Re: [Xen-devel] [PATCH] tools/libxencall: enforce proper alignment of hypercall buffers

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Jul 10, 2018 at 03:25:07PM +0200, Juergen Gross wrote:
> On 08/06/18 12:25, Juergen Gross wrote:
> > On 08/06/18 12:09, Andrew Cooper wrote:
> >> On 08/06/18 10:51, Juergen Gross wrote:
> >>> xencall_alloc_buffer() is used throughout Xen tools for allocating
> >>> hypercall buffers. Allocation is done at page granularity. For simple
> >>> administration each allocated set of pages contains a small header
> >>> holding the number of pages of that set. The hypercall buffer is
> >>> located directly after the 4 byte sized header, leading to a wrong
> >>> alignment for e.g. pointers.
> >>>
> >>> Repair that by using a 16 byte sized header enforcing the same
> >>> alignment as malloc().
> >>>
> >>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> >>> ---
> >>> Not sure whether this wants to be backported. In case the answer is
> >>> "yes" I think it should be part of 4.11.
> >>> ---
> >>>  tools/libs/call/buffer.c | 1 +
> >>>  1 file changed, 1 insertion(+)
> >>>
> >>> diff --git a/tools/libs/call/buffer.c b/tools/libs/call/buffer.c
> >>> index 2d8fc29ac6..0b6af2db60 100644
> >>> --- a/tools/libs/call/buffer.c
> >>> +++ b/tools/libs/call/buffer.c
> >>> @@ -151,6 +151,7 @@ void xencall_free_buffer_pages(xencall_handle *xcall, 
> >>> void *p, size_t nr_pages)
> >>>  
> >>>  struct allocation_header {
> >>>      int nr_pages;
> >>
> >> This hunk reveals another bug...
> >>
> >> There are a rather large number of size_t => int truncations in the
> >> code, which is surely going to catch people by surprise.  (Again - I was
> >> certain I commented on this during the original review of this library.)
> > 
> > On the practical side I don't think this really matters here. Passing
> > more than 2GB as a hypercall buffer is nothing we want to support...
> > 
> >>
> >>> +    int pad[3];
> >>
> >> This brings it to 16 byte alignment, but is that enough?  An xsave state
> >> block in x86 for example has 64 byte alignment.  I suppose we mostly
> >> care about ARM here, as its memcpy() will be most impacted by this
> >> misalignment.
> > 
> > Special purpose buffers can still be directly allocated via
> > xencall_alloc_buffer_pages(). I don't think we'll need alignment better
> > than malloc().
> > 
> >> Irrespective, might be easier to accomplish with an
> >> __attribute__((__aligned__($N))) rather than explicit padding?
> > 
> > I don't mind which way to go here. I'll leave the decision for the
> > maintainers. :-)
> 
> Ping?

Either is fine. I will apply this patch as-is.

Wei.

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