linux-kernel - Re: [PATCH] x86/kbuild: enable modversions for symbols exported from asm

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Message-ID: <20161210021529.4a6e684f@roar.ozlabs.ibm.com>
Date:   Sat, 10 Dec 2016 02:15:29 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Ian Campbell <ijc@...lion.org.uk>
Cc:     Michal Marek <mmarek@...e.com>,
        Ben Hutchings <ben@...adent.org.uk>,
        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>,
        Debian kernel maintainers <debian-kernel@...ts.debian.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dodji Seketeli <dodji@...eteli.org>
Subject: Re: [PATCH] x86/kbuild: enable modversions for symbols exported
 from asm

On Fri, 09 Dec 2016 15:21:33 +0000
Ian Campbell <ijc@...lion.org.uk> wrote:

> On Fri, 2016-12-09 at 13:33 +1000, Nicholas Piggin wrote:
> > 
> > Well I simply tested the outcome. If you have:
> > 
> > struct blah {
> >   int x;
> > };
> > int foo(struct blah *blah)
> > {
> >   return blah->x;
> > }
> > EXPORT(foo);
> > 
> > $ nm vmlinux | grep __crc_foo
> > 00000000a0cf13a0 A __crc_foo
> > 
> > Now change to
> > 
> > struct blah {
> >   int y;
> >   int x;
> > };
> > 
> > $ nm vmlinux | grep __crc_foo
> > 00000000a0cf13a0 A __crc_foo
> > 
> > It just doesn't catch these things.  
> 
> I found the same when I just added your snippet to init/main.c.
> 
> _But_ when I moved the struct into include/types.h (which happened to
> be included by init/main.c) then, with just x in the struct:
> 
>     $ make -s init/main.{o,symtypes} && grep -E foo\|blah init/main.symtypes && nm init/main.o  | grep __crc_foo
>     s#blah struct blah { int x ; } 
>     foo int foo ( s#blah * ) 
>     000000000cd0312e A __crc_foo
> 
> but adding y:
> 
>     $ make -s init/main.{o,symtypes} && grep -E foo\|blah init/main.symtypes && nm init/main.o  | grep __crc_foo
>     s#blah struct blah { int x ; int y ; } 
>     foo int foo ( s#blah * ) 
>     00000000eda220c6 A __crc_foo
> 
> So it does catch things in that case.
> 
> With struct blah inline in main.c it was:
> 
>     $ make -s init/main.{o,symtypes} && grep -E foo\|blah init/main.symtypes && nm init/main.o  | grep __crc_foo
>     s#blah struct blah { UNKNOWN } 
>     foo int foo ( s#blah * ) 
>     00000000a0cf13a0 A __crc_foo
> 
> So I suppose it only cares about structs which are in headers, which I
> guess makes sense. I think it is working in at least one of the
> important cases.

Aha thanks, well that's my mistake. Clever little bugger, isn't it? Okay
it's not so useless as I first thought!

That said, a dwarf based checker tool should be able to do as good a job
(maybe a bit better because report is very informative and it may pick up
compiler alignments or padding options). So I still think it's worth
looking at those if we can remove modversions.

Thanks,
Nick