linux-kernel - Re: perf: fuzzer BUG: KASAN: stack-out-of-bounds in __unwind

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Message-ID: <20161129171038.GN3092@twins.programming.kicks-ass.net>
Date:   Tue, 29 Nov 2016 18:10:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>
Subject: Re: perf: fuzzer BUG: KASAN: stack-out-of-bounds in __unwind_start

On Tue, Nov 29, 2016 at 05:29:20PM +0100, Petr Mladek wrote:

> > > People are very busy polishing the turd we call printk, but from where
> > > I'm sitting its terminally and unfixably broken.
> 
> I still hope that we could do better :-)

How? The console drivers are a complete trainwreck, you simply cannot
build anything sensible ontop of a trainwreck.

And from what I understood from talking to someone (I again forgot who)
at LPC, the whole reason people were poking at this is that the block
layer (or something thereabouts) prints a gazillion lines of crap when
you attach a stupid amount of devices (through FC or other SAN like
things).

The way we've 'fixed' that in the scheduler (a fairly long time ago)
when SGI complained about our printks taking too long (because they had
4096 CPUs), is to simply remove the printks (they're now hidden behind
the sched_debug boot param).

In any case, as long as printk has a globally serialized 'log', it, per
design, will be worse than the console drivers its build upon. And them
being shit precludes the entire stack from being useful.

It mostly works, most of the time, and that seems to be what Linus
wants, since its really the best we can have given the constraints. But
for debugging, when you have a UART, it totally blows.