Date: Tue, 11 Nov 2008 11:00:50 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: adobriyan@gmail.com, kenchen@google.com, linux-kernel@vger.kernel.org,
       a.p.zijlstra@chello.nl, Arjan van de Ven <arjan@infradead.org>
Subject: Re: [patch] add /proc/pid/stack to dump task's stack trace
Message-ID: <20081111100050.GA31766@elte.hu>
References: <20081107074147.GA26607@elte.hu> <20081107075925.GA1825@elte.hu> <20081107082003.GA15800@x200.localdomain> <20081107083249.GD4435@elte.hu> <20081107084943.GA16147@x200.localdomain> <20081107085352.GA27245@elte.hu> <20081107090304.GA30540@elte.hu> <20081107011659.98d3788a.akpm@linux-foundation.org> <20081107092902.GB7787@elte.hu> <20081110155410.be7552e3.akpm@linux-foundation.org>
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* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 7 Nov 2008 10:29:02 +0100
> Ingo Molnar <mingo@elte.hu> wrote:
> 
> > > I guess the 0400 mode on that file will suffice...
> > 
> > correct, 0400 is used already in the present patch:
> > 
> >  phoenix:~> cat /proc/1/stack
> >  cat: /proc/1/stack: Permission denied
> > 
> > but that is _not_ enough, it should be narrowed even more, to the 
> > boundaries that i pointed out in my first review feedback mail, and 
> > which is not implemented yet:
> > 
> >  1) only root should be allowed to do this - i.e. file needs to be 
> >     root-owned.
> > 
> >  2) there also needs to be a .config entry for folks to be able to
> >     turn it off altogether - just like folks can turn off sysrq-t 
> >     dumping via the .config.
> 
> Doing the above is desirable for another reason: given our rather 
> erratic history with the stack backtracer, this /proc file is 
> possibly a convenient way of oopsing the kernel, sending of off into 
> la-la-land, etc.

the stack tracer is rock-solid on x86 since Arjan started cleaning up 
the backtracing mess which we indeed had in x86 for years:

- adding frame-pointer support to 64-bit to improve the
  quality of stack-traces

- eliminating the broken and fragile dwarf2-unwinder

- expanding the use of the generic stacktrace infrastructure to
  lockdep, ftrace and other areas of code

if you know about any remaining fragility please holler, we havent had 
a backtracer induced oops for a long time :-)

	Ingo
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