Re: linux-next: add utrace tree

From: Linus Torvalds
Date: Wed Jan 27 2010 - 05:44:58 EST

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.

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