Re: [PATCH v1 7/10] Uprobes Implementation

[Ananth N Mavinakayanahalli <ananth@in.ibm.com>]

On Tue, Mar 23, 2010 at 02:46:52PM +0100, Peter Zijlstra wrote:
> On Tue, 2010-03-23 at 17:53 +0530, Srikar Dronamraju wrote:
 
> > > I would still prefer to see something like:
> > > 
> > >  vma:offset, instead of tid:vaddr
> > >  
> > > You want to probe a symbol in a DSO, filtering per-task comes after that
> > > if desired.
> > > 
> 
> > do you mean the user should be specifying 357c200000:74b80 to denote
> > 000000357c274b80? or /lib64/libc.so.6:74b80
> > And we trace all the process which have mapped this address?
> 
> Well userspace would simply specify something like: /lib/libc.so:malloc,
> we'd probably communicate that to the kernel using a filedesc and
> offset.
> 
> And yes, all processes that share that DSO, consumers can install
> filters.

Well, rewind back to 2006 to the first edition of uprobes; it had the
'global' tracing feature like what you indicate here, although Andrew
wouldn't want to be reminded of *how* that was done (hooking
readpages()) :-)

At the time, global tracing was vehemently vetoed in favour of a per-process
approach.

Now the question is, where the complexity needs to be.
 
> > > Also, like we discussed in person, I think we can do away with the
> > > handler_in_interrupt thing by letting the handler have an error return
> > > value and doing something like:
> > > 
> > > do_int3:
> > > 
> > >   uprobe = find_probe_point(addr);
> > > 
> > >   pagefault_disable();
> > >   err = uprobe->handler(uprobe, regs);
> > >   pagefault_enable();
> > > 
> > >   if (err == -EFAULT) {
> > >     /* set TIF flag and call the handler again from
> > >        task context */
> > >   }
> > > 
> > > This should allow the handler to optimistically access memory from the
> > > trap handler, but in case it does need to fault pages in we'll call it
> > > from task context.
> > 
> > Okay but what if the handler is coded to sleep.
> 
> Don't do that ;-)
> 
> What reason would you have to sleep from a int3 anyway? You want to log
> bits and get on with life, right? The only interesting case is faulting
> when some memory references you want are not currently available, and
> that can be done as suggested.

With the TIF method, you get to the probed process' task context in 
do_notify_resume(), and have sufficient flexibility for non-perf users, 
like gdb, 'cos what uprobes provides now, is close to what Tom Tromey
asked for gdb's usage.

Ananth