From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [BUGFIX][PATCH 4/4] XEN_DOMCTL_gdbsx_guestmemio:
 always do the copyback.
Date: Sun, 5 Jan 2014 23:35:24 +0000
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To: Don Slutz <dslutz@verizon.com>, xen-devel@lists.xen.org
Cc: Keir Fraser <keir@xen.org>, Ian Campbell <ian.campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, George Dunlap <george.dunlap@eu.citrix.com>, Ian Jackson <ian.jackson@eu.citrix.com>, Jan Beulich <jbeulich@suse.com>
List-Id: xen-devel@lists.xenproject.org

On 04/01/2014 17:52, Don Slutz wrote:
> The gdbsx code expects that domctl->u.gdbsx_guest_memio.remain is
> returned.
>
> Without this gdb does not report an error.
>
> With this patch and using a 1G hvm domU:
>
> (gdb) x/1xh 0x6ae9168b
> 0x6ae9168b:     Cannot access memory at address 0x6ae9168b
>
> Signed-off-by: Don Slutz <dslutz@verizon.com>
> ---
>  xen/arch/x86/domctl.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
> index ef6c140..4aa751f 100644
> --- a/xen/arch/x86/domctl.c
> +++ b/xen/arch/x86/domctl.c
> @@ -997,8 +997,7 @@ long arch_do_domctl(
>              domctl->u.gdbsx_guest_memio.len;
>  
>          ret = gdbsx_guest_mem_io(domctl->domain, &domctl->u.gdbsx_guest_memio);
> -        if ( !ret )
> -           copyback = 1;
> +        copyback = 1;

This whole hypercall implementation looks suspect.

gdbsx_guest_mem_io() itself writes to the 'remain' field, making the
statement crossing the top of this context redundant.

Furthermore, ret is strictly 0 in the case that 'remain' is 0, or
-EFAULT if 'remain' is non-0.

While this does change the ABI of a hypercall, I think it is reasonable
to do, as domctl.h does imply that 'remain' is an output parameter,
without specifying anything about its behaviour in the case of an error.

~Andrew