linux-kernel - Re: [PATCH] livepatch: Cleanup page permission changes

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Message-ID: <20151105151759.GC28254@treble.redhat.com>
Date:	Thu, 5 Nov 2015 09:17:59 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jiri Kosina <jikos@...nel.org>
Cc:	Miroslav Benes <mbenes@...e.cz>,
	Seth Jennings <sjenning@...hat.com>,
	Vojtech Pavlik <vojtech@...e.com>,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
	"Cyril B." <cbay@...aysdata.com>
Subject: Re: [PATCH] livepatch: Cleanup page permission changes

On Thu, Nov 05, 2015 at 10:40:26AM +0100, Jiri Kosina wrote:
> On Thu, 5 Nov 2015, Jiri Kosina wrote:
> 
> > > > > +#ifdef CONFIG_DEBUG_SET_MODULE_RONX
> > > > > +static void set_page_attributes(void *start, void *end,
> > > > > +				int (*set)(unsigned long start, int num_pages))
> > > > > +{
> > > > > +	unsigned long begin_pfn = PFN_DOWN((unsigned long)start);
> > > > > +	unsigned long end_pfn = PFN_DOWN((unsigned long)end);
> > > > > +
> > > > > +	if (end_pfn > begin_pfn)
> > > > > +		set(begin_pfn << PAGE_SHIFT, end_pfn - begin_pfn);
> > > > > +}
> > 
> > BTW is there any reason not to make use of the function from module.c 
> > which does exactly the same, instead of copy pasting it all around?
> > 
> > > > > +static void set_module_ro_rw(struct module *mod)
> > > > > +{
> > > > > +	set_page_attributes(mod->module_core,
> > > > > +			    mod->module_core + mod->core_ro_size,
> > > > > +			    set_memory_rw);
> > > > > +}
> > > > > +static void set_module_ro_ro(struct module *mod)
> > > > 
> > > > Honestly, I find both the function names above horrible and not really 
> > > > self-explanatory (especially the _ro_ro variant). At least comment, 
> > > > explaining what they are actually doing, or picking up a better name, 
> > > > would make the code much more self-explanatory in my eyes.
> > > 
> > > Being the patch author, naturally the function names make sense to me.
> > 
> > :)
> > 
> > > set_module_ro_ro() means "set the module's read-only area to have 
> > > read-only permissions."
> > >
> > > Do you have any suggestions for a better name?
> > 
> > I'd even say it's superfluous to have the functions at the first place, 
> > and just calling set_page_attributes() directly makes the intent clear 
> > enough already.
> 
> To make my proposal more clear:
> 
> - move set_page_attributes() to module.h and provide empty stub for 
>   !CONFIG_DEBUG_SET_MODULE_RONX case (and probably rename it to something 
>   like module_set_page_attributes() to avoid namespace conflicts with mm 
>   code)
>
> - make use of that function both from module.c (where it's already being 
>   used) and livepatch.c, where it'd be called directly

Ok, I'll use the module.c version of set_page_attributes() and get rid
of the set_module_ro_(ro|rw) functions.

I'd rather keep set_page_attributes() named as it already is, because
there's nothing module-specific about it.  It just happens to currently
live in module.c.

-- 
Josh
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