linux-kernel - Re: [PATCH 2/3 v4] livepatch: add old_sympos as disambiguator field to klp

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Date:	Mon, 16 Nov 2015 13:11:00 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Chris J Arges <chris.j.arges@...onical.com>,
	live-patching@...r.kernel.org, jeyu@...hat.com,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3 v4] livepatch: add old_sympos as disambiguator field
 to klp_reloc

On Fri 2015-11-13 10:59:34, Josh Poimboeuf wrote:
> On Fri, Nov 13, 2015 at 02:54:42PM +0100, Petr Mladek wrote:
> > On Thu 2015-11-12 13:19:17, Josh Poimboeuf wrote:
> > > On Thu, Nov 12, 2015 at 03:31:58PM +0100, Petr Mladek wrote:
> > > > On Wed 2015-11-11 11:57:31, Josh Poimboeuf wrote:
> > > > > On Wed, Nov 11, 2015 at 10:29:00AM -0600, Chris J Arges wrote:
> > > > > > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > > > > > index 26f9778..4eb8691 100644
> > > > > > --- a/kernel/livepatch/core.c
> > > > > > +++ b/kernel/livepatch/core.c
> > > > > > @@ -261,7 +222,7 @@ static int klp_find_verify_func_addr(struct klp_object *obj,
> > > > > >   * object is either vmlinux or the kmod being patched).
> > > > > >   */
> > > > > >  static int klp_find_external_symbol(struct module *pmod, const char *name,
> > > > > > -				    unsigned long *addr)
> > > > > > +				    unsigned long *addr, unsigned long sympos)
> > > > > >  {
> > > > > >  	const struct kernel_symbol *sym;
> > > > > >  
> > > > > 
> > > There are two cases for external symbols:
> > > 
> > > 1. Accessing a global symbol in another .o file in the patch module.
> > >    For an example of a patch which does this, see:
> > > 
> > >      https://github.com/dynup/kpatch/blob/master/test/integration/f22/module-call-external.patch
> > > 
> > >    In that example, notice that kpatch_string() function is global (not
> > >    static), and is not exported.  It *is* actually a real world
> > >    scenario.
> > 
> > Mirek helped me to understand it. The symbol is exported if you
> > compile the above patch from sources. kpatch produces the patch by
> > pecking out the newly created symbols without looking if they
> > are newly exported. I hope that we got it right.
> 
> Hm, I don't really follow what you're saying.  Are we using different
> definitions of 'exported'?

Yeah, I messed the meaning of the various terms. Also I did not have a
good picture about the difference in handling global symbols in normal
userspace binaries and kernel.


> By exported, I mean the use of the EXPORT_SYMBOL macro which makes the
> symbol available for use by other modules.  The above patch doesn't use
> the EXPORT_SYMBOL macro, so the kpatch_string symbol isn't exported, and
> can't be used by other kernel modules.
>
> However, the symbol *is* global and can be used by other .o files within
> the patch module.

Thanks you and Mirek for explanation. I hope that I will send less
confusing mails from now on.

Best Regards,
Petr
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