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Re: [Xen-devel] [PATCH v2 3/4] xen: introduce __symbol

On 17/10/2018 15:31, 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.

The C standard forbids for both comparisons and substraction (see C Standard, 6.5.6 [ISO/IEC 9899:2011] and [1]). Also, it is probably worth to give a pointer to the paragraph in the standard to help finding more details.


[1] https://wiki.sei.cmu.edu/confluence/display/c/ARR36-C.+Do+not+subtract+or+compare+two+pointers+that+do+not+refer+to+the+same+array

_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>
CC: JBeulich@xxxxxxxx
CC: andrew.cooper3@xxxxxxxxxx
CC: wei.liu2@xxxxxxxxxx
Changes in v2:
- do not cast return to char*
- move to common header
  xen/include/xen/compiler.h | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index ff6c0f5..850b0d0 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -99,6 +99,12 @@
      __asm__ ("" : "=r"(__ptr) : "0"(ptr));      \
      (typeof(ptr)) (__ptr + (off)); })
+ * Use RELOC_HIDE with symbols such as _stext and _etext to avoid errors
+ * on comparing pointers to different objects
+ */
+#define __symbol(x)         (RELOC_HIDE((unsigned long)(x), 0))
  # define ASM_FLAG_OUT(yes, no) yes

Julien Grall

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