Re: [Xen-devel] [PATCH v2 06/25] libelf-loader: introduce elf_load_image

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2011-12-09 at 15:12 +0000, Jan Beulich wrote:
> >>> On 09.12.11 at 14:40, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Fri, 2011-12-09 at 13:33 +0000, Jan Beulich wrote:
> >> >>> On 09.12.11 at 14:13, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> >> > Implement a new function, called elf_load_image, to perform the actually
> >> > copy of the elf image and clearing the padding.
> >> > The function is implemented as memcpy and memset when the library is
> >> > built as part of the tools, but it is implemented as copy_to_user and
> >> > clear_user when built as part of Xen, so that it can be safely called
> >> > with an HVM style dom0.
> >> 
> >> I meant to ask this on the first round already, but apparently forgot:
> >> What is it that prevents memcpy()/memset() from being used for a
> >> HVM style Dom0? {clear,copy_to}_user() still expect the guest memory
> >> to be visible in the hypervisor's virtual address space - how could a
> >> fault happen here? And if you have to take precautions for a fault,
> >> shouldn't the calling code check the respective return values?
> > 
> > HVM guest memory is not (necessarily) mapped in the hypervisor page
> > tables, it needs to be mapped on demand. Also the source/target (delete
> > as appropriate) is a guest virtual address so even if the RAM happened
> > to be mapped it would (likely) not be in the same place so at a minimum
> > we need to translate things.
> > 
> > This is what copy_{to,from}_user does for an HVM guest even on X86,
> > where copy_to_user becomes copy_to_user_hvm as appropriate. 
> 
> But that's not true - the distinction of hvm vs pv is at the
> copy_to_guest() layer (raw_copy_to_guest() in the x86 case). So
> maybe the patch meant to use those interfaces (and then we'd need
> a clear_guest() too, which should also have been obvious by the fact
> that the prior patch only introduced clear_user(), but no hvm variant
> of it)?

This code is also compiled in userspace which doesn't have copy_to_user
and in that case we need to use memcpy.

Ian.


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