Re: [Xen-devel] VMI: singlestep event not received

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 4/24/19 6:25 PM, Tamas K Lengyel wrote:
> On Mon, Apr 22, 2019 at 6:00 PM Mathieu Tarral
> <mathieu.tarral@xxxxxxxxxxxxxx> wrote:
> > On Monday 22 April 2019 11:22, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> 
> > wrote:
> >
> > > On 4/22/19 1:26 AM, Mathieu Tarral wrote:
> > >
> > > > The problem is that RtlUserThreadStart is paged-out, so i'm trying to reach it 
> > > > via singlestepping as a backup solution.
> > > FWIW, you can just use xc_hvm_inject_trap() with a TRAP_page_fault
> > > parameter to cause the OS to bring a paged out page back into main memory.
> > The problem is that i'm fully based on LibVMI, which doesn't provide an API for 
> > page injection yet.
> >
> > I think Tamas was waiting for a full support of pagefault injection on Xen 
> > before opening an API.
> >
> > Maybe we should open an issue to track the progress about it.
> >
> > Thanks anyway for the suggestion !
> Not having a LibVMI wrapper should not prevent you from using a libxc
> function directly. There are many corner-cases regarding when it is
> safe to inject a pagefault into the guest to bring back pages and that
> logic is not trivial enough to be implemented in LibVMI. It's way too
> specific to the usecase to decide when it's safe to do that. So in
> this case, if you know you are in ring3 of your target process you can
> just call the libxc function directly.
That's very very true, however I think we should assume that libraries 
such as libvmi and libbdvmi are used by client code smart enough to 
figure out, using said libraries, enough things about what's appropriate 
(or not) to do, and  let them do it rather than second-guess them. 
Hence, in libbdvmi I've wrapped that, and added a comment basically 
saying what you're saying above:
https://github.com/bitdefender/libbdvmi/blob/master/src/xendriver.cpp#L542

If we end up having the client code call backend-specific code directly 
we're not, after all, really abstracting it with our wrapper libraries.

Thanks,
Razvan

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