Re: [3/3 PATCH] Kprobes: User space probes support- single stepping out-of-line

[Suparna Bhattacharya]

On Tue, Mar 21, 2006 at 02:05:21AM -0800, Andrew Morton wrote:
> Prasanna S Panchamukhi <prasanna@xxxxxxxxxx> wrote:
> >
> > > 
> >  > > > > +	addr = (kprobe_opcode_t *)kmap_atomic(page, KM_USER1);
> >  > > > > +	addr = (kprobe_opcode_t *)((unsigned long)addr +
> >  > > > > +				 (unsigned long)(uprobe->offset & ~PAGE_MASK));
> >  > > > > +	*addr = opcode;
> >  > > > > +	/*TODO: flush vma ? */
> >  > > > 
> >  > > > flush_dcache_page() would be needed.
> >  > > > 
> >  > > > But then, what happens if the page is shared by other processes?  Do they
> >  > > > all start taking debug traps?
> >  > > 
> >  > > Yes, you are right. I think single stepping inline was a bad idea, disarming
> >  > > the probe looks to be a better option
> >  > > 
> >  > 
> >  > You skipped my second question?
> > 
> >  There is a small window in which other processor will not be able to see
> >  the breakpoint if we are single step inline. But since single stepping inline
> >  is a bad idea, we will disarm the probe forever (replace with original instrcution) if we cannot single step out-of-line.
> >  Any suggestions?
> 
> This doesn't appear to be working.
> 
> Let's go back in time and pretend that these patches were never written,
> OK?  We're standing around the water cooler saying "hey, wouldn't it be
> cool if someone did X".  You guys are way too far into this and you keep on
> leaving everyone else behind.  When I try to drag you up, you resist ;)
> 
> So let me rephrase, and thrash around in the dark a little more.
> 
> >From my reading of the code (and thus far that's my _only_ source of this
> information) it appears that a design decision has been made (for reasons
> which have yet to be disclosed) that the way to implement this (yet to be
> described) requirement is to set user breakpoints upon particular
> instructions within executables.  System-wide, for all processes and
> threads.
> 
> There are other things that could have been done.  For example, you might
> have chosen to set breakpoints upon a particular virtual address within a
> heavyweight process.  That's a process-oriented viewpoint, rather than a
> file-oriented one, if you like.
> 
> This raises interesting questions, like
> 
> - How come that decision was made?  Why _is_ this an executable-oriented
>   rather than process-oriented thing?  Richard has covered that somewhat.
> 
> - What happens if the executable is writeably mapped?
> 
> - What happens if someone writes to the executable?  (I think both
>   of these were disallowed in the implementation-which-is-not-to-be-named).
> 
> - What happens if different processes map the executable at different
>   addresses?
> 
> - Various other things which you've thought of and I haven't and which it
>   would be REALLY interesting to hear about.
> 
> But this is just one example.  I don't think I'm being too picky here - my
> reaction on seeing all this stuff was, basically, "wtf is all this code
> for?".  References to dprobes won't help, sorry - I've never looked at
> dprobes and I don't know anyone apart from you guys who has.
> 
> What I'm asking you for is a description of what problem we're trying to
> solve and how this code solves that problem.  It is hard, it is inefficient
> and, worse, it is error-prone for us to try to work all that out from a
> particular implementation.

Prasanna, I guess putting together a short writeup on the problem
description and the thinking behind the design decisions, known flaws etc,
in the form of a Documentation patch may help for starters. These questions
are likely to come up everytime anyone looks at the code.

The key thinking behind a lot of the design decisions was the
need for a very low overhead probe mechanism that would allow thousands of
active probes on the system and could detect any instance which hits the probe,
including probes on shared libraries which may be loaded by lots of
processes. Thus, (1) no forcing copy-on-write pages, (2) no forcing
in executable pages in memory just to place a probe on them (hence
zero overhead for probes which are very unlikely to be hit), (3) no
restrictions on evicting a page with a probe on it from memory (4) probes
being tracked by an (inode, offset) tuple rather than by virtual address
so that they can be shared across all processes mapping the executable/library
even at different virtual addresses, etc.

Regards
Suparna

-- 
Suparna Bhattacharya (suparna@xxxxxxxxxx)
Linux Technology Center
IBM Software Lab, India

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