linux-kernel - Re: [PATCH 1/2] livepatch: Initialize shadow variables by init function safely

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Date:   Wed, 4 Apr 2018 15:08:50 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Joe Lawrence <joe.lawrence@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, Jessica Yu <jeyu@...nel.org>,
        Nicolai Stange <nstange@...e.de>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] livepatch: Initialize shadow variables by init
 function safely

On Wed 2018-03-14 14:44:36, Josh Poimboeuf wrote:
> On Wed, Mar 14, 2018 at 03:27:02PM -0400, Joe Lawrence wrote:
> > On Tue, Mar 13, 2018 at 04:54:47PM +0100, Petr Mladek wrote:
> > > The existing API allows to pass a sample data to initialize the shadow
> > > data. It works well when the data are position independent. But it fails
> > > miserably when we need to set a pointer to the shadow structure itself.
> > > 
> > > Unfortunately, we might need to initialize the pointer surprisingly
> > > often because of struct list_head. It is even worse because the list
> > > might be hidden in other common structures, for example, struct mutex,
> > > struct wait_queue_head.
> > > 
> > > This patch makes the API more safe. A custom init function and data
> > > are passed to klp_shadow_*alloc() functions instead of the sample data.
> > 
> > Yup, this looks kinda familiar, I remember tinkering with the same idea
> > last year [1] before settling on the simpler API.
> > 
> > [1] https://github.com/torvalds/linux/compare/master...joe-lawrence:shadow_variables_v2_c
> > 
> > > Note that the init_data are not longer a template for the shadow->data.
> > > It might point to any data that might be necessary when the init
> > > function is called.
> > 
> > I'm not opposed to changing the API, but I was wondering if you had
> > thought about expanding it as an alternative? 
> > 
> > When working on this last summer, I remember holding onto to some less
> > than intuitive naming conventions so that I could support a basic API
> > and an extended API with bells and whistles like this patchset
> > implements.  It didn't seem too difficult to layer the basic API ontop
> > of one like this (see [1] for example), so maybe that's an option to
> > keep basic shadow variable usage a little simpler.  /two cents
> 
> I like Petr's new API.  It's not a big deal to just pass a couple of
> NULLs if you don't need the callback.
> 
> And I prefer fewer functions anyway -- maybe it's my functionitis
> allergies acting up again.

Yeah, I think that that two APIs might cause confusion. Especially
because *data and *init_data have different meaning. I would prefer
to keep only the first one.


> > Perhaps shadow variables are another candidate for some kind of
> > kselftest?
> 
> Indeed!

It would be great.


Best Regards,
Petr

PS: Thanks all for the feedback.