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Subject: Re: [PATCH v5 3.1.0-rc4-tip 17/26]   x86: arch specific hooks for
 pre/post singlestep handling.
From: Peter Zijlstra <peterz@infradead.org>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
        Linux-mm <linux-mm@kvack.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Hugh Dickins <hughd@google.com>, Christoph Hellwig <hch@infradead.org>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Jonathan Corbet <corbet@lwn.net>,
        Oleg Nesterov <oleg@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        Roland McGrath <roland@hack.frob.com>,
        Andi Kleen <andi@firstfloor.org>,
        Andrew Morton <akpm@linux-foundation.org>
Date: Tue, 27 Sep 2011 13:44:12 +0200
In-Reply-To: <20110926163426.GA15435@linux.vnet.ibm.com>
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On Mon, 2011-09-26 at 22:04 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@infradead.org> [2011-09-26 16:23:53]:
> 
> > On Tue, 2011-09-20 at 17:33 +0530, Srikar Dronamraju wrote:
> > > +fail:
> > > +       pr_warn_once("uprobes: Failed to adjust return address after"
> > > +               " single-stepping call instruction;"
> > > +               " pid=%d, sp=%#lx\n", current->pid, sp);
> > > +       return -EFAULT; 
> > 
> > So how can that happen? Single-Step while someone unmapped the stack?
> 
> We do a copy_to_user, copy_from_user just above this,

I saw that,

>  Now if either of
> them fail, we have no choice but to Bail out.

Agreed,

>  What caused this EFault may not be under Uprobes's Control.

I never said it was.. All I asked is what (outside of uprobe) was done
to cause this, and why is this particular error important enough to
warrant a warn.