[PATCH v2] xen/x86: Zero out .bss for PV guests

[PATCH v2] xen/x86: Zero out .bss for PV guests

[Boris Ostrovsky]

Baremetal kernels clear .bss early in the boot but Xen PV guests don't
execute that code. They have been able to run without problems because
Xen domain builder happens to give out zeroed pages. However, since this
is not really guaranteed, .bss should be explicitly cleared.

(Since we introduce macros for specifying 32- and 64-bit registers we
can get rid of ifdefs in startup_xen())

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: stable@vger.kernel.org
---
 arch/x86/xen/xen-head.S | 29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index b65f59a..2af87d1 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -35,16 +35,31 @@
 #define PVH_FEATURES (0)
 #endif
 
-	__INIT
-ENTRY(startup_xen)
-	cld
 #ifdef CONFIG_X86_32
-	mov %esi,xen_start_info
-	mov $init_thread_union+THREAD_SIZE,%esp
+#define REG(register)	%e##register
+#define WSIZE_SHIFT	2
+#define STOS		stosl
 #else
-	mov %rsi,xen_start_info
-	mov $init_thread_union+THREAD_SIZE,%rsp
+#define REG(register)	%r##register
+#define WSIZE_SHIFT	3
+#define STOS		stosq
 #endif
+
+	__INIT
+ENTRY(startup_xen)
+	cld
+
+	/* Clear .bss */
+	xor REG(ax),REG(ax)
+	mov $__bss_start,REG(di)
+	mov $__bss_stop,REG(cx)
+	sub REG(di),REG(cx)
+	shr $WSIZE_SHIFT,REG(cx)
+	rep STOS
+
+	mov REG(si),xen_start_info
+	mov $init_thread_union+THREAD_SIZE,REG(sp)
+
 	jmp xen_start_kernel
 
 	__FINIT
-- 
2.1.0

Re: [PATCH v2] xen/x86: Zero out .bss for PV guests

[Brian Gerst]

On Wed, Feb 24, 2016 at 10:19 AM, Boris Ostrovsky
<boris.ostrovsky@oracle.com> wrote:
> Baremetal kernels clear .bss early in the boot but Xen PV guests don't
> execute that code. They have been able to run without problems because
> Xen domain builder happens to give out zeroed pages. However, since this
> is not really guaranteed, .bss should be explicitly cleared.
>
> (Since we introduce macros for specifying 32- and 64-bit registers we
> can get rid of ifdefs in startup_xen())
>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: stable@vger.kernel.org
> ---
>  arch/x86/xen/xen-head.S | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index b65f59a..2af87d1 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -35,16 +35,31 @@
>  #define PVH_FEATURES (0)
>  #endif
>
> -       __INIT
> -ENTRY(startup_xen)
> -       cld
>  #ifdef CONFIG_X86_32
> -       mov %esi,xen_start_info
> -       mov $init_thread_union+THREAD_SIZE,%esp
> +#define REG(register)  %e##register
> +#define WSIZE_SHIFT    2
> +#define STOS           stosl
>  #else
> -       mov %rsi,xen_start_info
> -       mov $init_thread_union+THREAD_SIZE,%rsp
> +#define REG(register)  %r##register
> +#define WSIZE_SHIFT    3
> +#define STOS           stosq
>  #endif
> +
> +       __INIT
> +ENTRY(startup_xen)
> +       cld
> +
> +       /* Clear .bss */
> +       xor REG(ax),REG(ax)
> +       mov $__bss_start,REG(di)
> +       mov $__bss_stop,REG(cx)
> +       sub REG(di),REG(cx)
> +       shr $WSIZE_SHIFT,REG(cx)
> +       rep STOS
> +
> +       mov REG(si),xen_start_info
> +       mov $init_thread_union+THREAD_SIZE,REG(sp)
> +
>         jmp xen_start_kernel
>
>         __FINIT

Use the macros in <asm/asm.h> instead of defining your own.  Also,
xorl %eax,%eax is good for 64-bit too, since the upper bits are
cleared.

--
Brian Gerst

Re: [PATCH v2] xen/x86: Zero out .bss for PV guests

[Boris Ostrovsky]

On 02/24/2016 11:05 AM, Brian Gerst wrote:
>
> Use the macros in <asm/asm.h> instead of defining your own.  Also,
> xorl %eax,%eax is good for 64-bit too, since the upper bits are
> cleared.

I suspected this would have to be defined somewhere but couldn't find 
it. Thanks!

-boris

Re: [Xen-devel] [PATCH v2] xen/x86: Zero out .bss for PV guests

[Andrew Cooper]

On 24/02/16 15:19, Boris Ostrovsky wrote:
> Baremetal kernels clear .bss early in the boot but Xen PV guests don't
> execute that code. They have been able to run without problems because
> Xen domain builder happens to give out zeroed pages. However, since this
> is not really guaranteed, .bss should be explicitly cleared.
>
> (Since we introduce macros for specifying 32- and 64-bit registers we
> can get rid of ifdefs in startup_xen())
>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: stable@vger.kernel.org
> ---
>  arch/x86/xen/xen-head.S | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index b65f59a..2af87d1 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -35,16 +35,31 @@
>  #define PVH_FEATURES (0)
>  #endif
>  
> -	__INIT
> -ENTRY(startup_xen)
> -	cld
>  #ifdef CONFIG_X86_32
> -	mov %esi,xen_start_info
> -	mov $init_thread_union+THREAD_SIZE,%esp
> +#define REG(register)	%e##register
> +#define WSIZE_SHIFT	2
> +#define STOS		stosl
>  #else
> -	mov %rsi,xen_start_info
> -	mov $init_thread_union+THREAD_SIZE,%rsp
> +#define REG(register)	%r##register
> +#define WSIZE_SHIFT	3
> +#define STOS		stosq
>  #endif
> +
> +	__INIT
> +ENTRY(startup_xen)
> +	cld
> +
> +	/* Clear .bss */
> +	xor REG(ax),REG(ax)

If we are nitpicking,

This should be xor %eax, %eax even in 64bit.  Functionally identical,
and shorter to encode.

~Andrew

Re: [Xen-devel] [PATCH v2] xen/x86: Zero out .bss for PV guests

[Boris Ostrovsky]

On 02/24/2016 12:26 PM, Andrew Cooper wrote:
> On 24/02/16 15:19, Boris Ostrovsky wrote:
>> +	/* Clear .bss */
>> +	xor REG(ax),REG(ax)
> If we are nitpicking,
>
> This should be xor %eax, %eax even in 64bit.  Functionally identical,
> and shorter to encode.

Right, Brian Gerst pointed this out too in another message.

-boris