Re: [PATCH 3/6] x86/dumpstack: make printk_stack_address() more generally useful

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Steven Rostedt <rostedt@goodmis.org>,
	Brian Gerst <brgerst@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Byungchul Park <byungchul.park@lge.com>,
	Nilay Vaish <nilayvaish@gmail.com>
Subject: Re: [PATCH 3/6] x86/dumpstack: make printk_stack_address() more generally useful
Date: Thu, 25 Aug 2016 12:49:04 -0500	[thread overview]
Message-ID: <20160825174904.vfam55lpo7geczqf@treble> (raw)
In-Reply-To: <20160824193707.qw7vii3l4ggcfl6d@treble>

On Wed, Aug 24, 2016 at 02:37:07PM -0500, Josh Poimboeuf wrote:
> On Wed, Aug 24, 2016 at 02:37:21PM -0400, Linus Torvalds wrote:
> > On Wed, Aug 24, 2016 at 2:22 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > I actively disable KASLR on my dev box and feed these hex numbers into
> > > addr2line -ie vmlinux to find where in the function we are.
> > >
> > > Having the option to make %pB generate them works for me.
> > 
> > Yeah, considering that this is the only place this is used, changing
> > %pB sounds quite reasonable.
> 
> There's now another use of '%pB' in proc_pid_stack() in the tip tree: I
> changed it to '%pB' from '%pS'.  But I think the modified '%pB' would
> work there as well.
> 
> > We could perhaps make %pB show the hex numbers and address (so pB
> > would expand to "[<hex>] symbolname".if
> > 
> >  (a) not randomizing (so the hex numbers _may_ be useful)
> > 
> >  (b) kptr_restrict is 0 (so the hex numbers are "safe" in the dmesg)
> > 
> > and fall back to just the symbolic name if either of those aren't true?
> 
> Do we really need to check for both?  '%pK' only checks kptr_restrict.
> I'd think we should be consistent with that.  And maybe there are some
> scenarios where the actual text addresses provide useful debug
> information if KASLR is enabled and kptr_restrict is zero.

So I was looking at implementing this, and I noticed that '%pK' prints
"pK-error" if it's called from interrupt context when kptr_restrict==1.
Because checking CAP_SYSLOG would be meaningless in that case.

I don't really understand the point of the "pK-error" thing.  Any reason
why we can't print zero, i.e., just degrade the kptr_restrict from 1 to
2 in an interrupt?

That would make the '%pK' code simpler and usable from interrupt
context.  Also it would make its behavior consistent with the proposed
'%pB' changes, and the kptr_restrict code could be shared between '%pK'
and '%pB'.

Kess (or others), any objections if I make that change?

-- 
Josh