compiler: Remove uninitialized_var() macro

Using uninitialized_var() is dangerous as it papers over real bugs[1]
(or can in the future), and suppresses unrelated compiler warnings
(e.g. "unused variable"). If the compiler thinks it is uninitialized,
either simply initialize the variable or make compiler changes.

As recommended[2] by[3] Linus[4], remove the macro. With the recent
change to disable -Wmaybe-uninitialized in v5.7 in commit 78a5255ffb6a
("Stop the ad-hoc games with -Wno-maybe-initialized"), this is likely
the best time to make this treewide change.

[1] https://lore.kernel.org/lkml/[e-mail]/
[2] https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=[e-mail]/
[3] https://lore.kernel.org/lkml/[e-mail]/
[4] https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=[e-mail]/

Suggested-by: Linus Torvalds <[e-mail]>
Reviewed-by: Bart van Assche <[e-mail]>
Reviewed-by: Miguel Ojeda <[e-mail]>
Tested-by: Nathan Chancellor <[e-mail]>
Tested-by: Sedat Dilek <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

include/linux/compiler-clang.h

···
 
 /* Compiler specific definitions for Clang compiler */
 
-#define uninitialized_var(x) x = *(&(x))
-
 /* same as gcc, this was present in clang-2.6 so we can assume it works
  * with any version that can compile the kernel
  */

include/linux/compiler-gcc.h

···
 	(typeof(ptr)) (__ptr + (off));					\
 })
 
-/*
- * A trick to suppress uninitialized variable warning without generating any
- * code
- */
-#define uninitialized_var(x) x = x
-
 #ifdef CONFIG_RETPOLINE
 #define __noretpoline __attribute__((__indirect_branch__("keep")))
 #endif

tools/include/linux/compiler.h

···
 # define noinline
 #endif
 
-#define uninitialized_var(x) x = *(&(x))
-
 #include <linux/types.h>
 
 /*

tools/virtio/linux/kernel.h

···
 	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
 	(type *)( (char *)__mptr - offsetof(type,member) );})
 
-#define uninitialized_var(x) x = x
-
 # ifndef likely
 #  define likely(x)	(__builtin_expect(!!(x), 1))
 # endif