linux-kernel - Re: [PATCH] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags

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Message-ID: <20191121165320.GA4905@willie-the-truck>
Date:   Thu, 21 Nov 2019 16:53:20 +0000
From:   Will Deacon <will@...nel.org>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org,
        Matthias Maennich <maennich@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH] export.h: reduce __ksymtab_strings string duplication by
 using "MS" section flags

Hi Jessica,

On Thu, Nov 21, 2019 at 05:09:20PM +0100, Jessica Yu wrote:
> +++ Rasmus Villemoes [21/11/19 11:51 +0100]:
> > On 20/11/2019 15.51, Jessica Yu wrote:
> > >  /*
> > > - * note on .section use: @progbits vs %progbits nastiness doesn't matter,
> > > - * since we immediately emit into those sections anyway.
> > > + * note on .section use: we specify @progbits vs %progbits since usage of
> > > + * "M" (SHF_MERGE) section flag requires it.
> > >   */
> > > +
> > > +#ifdef CONFIG_ARM
> > > +#define ARCH_PROGBITS %progbits
> > > +#else
> > > +#define ARCH_PROGBITS @progbits
> > > +#endif
> > 
> > Did you figure out a way to determine if ARM is the only odd one? I was
> > just going by gas' documentation which mentions ARM as an example, but
> > doesn't really provide a way to know what each arch uses. I suppose 0day
> > will tell us shortly.
> 
> I *think* so. At least, I was going off of
> drivers/base/firmware_loader/builtin/Makefile and
> scripts/recordmcount.pl, which were the only other places that I found
> that reference the %progbits vs @progbits oddity. They only use
> %progbits in the case of CONFIG_ARM and @progbits for other
> arches. I wasn't sure about arm64, but I looked at the assembly files
> gcc produced and it looked like @progbits was used there. Added some
> arm64 people to CC since they would know :-)

The '@' character is a comment delimiter for 32-bit ARM assembly, so that's
why you end up having to use a different character there. This isn't the
case for arm64, where you need to use standard C/C++ comment delimiters
instead and so '@...gbits' should work correctly.

Hope that helps,

Will