Le 18/03/2025 à 16:59, Will Deacon a écrit :

On Thu, Mar 13, 2025 at 05:40:59PM +0100, Alessandro Carminati wrote:

quoted

On Thu, Mar 13, 2025 at 1:25 PM Will Deacon [off-list ref] wrote:

quoted

On Thu, Mar 13, 2025 at 11:43:22AM +0000, Alessandro Carminati wrote:

quoted

diff --git a/arch/arm64/include/asm/bug.h b/arch/arm64/include/asm/bug.h
index 28be048db3f6..044c5e24a17d 100644
--- a/arch/arm64/include/asm/bug.h
+++ b/arch/arm64/include/asm/bug.h

@@ -11,8 +11,14 @@

  #include <asm/asm-bug.h>

+#ifdef HAVE_BUG_FUNCTION
+# define __BUG_FUNC  __func__
+#else
+# define __BUG_FUNC  NULL
+#endif
+
  #define __BUG_FLAGS(flags)                           \
-     asm volatile (__stringify(ASM_BUG_FLAGS(flags)));
+     asm volatile (__stringify(ASM_BUG_FLAGS(flags, %c0)) : : "i" (__BUG_FUNC));

Why is 'i' the right asm constraint to use here? It seems a bit odd to
use that for a pointer.

I received this code as legacy from a previous version.
In my review, I considered the case when HAVE_BUG_FUNCTION is defined:
Here, __BUG_FUNC is defined as __func__, which is the name of the
current function as a string literal.
Using the constraint "i" seems appropriate to me in this case.

However, when HAVE_BUG_FUNCTION is not defined:
__BUG_FUNC is defined as NULL. Initially, I considered it literal 0,
but after investigating your concern, I found:

$ echo -E "#include <stdio.h>\n#include <stddef.h>\nint main()
{\nreturn 0;\n}" | aarch64-linux-gnu-gcc -E -dM - | grep NULL
#define NULL ((void *)0)

I realized that NULL is actually a pointer that is not a link time
symbol, and using the "i" constraint with NULL may result in undefined
behavior.

Would the following alternative definition for __BUG_FUNC be more convincing?

#ifdef HAVE_BUG_FUNCTION
     #define __BUG_FUNC __func__
#else
     #define __BUG_FUNC (uintptr_t)0
#endif

Let me know your thoughts.

Thanks for the analysis; I hadn't noticed this specific issue, it just
smelled a bit fishy. Anyway, the diff above looks better, thanks.

That propably deserves a comment.

Doesn't sparse and/or checkpatch complain about 0 being used in lieu of 
NULL ?

By the way I had similar problem in the past with GCC not seeing NULL as 
a __builtin_constant_p(), refer commit 1d8f739b07bd ("powerpc/kuap: Fix 
set direction in allow/prevent_user_access()")

Christophe