linux-kernel - Re: [RFC, PATCH, v3.9] default exported asm symbols to zero

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Message-ID: <1480739797.16599.118.camel@decadent.org.uk>
Date:   Sat, 03 Dec 2016 04:36:37 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Arnd Bergmann <arnd@...db.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Adam Borowski <kilobyte@...band.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Michal Marek <mmarek@...e.com>,
        Debian kernel maintainers <debian-kernel@...ts.debian.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alan Modra <amodra@...il.com>
Subject: Re: [RFC, PATCH, v3.9] default exported asm symbols to zero

On Fri, 2016-12-02 at 13:40 +0100, Arnd Bergmann wrote:
> With binutils-2.16 and before, a weak missing symbol was kept during the
> final link, and a missing CRC for an export would lead to that CRC
> being treated as zero implicitly. With binutils-2.17, the crc
> symbol gets dropped, and any module trying to use it will fail to
> load.
> 
> This sets the weak CRC symbol to zero explicitly, making it defined
> in vmlinux, which in turn lets us load the modules referring to
> that CRC.
> 
> The comment above the __CRC_SYMBOL macro suggests that this was
> always the intention, although it also seems that all symbols
> defined in C have a correct CRC these days, and only the exports
> that are now done in assembly need this.
> 
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> Not sure if this is the correct way of doing it, but this seems trivial
> enough and lets me build the kernel with missing CRCs with any binutils
> version.

I tried this along with Adam's patch on x86_64, with Debian's binutils
2.27.51.20161127.  The result was that the kernel's __kcrctab held 0
for several symbols, even though there was type information in asm-
prototypes.h and Module.symvers and the modules had a non-zero CRC for
those symbols.  With just Adam's patch, the kernel and modules agreed.

Ben.

> diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
> index 63554e9..59a3b2f 100644
> --- a/include/asm-generic/export.h
> +++ b/include/asm-generic/export.h
> @@ -54,6 +54,7 @@ KSYM(__kstrtab_\name):
>  KSYM(__kcrctab_\name):
> >  	__put KSYM(__crc_\name)
> >  	.weak KSYM(__crc_\name)
> > +	.set KSYM(__crc_\name), 0
> >  	.previous
>  #endif
>  #endif
> 
-- 
Ben Hutchings
Absolutum obsoletum. (If it works, it's out of date.) - Stafford Beer

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