linux-kernel - Re: [PATCH 3/6] x86/dumpstack: make printk_stack

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Date:   Wed, 24 Aug 2016 14:37:07 -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 02:37:21PM -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.

> And obviously, if KALLSYMS isn't enabled, you always show hex
> numbers.. That's already the case (but we might want to add the "[<>}'
> markers around the hex numbers just to make the user space automation
> we do have work).

Even if kptr_restrict is set?

-- 
Josh