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

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Message-Id: <1634538449.eah9b31bbz.astroid@bobo.none>
Date:   Mon, 18 Oct 2021 16:29:57 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Helge Deller <deller@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Kees Cook <keescook@...omium.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-arch@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 07/12] asm-generic: Define 'func_desc_t' to commonly
 describe function descriptors

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@...omium.org>
> Acked-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---

[...]

> 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.

Thanks,
Nick