Re: [PATCH v3 07/12] asm-generic: Define 'func_desc_t' to commonly describe function descriptors

[Christophe Leroy]

Le 18/10/2021 à 08:29, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of October 17, 2021 10:38 pm:
> > We have three architectures using function descriptors, each with its
> > own type and name.
> >
> > Add a common typedef that can be used in generic code.
> >
> > Also add a stub typedef for architecture without function descriptors,
> > to avoid a forest of #ifdefs.
> >
> > It replaces the similar 'func_desc_t' previously defined in
> > arch/powerpc/kernel/module_64.c
> >
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
>
> [...]
>
> > diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
> > index a918388d9bf6..33b51efe3a24 100644
> > --- a/include/asm-generic/sections.h
> > +++ b/include/asm-generic/sections.h
> > @@ -63,6 +63,9 @@ extern __visible const void __nosave_begin, __nosave_end;
> >   #else
> >   #define dereference_function_descriptor(p) ((void *)(p))
> >   #define dereference_kernel_function_descriptor(p) ((void *)(p))
> > +typedef struct {
> > +	unsigned long addr;
> > +} func_desc_t;
> >   #endif
> >   
>
> I think that deserves a comment. If it's just to allow ifdef to be
> avoided, I guess that's okay with a comment. Would be nice if you could
> cause it to generate a link time error if it was ever used like
> undefined functions, but I guess you can't. It's not a necessity though.

I tried to explain it in the commit message, but I can add a comment 
here in addition for sure.

By the way, it IS used in powerpc's module_64.c:

static func_desc_t func_desc(unsigned long addr)
{
	return (func_desc_t){addr};
}

static unsigned long func_addr(unsigned long addr)
{
	return func_desc(addr).addr;
}