On Thu, 17 Nov 2011 13:57:30 -0800
David Daney <ddaney.cavm@xxxxxxxxx> wrote:
> From: David Daney <david.daney@xxxxxxxxxx>
>
> It was pointed out by David Rientjes that the dummy values for
> HPAGE_MASK and HPAGE_SIZE are quite unsafe. It they are inadvertently
> used with !CONFIG_HUGETLB_PAGE, compilation would succeed, but the
> resulting code would surly not do anything sensible.
>
> Place BUG() in the these dummy definitions, as we do in similar
> circumstances in other places, so any abuse can be easily detected.
>
> Since the only sane place to use these symbols when
> !CONFIG_HUGETLB_PAGE is on dead code paths, the BUG() cause any actual
> code to be emitted by the compiler.
I assume you meant "omitted" here.
But I don't think it's true. Any such code would occur after testing
is_vm_hugetlb_page() or similar, and would have been omitted anyway.
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -111,8 +111,9 @@ static inline void copy_huge_page(struct page *dst, struct page *src)
> #define hugetlb_change_protection(vma, address, end, newprot)
>
> #ifndef HPAGE_MASK
> -#define HPAGE_MASK PAGE_MASK /* Keep the compiler happy */
> -#define HPAGE_SIZE PAGE_SIZE
> +/* Keep the compiler happy with some dummy (but BUGgy) values */
That's a quite poor comment. This?
--- a/include/linux/hugetlb.h~hugetlb-provide-safer-dummy-values-for-hpage_mask-and-hpage_size-fix
+++ a/include/linux/hugetlb.h
@@ -111,7 +111,11 @@ static inline void copy_huge_page(struct
#define hugetlb_change_protection(vma, address, end, newprot)
#ifndef HPAGE_MASK
-/* Keep the compiler happy with some dummy (but BUGgy) values */
+/*
+ * HPAGE_MASK and friends are defined if !CONFIG_HUGETLB_PAGE as an
+ * ifdef-avoiding convenience. However they should never be evaluated at
+ * runtime if !CONFIG_HUGETLB_PAGE.
+ */
#define HPAGE_MASK ({BUG(); 0; })
#define HPAGE_SIZE ({BUG(); 0; })
#define HPAGE_SHIFT ({BUG(); 0; })
_
> +#define HPAGE_MASK ({BUG(); 0; })
> +#define HPAGE_SIZE ({BUG(); 0; })
> #define HPAGE_SHIFT ({BUG(); 0; })
This change means that HPAGE_* cannot be evaluated at compile time. So
int foo = HPAGE_SIZE;
outside functions will explode. I guess that's OK - actually desirable
- as such code shouldn't have been compiled anyway.
