[PATCH 1/2] kernel.h: use parentheses around argument in u64_to_user_ptr()

[Jann Horn]

Use parentheses around uses of the argument in u64_to_user_ptr() to ensure
that the cast doesn't apply to part of the argument.

There are existing uses of the macro of the form `u64_to_user_ptr(A + B)`,
which expands to `(void __user *)(uintptr_t)A + B` (the cast applies to the
first operand of the addition, the addition is a pointer addition). This
happens to still work as intended, the semantic difference doesn't cause a
difference in behavior.
But I want to use u64_to_user_ptr() with a ternary operator in the
argument, like so: `u64_to_user_ptr(A ? B : C)`. This currently doesn't
work as intended.

Fixes: f09174c501f8 ("x86: add user_atomic_cmpxchg_inatomic at uaccess.h")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
Can we take this patch through the x86 tree with the following one, or
do we need to get this one through akpm's tree first?

 include/linux/kernel.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 34a5036debd3..2d14e21c16c0 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -47,8 +47,8 @@
 
 #define u64_to_user_ptr(x) (		\
 {					\
-	typecheck(u64, x);		\
-	(void __user *)(uintptr_t)x;	\
+	typecheck(u64, (x));		\
+	(void __user *)(uintptr_t)(x);	\
 }					\
 )
 
-- 
2.21.0.392.gf8f6787159e-goog