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 09:07:21 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Nicholas Piggin <npiggin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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



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.

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;
}