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

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Date:	Wed, 4 Nov 2015 17:12:52 -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 Wed, Nov 04, 2015 at 11:56:13PM +0100, Jiri Kosina wrote:
> On Tue, 3 Nov 2015, Josh Poimboeuf wrote:
> 
> > Subject: [PATCH] livepatch: Cleanup page permission changes
> > 
> > Calling set_memory_rw() and set_memory_ro() for every iteration of the
> > loop in klp_write_object_relocations() is messy and inefficient.  Change
> > all the RO pages to RW before the loop and convert them back to RO after
> > the loop.
> 
> Generally speaking, I like the patch and would like to have this in 4.4 
> still (if worse becomes worst and we don't make it in time for merge 
> window, this still qualifies for -rc bugfix).
> 
> > Suggested-by: Miroslav Benes <mbenes@...e.cz>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > ---
> >  arch/x86/kernel/livepatch.c | 25 ++-----------------------
> >  kernel/livepatch/core.c     | 42 +++++++++++++++++++++++++++++++++++++-----
> >  2 files changed, 39 insertions(+), 28 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/livepatch.c b/arch/x86/kernel/livepatch.c
> > index d1d35cc..1062eff 100644
> > --- a/arch/x86/kernel/livepatch.c
> > +++ b/arch/x86/kernel/livepatch.c
> > @@ -20,8 +20,6 @@
> >  
> >  #include <linux/module.h>
> >  #include <linux/uaccess.h>
> > -#include <asm/cacheflush.h>
> > -#include <asm/page_types.h>
> >  #include <asm/elf.h>
> >  #include <asm/livepatch.h>
> >  
> > @@ -38,8 +36,7 @@
> >  int klp_write_module_reloc(struct module *mod, unsigned long type,
> >  			   unsigned long loc, unsigned long value)
> >  {
> > -	int ret, numpages, size = 4;
> > -	bool readonly;
> > +	int size = 4;
> 
> BTW I don't see a reason to have 'size' signed here.

It was already signed to begin with, but I can change it to size_t.

> [ ... snip ... [
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -28,6 +28,7 @@
> >  #include <linux/list.h>
> >  #include <linux/kallsyms.h>
> >  #include <linux/livepatch.h>
> > +#include <asm/cacheflush.h>
> >  
> >  /**
> >   * struct klp_ops - structure for tracking registered ftrace ops structs
> > @@ -131,6 +132,33 @@ static bool klp_initialized(void)
> >  	return !!klp_root_kobj;
> >  }
> >  
> > +#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);
> > +}
> > +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?

-- 
Josh
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