On 3/18/25 08:59, Will Deacon wrote:
On Thu, Mar 13, 2025 at 05:40:59PM +0100, Alessandro Carminati wrote:
On Thu, Mar 13, 2025 at 1:25 PM Will Deacon <w...@kernel.org> wrote:
On Thu, Mar 13, 2025 at 11:43:22AM +0000, Alessandro Carminati wrote:
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.
It has been a long time, but I seem to recall that I ran into trouble when
trying to use a different constraint.
Guenter
