linux-kernel - Re: [patch] add /proc/pid/stack to dump task's stack trace

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Date:	Fri, 7 Nov 2008 20:51:57 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ken Chen <kenchen@...gle.com>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch] add /proc/pid/stack to dump task's stack trace

On Fri, Nov 07, 2008 at 10:29:02AM +0100, Ingo Molnar wrote:
> 
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Fri, 7 Nov 2008 10:03:04 +0100 Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > > task running on this CPU:
> > > 
> > >  # cat /proc/self/stack
> > >  [<ffffffff80216f79>] save_stack_trace_tsk+0x26/0x44
> > >  [<ffffffff802f59a5>] proc_pid_stack+0x6e/0xd3
> > >  [<ffffffff802f6da3>] proc_info_read+0x68/0xba
> > >  [<ffffffff802b2f17>] vfs_read+0xa9/0xe3
> > >  [<ffffffff802b301f>] sys_read+0x4c/0x73
> > >  [<ffffffff8020c23b>] system_call_fastpath+0x16/0x1b
> > >  [<ffffffffffffffff>] 0xffffffffffffffff
> > 
> > So we provide a means by which process A can sample process B's 
> > instruction pointer?  Even if it's in random.c or crypto code?  
> > There's a little project for someone.
> 
> yes - like "echo p > /proc/sysrq-trigger" and sysrq-t. Although unlike 
> sysrq-p, the IP itself isnt printed, just the stack trace - but 
> there's indeed a correlation.
> 
> > 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.

In the name of everything holy, don't add another config option.
It's a _tiny_ piece of code and you have select STACKTRACE via
fault injection, latencytop or lockdep before that.
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