[PATCH v4 07/14] arm64: Add support for suppressing warning backtraces

[Alessandro Carminati]

Hello Will,

On Thu, Mar 13, 2025 at 1:25 PM Will Deacon <will at 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.

>
> Will
>



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