linux-kernel - Re: linux-next: add utrace tree

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Date:	Wed, 27 Jan 2010 02:43:39 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Tom Tromey <tromey@...hat.com>,
	Kyle Moffett <kyle@...fetthome.net>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-next@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	utrace-devel@...hat.com, Thomas Gleixner <tglx@...utronix.de>,
	JimKeniston <jkenisto@...ibm.com>
Subject: Re: linux-next: add utrace tree

On Wed, 27 Jan 2010, Peter Zijlstra wrote:
> 
> Right, so you're going to love uprobes, which does exactly that. The
> current proposal is overwriting the target instruction with an INT3 and
> injecting an extra vma into the target process's address space
> containing the original instruction(s) and possible jumps back to the
> old code stream.

Just out of interest, how does it handle the threading issue?

Last I saw, at least some CPU people were _very_ nervous about overwriting 
instructions if another CPU might be just about to execute them.

Even the "overwrite only the first byte with 'int3'" made them go "umm, I 
need to talk to some core CPU people to see if that's ok". They mumble 
about possible CPU errata, I$ coherency, instruction retry etc.

I realize kprobes does this very thing, but kprobes is esoteric stuff and 
doesn't have much choice. In user space, you _could_ do the modification 
on a different physical page and then just switch the page table entry 
instead, and not get into the whole D$/I$ coherency thing at all.

				Linus
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