Date: Mon, 9 Nov 2015 22:54:34 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Chris J Arges <chris.j.arges@canonical.com>
Cc: live-patching@vger.kernel.org, jeyu@redhat.com,
        Seth Jennings <sjenning@redhat.com>, Jiri Kosina <jikos@kernel.org>,
        Vojtech Pavlik <vojtech@suse.com>, linux-api@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] livepatch: old_name,number scheme in livepatch sysfs
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On Mon, Nov 09, 2015 at 05:01:18PM -0600, Chris J Arges wrote:
> On 11/09/2015 02:56 PM, Josh Poimboeuf wrote:
> > I'd recommend splitting this up into two separate patches:
> > 
> > 1. introduce old_sympos
> > 2. change the sysfs interface
> > 
> > On Mon, Nov 09, 2015 at 10:16:05AM -0600, Chris J Arges wrote:
> >> In cases of duplicate symbols in vmlinux, old_sympos will be used to
> >> disambiguate instead of old_addr. Normally old_sympos will be 0, and
> >> default to only returning the first found instance of that symbol. If an
> >> incorrect symbol position is specified then livepatching will fail.
> > 
> > In the case of old_sympos == 0, instead of just returning the first
> > symbol it finds, I think it should ensure that the symbol is unique.  As
> > Miroslav suggested:
> > 
> >   0 - default, preserve more or less current behaviour. If the symbol is 
> >       unique there is no problem. If it is not the patching would fail.
> >   1, 2, ... - occurrence of the symbol in kallsyms.
> >   
> >   The advantage is that if the user does not care and is certain that the 
> >   symbol is unique he doesn't have to do anything. If the symbol is not 
> >   unique he still has means how to solve it.
> > 
> 
> So one part that will be confusing here is as follows.
> 
> If '0' is specified for old_sympos, should the symbol be 'func_name,0'
> or 'func_name,1' provided we have a unique symbol? We could also default
> to 'what the user provides', but this seems odd.

I don't feel strongly either way, but I think using the same number the
user provides is fine, since it makes the sysfs interface consistent
with the old_sympos usage.

> Another option would be to use no postfix when 0 is given, and only
> introduce the ',n' postfix if old_sympos is > 0.

IMO always having a suffix is good, as it makes parsing less surprising
and less error-prone.

> >>  static int klp_write_object_relocations(struct module *pmod,
> >> @@ -307,7 +318,7 @@ static int klp_write_object_relocations(struct module *pmod,
> >>  			else
> >>  				ret = klp_find_object_symbol(obj->mod->name,
> >>  							     reloc->name,
> >> -							     &reloc->val);
> >> +							     &reloc->val, 0);
> > 
> > I think it would be a good idea to also add old_sympos to klp_reloc so
> > the relocation code is consistent with the klp_func symbol addressing.
> >
> 
> So you are thinking as an optional external field as well? I'll have to
> look at this a bit more but makes sense to me.

Yeah, the semantics would be the same as klp_func.old_sympos.  We could
add a new klp_reloc.sympos and make klp_reloc.val a private field.

-- 
Josh
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