struct dom0_vga_console_info has been extended in the past, and it may
be extended again. The use in PV Dom0's start info already covers for
that by supplying the size of the provided data. For the recently
introduced platform-op size needs providing similarly. Go the easiest
available route and simply supply size via the hypercall return value.

While there also add a build-time check that possibly future growth of
the struct won't affect xen_platform_op_t's size.

Fixes: 4dd160583c79 ("x86/platform: introduce hypercall to get initial video console settings")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
Noticed while trying to actually use the new hypercall in Linux.

--- a/xen/arch/x86/platform_hypercall.c
+++ b/xen/arch/x86/platform_hypercall.c
@@ -841,6 +841,8 @@ ret_t do_platform_op(
 
 #ifdef CONFIG_VIDEO
     case XENPF_get_dom0_console:
+        BUILD_BUG_ON(sizeof(op->u.dom0_console) > sizeof(op->u.pad));
+        ret = sizeof(op->u.dom0_console);
         if ( !fill_console_start_info(&op->u.dom0_console) )
         {
             ret = -ENODEV;

On Mon, Feb 13, 2023 at 03:51:50PM +0100, Jan Beulich wrote:
> struct dom0_vga_console_info has been extended in the past, and it may
> be extended again. The use in PV Dom0's start info already covers for
> that by supplying the size of the provided data. For the recently
> introduced platform-op size needs providing similarly. Go the easiest
> available route and simply supply size via the hypercall return value.
> 
> While there also add a build-time check that possibly future growth of
> the struct won't affect xen_platform_op_t's size.
> 
> Fixes: 4dd160583c79 ("x86/platform: introduce hypercall to get initial video console settings")
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

With the added comment.

Thanks, Roger.

On 13.02.2023 15:51, Jan Beulich wrote:
> struct dom0_vga_console_info has been extended in the past, and it may
> be extended again. The use in PV Dom0's start info already covers for
> that by supplying the size of the provided data. For the recently
> introduced platform-op size needs providing similarly. Go the easiest
> available route and simply supply size via the hypercall return value.
> 
> While there also add a build-time check that possibly future growth of
> the struct won't affect xen_platform_op_t's size.
> 
> Fixes: 4dd160583c79 ("x86/platform: introduce hypercall to get initial video console settings")
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> Noticed while trying to actually use the new hypercall in Linux.
> 
> --- a/xen/arch/x86/platform_hypercall.c
> +++ b/xen/arch/x86/platform_hypercall.c
> @@ -841,6 +841,8 @@ ret_t do_platform_op(
>  
>  #ifdef CONFIG_VIDEO
>      case XENPF_get_dom0_console:
> +        BUILD_BUG_ON(sizeof(op->u.dom0_console) > sizeof(op->u.pad));
> +        ret = sizeof(op->u.dom0_console);
>          if ( !fill_console_start_info(&op->u.dom0_console) )
>          {
>              ret = -ENODEV;
> 

Thinking about it, I've added the hunk below.

Jan

--- a/xen/include/public/platform.h
+++ b/xen/include/public/platform.h
@@ -605,7 +605,11 @@ struct xenpf_symdata {
 typedef struct xenpf_symdata xenpf_symdata_t;
 DEFINE_XEN_GUEST_HANDLE(xenpf_symdata_t);
 
-/* Fetch the video console information and mode setup by Xen. */
+/*
+ * Fetch the video console information and mode setup by Xen.  A non-
+ * negative return value indicates the size of the (part of the) structure
+ * which was filled.
+ */
 #define XENPF_get_dom0_console 64
 typedef struct dom0_vga_console_info xenpf_dom0_console_t;
 DEFINE_XEN_GUEST_HANDLE(xenpf_dom0_console_t);