Subject: Re: [PATCH v3 2/2] powerpc/mm: Tracking vDSO remap

From: Ingo Molnar <[email-masked]>
Date: Thu, 26 Mar 2015 15:17:31 +0100

* Laurent Dufour <[email-masked]> wrote:

> > I argue we should use the right condition to clear vdso_base: if 
> > the vDSO gets at least partially unmapped. Otherwise there's 
> > little point in the whole patch: either correctly track whether 
> > the vDSO is OK, or don't ...
> 
> That's a good option, but it may be hard to achieve in the case the 
> vDSO area has been splitted in multiple pieces.
>
> Not sure there is a right way to handle that, here this is a best 
> effort, allowing a process to unmap its vDSO and having the 
> sigreturn call done through the stack area (it has to make it 
> executable).
> 
> Anyway I'll dig into that, assuming that the vdso_base pointer 
> should be clear if a part of the vDSO is moved or unmapped. The 
> patch will be larger since I'll have to get the vDSO size which is 
> private to the vdso.c file.

At least for munmap() I don't think that's a worry: once unmapped 
(even if just partially), vdso_base becomes zero and won't ever be set 
again.

So no need to track the zillion pieces, should there be any: Humpty 
Dumpty won't be whole again, right?

> > There's also the question of mprotect(): can users mprotect() the 
> > vDSO on PowerPC?
> 
> Yes, mprotect() the vDSO is allowed on PowerPC, as it is on x86, and 
> certainly all the other architectures. Furthermore, if it is done on 
> a partial part of the vDSO it is splitting the vma...

btw., CRIU's main purpose here is to reconstruct a vDSO that was 
originally randomized, but whose address must now be reproduced as-is, 
right?

In that sense detecting the 'good' mremap() as your patch does should 
do the trick and is certainly not objectionable IMHO - I was just 
wondering whether we could make a perfect job very simply.

Thanks,

	Ingo