[Xen-devel] [PATCH 1/2] xen: Fix page <-> pfn conversion on 32 bit systems

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

The xen functions to convert between pages and pfns fail due to an
overflow on systems where a physical address may not fit in an unsigned
long (e.g. x86 32 bit PAE systems). Rework the conversion to avoid
overflow. This should also result in simpler object code.

This bug manifested itself as disk corruption with Linux 4.4 when using
blkfront in a Xen HVM x86 32 bit guest with more than 4 GiB of memory.

Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
---
 include/xen/page.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/xen/page.h b/include/xen/page.h
index 96294ac..9dc46cb 100644
--- a/include/xen/page.h
+++ b/include/xen/page.h
@@ -15,9 +15,9 @@
  */
 
 #define xen_pfn_to_page(xen_pfn)       \
-       ((pfn_to_page(((unsigned long)(xen_pfn) << XEN_PAGE_SHIFT) >> 
PAGE_SHIFT)))
+       (pfn_to_page((unsigned long)(xen_pfn) >> (PAGE_SHIFT - XEN_PAGE_SHIFT)))
 #define page_to_xen_pfn(page)          \
-       (((page_to_pfn(page)) << PAGE_SHIFT) >> XEN_PAGE_SHIFT)
+       ((page_to_pfn(page)) << (PAGE_SHIFT - XEN_PAGE_SHIFT))
 
 #define XEN_PFN_PER_PAGE       (PAGE_SIZE / XEN_PAGE_SIZE)
 
-- 
2.4.3


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