[Qemu-devel] [RFC] spapr_drc: Fix potential undefined behaviour

[Qemu-devel] [RFC] spapr_drc: Fix potential undefined behaviour

[David Gibson]

The DRC_INDEX_ID_MASK macro does a left shift on ~0, which is a signed
quantity, and therefore undefined behaviour according to the C spec.  In
particular this causes warnings from the clang sanitizer.

This fixes it by calculating the same mask without using ~0 (I think the
new method is a more common idiom for generating masks anyway).  For good
measure I also use 1ULL to force the expression's type to unsigned long
long, which should be good for assigning to anything we're going to want
to.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 hw/ppc/spapr_drc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

I'm hoping to get some Reviewed-bys for this patch so that I'm ready
to merge my spapr-next queue.

diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
index ee87432..8cbcf4d 100644
--- a/hw/ppc/spapr_drc.c
+++ b/hw/ppc/spapr_drc.c
@@ -32,7 +32,7 @@
 
 #define DRC_CONTAINER_PATH "/dr-connector"
 #define DRC_INDEX_TYPE_SHIFT 28
-#define DRC_INDEX_ID_MASK (~(~0 << DRC_INDEX_TYPE_SHIFT))
+#define DRC_INDEX_ID_MASK ((1ULL << DRC_INDEX_TYPE_SHIFT) - 1)
 
 static sPAPRDRConnectorTypeShift get_type_shift(sPAPRDRConnectorType type)
 {
-- 
2.4.3

Re: [Qemu-devel] [RFC] spapr_drc: Fix potential undefined behaviour

[Alexey Kardashevskiy]

On 09/01/2015 01:17 PM, David Gibson wrote:
> The DRC_INDEX_ID_MASK macro does a left shift on ~0, which is a signed
> quantity, and therefore undefined behaviour according to the C spec.  In
> particular this causes warnings from the clang sanitizer.
>
> This fixes it by calculating the same mask without using ~0 (I think the
> new method is a more common idiom for generating masks anyway).  For good
> measure I also use 1ULL to force the expression's type to unsigned long
> long, which should be good for assigning to anything we're going to want
> to.
>
> Reported-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>	

> ---
>   hw/ppc/spapr_drc.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> I'm hoping to get some Reviewed-bys for this patch so that I'm ready
> to merge my spapr-next queue.
>
> diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
> index ee87432..8cbcf4d 100644
> --- a/hw/ppc/spapr_drc.c
> +++ b/hw/ppc/spapr_drc.c
> @@ -32,7 +32,7 @@
>
>   #define DRC_CONTAINER_PATH "/dr-connector"
>   #define DRC_INDEX_TYPE_SHIFT 28
> -#define DRC_INDEX_ID_MASK (~(~0 << DRC_INDEX_TYPE_SHIFT))
> +#define DRC_INDEX_ID_MASK ((1ULL << DRC_INDEX_TYPE_SHIFT) - 1)
>
>   static sPAPRDRConnectorTypeShift get_type_shift(sPAPRDRConnectorType type)
>   {
>


-- 
Alexey