linux-kernel - Re: [PATCH v5 3.1.0-rc4-tip 26/26] uprobes: queue signals while thread is singlestepping.

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Message-ID: <20111011173840.GE16268@linux.vnet.ibm.com>
Date:	Tue, 11 Oct 2011 23:08:40 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3.1.0-rc4-tip 26/26]   uprobes: queue signals while
 thread is singlestepping.

> > HOWEVER! I simply do not know what should we do if the probed insn
> > is something like asm("1:; jmp 1b;"). IIUC, in this sstep_complete()
> > never returns true. The patch also adds the fatal_signal_pending()
> > check to make this task killlable, but the problem is: whatever we do,
> > I do not think it is correct to disable/delay the signals in this case.
> > With any approach.
> >
> > What do you think? Maybe we should simply disallow to probe such insns?
> 
> Or. Could you explain why we can't simply remove the
> "if (vaddr == current->utask->xol_vaddr)" check from sstep_complete() ?


Yes, we could remove the check and rely on just the DIE_DEBUG to say
that singlestep has occurred. This was mostly needed when we were not
handling signals on singlestep.

> In some sense, imho this looks more correct for "rep" or jmp/call self.
> The task will trap again on the same (original) address, and
> handler_chain() will be called to notify the consumers.
> 
> But. I am really, really ignorant in this area, I am almost sure this
> is not that simple.
> 

Thats being modest.

-- 
Thanks and Regards
Srikar
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