linux-kernel - Re: [PATCH] [GIT PULL] tracing: allow to change permissions for text with dynamic ftrace enabled

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Message-Id: <1256760953.26028.1770.camel@gandalf.stny.rr.com>
Date:	Wed, 28 Oct 2009 16:15:53 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] [GIT PULL] tracing: allow to change permissions for
 text with dynamic ftrace enabled

On Wed, 2009-10-28 at 11:56 -0800, Suresh Siddha wrote:
> On Tue, 2009-10-27 at 16:31 -0700, Steven Rostedt wrote: 
> > On Tue, 2009-10-27 at 15:23 -0800, Suresh Siddha wrote:
> > 
> > > > 
> > > > I'll test it out, and if it does work, you can write up a formal patch
> > > > and remove the !define that I added.
> > > 
> > > I just saw one more place calling do_ftrace_mod_code(). So moved this
> > > check inside the do_ftrace_mod_code(). Does this cover all the cases?
> > > Thanks.
> > 
> > For ftrace, probably. But I just realized this may have other
> > consequences. The CPA_DEBUG tests setting a bit in the page table at
> > random locations.
> 
> CPA_DEBUG is setting/clearing unused bit 9. It has no interaction with
> RW bits. Though the random page and random length selections may end up

Actually it does. When you call the clear_page_attribute, and go to set
or clear bit 9, the static_protections() function is called against the
new_prot. This will clear the RW bit if it is set, even if the original
caller did not touch that bit.

> splitting the large page mappings for kernel text. This is a different
> issue that needs to be addressed (perhaps I can ignore this bit 9 as
> well for the kernel text mapping. In general it sounds like a bad idea
> to break any large page mappings to small page mappings for cpa debug
> tests, especially when we don't restore the large page mapping when we
> clear those unused bits once the test is done. Will look into this).
> 
> > The code you added makes any change to kernel text
> > page attributes remove the RW bit.
> 
> Only for CONFIG_DEBUG_RODATA.
> 
> > On boot up, it is considered that
> > kernel text is writable.
> 
> For CONFIG_DEBUG_RODATA, during the boot we change it to RO.

We do that at the end of boot up. I'm sure there's a reason for this :-/

> 
> > Perhaps you need to add a check in your if statement to only prevent
> > that bit when kernel_set_to_readonly is not true.
> 
> Are you worried that kernel_set_to_readonly might not be true but we
> might have the kernel text mapping read-only?

Yes, because the code clears the RW bit whenever something changes any
attribute of that page table. Including bit 9.

> 
> As ftrace is now using kernel identity mapping on 64-bit, we can change
> the semantics of kernel_set_to_readonly to reflect the status of kernel
> identity mapping for 64-bit. I can update the comments.

Well, the identity mapping should still be set to readonly after boot
up, right?

-- Steve


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