Re: [PATCH][next] xen: privcmd: Replace zero-length array with flex-array member and use __counted_by

["Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx>]

On 11/16/23 15:08, Kees Cook wrote:
> On Thu, Nov 16, 2023 at 12:54:59PM -0600, Gustavo A. R. Silva wrote:
> > Fake flexible arrays (zero-length and one-element arrays) are deprecated,
> > and should be replaced by flexible-array members. So, replace
> > zero-length array with a flexible-array member in `struct
> > privcmd_kernel_ioreq`.
> >
> > Also annotate array `ports` with `__counted_by()` to prepare for the
> > coming implementation by GCC and Clang of the `__counted_by` attribute.
> > Flexible array members annotated with `__counted_by` can have their
> > accesses bounds-checked at run-time via `CONFIG_UBSAN_BOUNDS` (for array
> > indexing) and `CONFIG_FORTIFY_SOURCE` (for strcpy/memcpy-family functions).
> >
> > This fixes multiple -Warray-bounds warnings:
> > drivers/xen/privcmd.c:1239:30: warning: array subscript i is outside array 
> > bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> > drivers/xen/privcmd.c:1240:30: warning: array subscript i is outside array 
> > bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> > drivers/xen/privcmd.c:1241:30: warning: array subscript i is outside array 
> > bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> > drivers/xen/privcmd.c:1245:33: warning: array subscript i is outside array 
> > bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> > drivers/xen/privcmd.c:1258:67: warning: array subscript i is outside array 
> > bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> >
> > This results in no differences in binary output.
> >
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> Looks right to me. I can see the allocation:
Yep, I always check for that; in particular, the 'counter' assignment. :)

Do you want me to mention it in the changelog text?

>          size = struct_size(kioreq, ports, ioeventfd->vcpus);
>          kioreq = kzalloc(size, GFP_KERNEL);
>          if (!kioreq)
>                  return ERR_PTR(-ENOMEM);
>
>          kioreq->dom = ioeventfd->dom;
>          kioreq->vcpus = ioeventfd->vcpus;
>
>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Thanks!
--
Gustavo