Subject: Re: linux-next: add utrace tree

From: Linus Torvalds <[email-masked]>
Date: Wed, 27 Jan 2010 02:43:39 -0800 (PST)

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