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

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Date:   Sat, 03 Dec 2016 11:43:07 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Ben Hutchings <ben@...adent.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 Saturday, December 3, 2016 4:36:37 AM CET Ben Hutchings wrote:
> 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.

Can you be more specific? Which symbols are those? I would have expected
modpost to generate Module.symvers from the vmlinux file, so I wonder
where that difference comes from.

	Arnd