[ Added Arjan ]

On Wed, 2010-01-27 at 02:43 -0800, Linus Torvalds wrote:
> 
> 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.

I think the issue was that ring 0 was never meant to do that, where as,
ring 3 does it all the time. Doesn't the dynamic library modify its
text?

-- Steve

> 
> 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