Re: linux-next: add utrace tree

["H. Peter Anvin" <hpa@zytor.com>]

On 01/27/2010 02:43 AM, 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.
> 
> 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.
> 

We actually went through a review of that here at Intel.  We do not yet
have an *official* answer (in order for us to have that we have to have
it approved by the architecture committee and published in the SDM), but
to the best of our current knowledge (and I'm allowed to say this) the
int3 method followed by global IPIs should be safe for modifying *one
(atomic) instruction*.  This is a specific case of a more general rule,
but I don't want to disclose the whole rule until it has been officially
approved.

> 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.

On the more general rule of interpretation: I'm really concerned about
having a bunch of partially-capable x86 interpreters all over the
kernel.  x86 is *hard* to emulate, and it will only get harder as the
architecture evolves.

	-hpa