[Xen-devel] [PATCH v4 1/2] xen: introduce SYMBOL

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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
code.

RELOC_HIDE is needed when accessing symbols such as _stext and _etext
because the C standard forbids for both comparisons and substraction
(see C Standard, 6.5.6 [ISO/IEC 9899:2011] and [1]).

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.

[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

Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Julien Grall <julien.grall@xxxxxxx>
CC: JBeulich@xxxxxxxx
CC: andrew.cooper3@xxxxxxxxxx
CC: wei.liu2@xxxxxxxxxx
---
Changes in v4:
- add acked-bys
- remove unneeded parenthesis

Changes in v3:
- improve commit message
- rename __symbol to SYMBOL to avoid name space violations

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..344efff 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
+ * undefined behavior comparing pointers to different objects
+ */
+#define SYMBOL(x)         RELOC_HIDE((unsigned long)(x), 0)
+
 #ifdef __GCC_ASM_FLAG_OUTPUTS__
 # define ASM_FLAG_OUT(yes, no) yes
 #else
-- 
1.9.1


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