linux-kernel - Re: [PATCH v3] livepatch: old_name,number scheme in livepatch sysfs directory

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Message-ID: <20151110134014.GA3163@treble.redhat.com>
Date:	Tue, 10 Nov 2015 07:40:14 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
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-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] livepatch: old_name,number scheme in livepatch sysfs
 directory

On Tue, Nov 10, 2015 at 09:49:09AM +0100, Miroslav Benes wrote:
> On Mon, 9 Nov 2015, Josh Poimboeuf wrote:
> 
> > 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.
> 
> I think it should be func_name,1 even if '0' is specified and the symbol 
> is unique. Because if we say that 1, 2, ... is the occurrence of the 
> symbol in kallsyms it should stay that way everywhere. Hence for 
> old_sympos == 0 it is func_name,1 in sysfs; for 1 it is still func_name,1; 
> for 2 it is func_name,2 and so on.
> 
> And I'd add this to sysfs documentation.

That makes sense, sounds fine to me.

-- 
Josh
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