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Message-ID: <20160825174904.vfam55lpo7geczqf@treble>
Date: Thu, 25 Aug 2016 12:49:04 -0500
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...nel.org>,
"H . Peter Anvin" <hpa@...or.com>,
the arch/x86 maintainers <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Andy Lutomirski <luto@...capital.net>,
Steven Rostedt <rostedt@...dmis.org>,
Brian Gerst <brgerst@...il.com>,
Kees Cook <keescook@...omium.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Byungchul Park <byungchul.park@....com>,
Nilay Vaish <nilayvaish@...il.com>
Subject: Re: [PATCH 3/6] x86/dumpstack: make printk_stack_address() more
generally useful
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@...radead.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
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