[RFC PATCH 1/3] modpost: fix TO_NATIVE() with expressions and consts

[Alexander Lobakin]

Macro TO_NATIVE() directly takes a reference to its argument @x
without making an intermediate variable. This makes compilers
emit build warnings and errors if @x is an expression or a deref
of a const pointer (when target Endianness != host Endianness):

>> scripts/mod/modpost.h:87:18: error: lvalue required as unary '&' operand
      87 |         __endian(&(x), &(__x), sizeof(__x));                    \
         |                  ^
   scripts/mod/sympath.c:19:25: note: in expansion of macro 'TO_NATIVE'
      19 | #define t(x)            TO_NATIVE(x)
         |                         ^~~~~~~~~
   scripts/mod/sympath.c:100:31: note: in expansion of macro 't'
     100 |                 eh->e_shoff = t(h(eh->e_shoff) + off);

>> scripts/mod/modpost.h:87:24: warning: passing argument 2 of '__endian'
discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
      87 |         __endian(&(x), &(__x), sizeof(__x));                    \
         |                        ^~~~~~
   scripts/mod/sympath.c:18:25: note: in expansion of macro 'TO_NATIVE'
      18 | #define h(x)            TO_NATIVE(x)
         |                         ^~~~~~~~~
   scripts/mod/sympath.c:178:48: note: in expansion of macro 'h'
     178 |              iter < end; iter = (void *)iter + h(eh->e_shentsize)) {

Create a temporary variable, assign @x to it and don't use @x after
that. This makes it possible to pass expressions as an argument.
Also, do a cast-away for the second argument when calling __endian()
to avoid 'discarded qualifiers' warning, as typeof() preserves
qualifiers and makes compilers think that we're passing pointer
to a const.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@xxxxxxxxxxxxxxx # 4.9+
Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
---
 scripts/mod/modpost.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/scripts/mod/modpost.h b/scripts/mod/modpost.h
index 1178f40a73f3..29f440f18414 100644
--- a/scripts/mod/modpost.h
+++ b/scripts/mod/modpost.h
@@ -83,9 +83,10 @@ static inline void __endian(const void *src, void *dest, unsigned int size)
 
 #define TO_NATIVE(x)						\
 ({								\
-	typeof(x) __x;						\
-	__endian(&(x), &(__x), sizeof(__x));			\
-	__x;							\
+	typeof(x) __src = (x);					\
+	typeof(__src) __dst;					\
+	__endian(&__src, (void *)&__dst, sizeof(__src));	\
+	__dst;							\
 })
 
 #else /* endianness matches */
-- 
2.37.2