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Re: [Xen-devel] [PATCH 3/4] xen: introduce __symbol


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On 15/10/2018 10:56, Stefano Stabellini wrote:
Introduce a macro, __symbol, which is a simple wrapper around RELOC_HIDE
to be used everywhere symbols such as _stext and _etext are used in the

RELOC_HIDE is needed when accessing symbols such as _stext and _etext
because the C standard forbids comparisons between pointers pointing to
different objects. _stext, _etext, etc. are all pointers to different
objects from ANCI C point of view.

To work around potential C compiler issues (which have actually
been found, see the comment on top of RELOC_HIDE in Linux), and to help
with certifications, let's introduce some syntactic sugar to be used in
following patches.

Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
  xen/include/asm-arm/mm.h   | 6 ++++++
  xen/include/asm-x86/page.h | 6 ++++++
  2 files changed, 12 insertions(+)

diff --git a/xen/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
index 940b74b..02ce05a 100644
--- a/xen/include/asm-arm/mm.h
+++ b/xen/include/asm-arm/mm.h
@@ -284,6 +284,12 @@ static inline uint64_t gvirt_to_maddr(vaddr_t va, paddr_t 
  #define __mfn_to_virt(mfn) (maddr_to_virt((paddr_t)(mfn) << PAGE_SHIFT))
+ * Use RELOC_HIDE with symbols such as _stext and _etext to avoid errors
+ * on comparing pointers to different objects
+ */
+#define __symbol(x)         ((char *)RELOC_HIDE((unsigned long)(x), 0))

There are no different between arm and x86. A better place would be xen/compiler.h (or something common).

But, after this patch, there are even more chance the compiler will consider that 2 _symbol(...) will come from different objects. So how is this meant to help here?


Julien Grall

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