Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions

Andi Kleen
2009-02-28T16:38:16Z

On Fri, Feb 27, 2009 at 10:08:56PM +0100, Pavel Machek wrote:
> On Sun 2009-02-22 18:50:00, Andi Kleen wrote:
> > Steven Rostedt <rostedt@goodmis.org> writes:
> >
> > > From: Steven Rostedt <srostedt@redhat.com>
> > >
> > > Impact: keep kernel text read only
> > >
> > > Because dynamic ftrace converts the calls to mcount into and out of
> > > nops at run time, we needed to always keep the kernel text writable.
> > >
> > > But this defeats the point of CONFIG_DEBUG_RODATA. This patch converts
> > > the kernel code to writable before ftrace modifies the text, and converts
> > > it back to read only afterward.
> > >
> > > The conversion is done via stop_machine and no IPIs may be executed
> > > at that time. The kernel text is set to write just before calling
> > > stop_machine and set to read only again right afterward.
> >
> > The very old text poke code I had for this just used a dynamic
> > mapping elsewhere instead to modify the code. That's much less
> > intrusive than changing the complete mappings. Any reason you can't use
> > that too?
>
> Is it legal to have two mappings of same page with different
> attributes? IIRC some processors did not like that...

If you mean PAT caching attributes: correct it is not legal in x86 and
causes problems including data corruption.
If you mean other attributes like large page vs small page: it's normally legal,
with a few exceptions.

-Andi

--
ak@linux.intel.com -- Speaking for myself only.