linux-kernel - Re: [PATCH v3 2/4] asm-generic: Provide a fncpy() implementation

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Date:   Mon, 19 Jun 2017 04:11:05 +0300
From:   Yury Norov <ynorov@...iumnetworks.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@...r.kernel.org>, Keerthy J <j-keerthy@...com>,
        Arnd Bergmann <arnd@...db.de>, Dave Gerlach <d-gerlach@...com>,
        Tony Lindgren <tony@...mide.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        open list <linux-kernel@...r.kernel.org>,
        bcm-kernel-feedback-list@...adcom.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        linux-omap@...r.kernel.org, Shawn Guo <shawnguo@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 2/4] asm-generic: Provide a fncpy() implementation

> > +/*
> > + * Minimum alignment requirement for the source and destination addresses
> > + * for function copying.
> > + */
> > +#define FNCPY_ALIGN 8
> 
> >From now this is not arm-only, and it's possible that some architectures
> might want to redefine it in their arch/xxx/include/asm/fncpy.h files.
> So it will be easier for them if you'll wrap FNCPY_ALIGN here with #ifdef
> guards.
> 
> By the way, compiler already has an information on the proper alignment.
> Maybe it's better to use it as the default value here instead of the
> hardcoded value?
> 
> #ifndef FNCPY_ALIGN
> #define FNCPY_ALIGN ({void foo(); __alignof__(&foo);})
> #endif

Ah sorry, at first it should be like this:
#define FNCPY_ALIGN ({void foo(); __alignof__(foo);})

And at second, the correct version returns 1 always.

Even if I pass falign-functions=4096 to gcc, and I see that functions
are aligned accordingly in elf file, the macro returns 1 anyway. So if 
it doesn't work, the hardcoded '8' is the only option.

Yury