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

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Date:   Mon, 18 Oct 2021 19:16:25 +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 18, 2021 5:07 pm:
> 
> 
> 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@...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.
>> 
> 
> I tried to explain it in the commit message, but I can add a comment 
> here in addition for sure.

Thanks.

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

Ah yes of course. I guess the point is function descriptors don't exist 
so it should not be used (in general). powerpc module code knows what it
is doing, I guess it's okay for it to use it.

Thanks,
Nick