[PATCH v6 1/4] xen: introduce SYMBOL

[Stefano Stabellini <sstabellini@kernel.org>]

Introduce a macro, SYMBOL, which is similar to RELOC_HIDE, but it is
meant to be used everywhere symbols such as _stext and _etext are used
in the code. It can take an array type as a parameter, and it returns
the same type.

SYMBOL 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]) 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@xilinx.com>
CC: JBeulich@suse.com
CC: andrew.cooper3@citrix.com
CC: wei.liu2@citrix.com
---
Changes in v6:
- drop acks
- don't use RELOC_HIDE for the implementation
- return native type from SYMBOL

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 | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index ff6c0f5..d4c856c 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -99,6 +99,16 @@
     __asm__ ("" : "=r"(__ptr) : "0"(ptr));      \
     (typeof(ptr)) (__ptr + (off)); })
 
+/*
+ * Similar to RELOC_HIDE, but written to be used with symbols such as
+ * _stext and _etext to avoid undefined behavior comparing pointers to
+ * different objects. It can handle array types.
+ */
+#define SYMBOL(ptr)                               \
+  ({ unsigned long __ptr;                       \
+    __asm__ ("" : "=r"(__ptr) : "0"(ptr));      \
+    (typeof(*(ptr)) *) (__ptr); })
+
 #ifdef __GCC_ASM_FLAG_OUTPUTS__
 # define ASM_FLAG_OUT(yes, no) yes
 #else
-- 
1.9.1


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