>> the thing is that they are not. the kernel chooses to REPLACE my mapping.
>>>
>>> consider the user-space code:
>>>
>>> mmap(0xaaaa0000, 0x3000, MAP_FIXED, ...);
>>> mmap(0xaaaa1000, 0x4000, MAP_FIXED, ...);
>>>
>>> here, the second call to mmap will shorten the first mapping to 0x1000
>>> bytes and create one big vma with size 0x5000 bytes.
>>>
>>> is there a way to tell it that the second mmap MUST fail?
>>
>> There's an LSM exit point for mmap, you could perhaps do something there.
>>
>> What are you trying to achieve by forcing the second one to fail?
>
> puh... that is a good question :-)
>
> I'm writing my master's thesis on a new model of memory protection and need to
> have every memory mapping in userspace duplicated. I also have kind of a
> second PGD/PTD that allows finding this mirrored mapping.
>
> However, as the number of original mappings grows, I suddenly have the problem
> that the kernel tries to allocate a new mapping and picks the address of a
> mirrored memory page, which it shouldn't.
>
> Honestly, I don't understand why it does so, but it simply does... so
> basically I want these second mappings to stay in memory as long as the
> original page.
>
> What do you mean exactly with
>
>> There's an LSM exit point for mmap, you could perhaps do something there.
>
> ??
>
> By the way / @Jiri Kosina:
>
>> Which is exactly in compliance with what POSIX says about MAP_FIXED mmaps
>> - see http://opengroup.org/onlinepubs/007908799/xsh/mmap.html
>
> I know... I'm not even saying that this is a bug... I just wonder if there is
> some way to avoid this problem :-)
>
> Thanks for your help - i really appreciate that!!

You might need to keep track of your memory mappings and
not remap something that will overlap. You can mmap
a single page in which you write the addresses. If you
map it shared, all your processes can access it and
call a common memory-mapping procedure (that you write)
which keeps track of its kernel allocations for you.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.22.1 on an i686 machine (5588.30 BogoMips).
My book : http://www.AbominableFirebug.com/
_


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